THE STRUCTURE AND PRINCIPLES OF THE IMMUNE SYSTEM

Structure and principles of function of the immune system. Immunology of ages. Current assessing of the immune status and the degree of immune disorders. Methods of laboratory immunodiagnosis. Features immunity parameters in different age groups. Values of local immunity factors in the formation of systemic immune response. Oral microbiota, immune system of oral mucosa.

Structure and principles of functioning of the immune system

Clinical immunology is a section of immunology, which studies the functions of the immune system in the normal condition and pathology.

Tasks of the clinical immunology: the study of functions of the immune system in the normal condition; the study of functions of the immune system in pathology (patients with the autoimmune diseases, allergies, cancer, in complications after surgical interventions, in diseases of the internal organs); development, study and classification of the immunodeficiency states; the study of infections of the immune system (HIV); development of the methods of diagnosis and evaluation of the immune status; development of the methods of correction of the immune status; search for and development of the immunotropic preparations; training of the staff.

The object of the studies of clinical immunologists is the immune (antigenic- structural) homeostasis, its regulation under the normal conditions and in pathology, as well as pathologic processes, different diseases, caused by disturbance of this homeostatic function, which can be defined as diseases of the immune system or immunopathology.

At present, the experts of WHO (World Health Organization) isolated five groups of diseases, directly associated with disturbance of functioning and pathology of the immune system:

– the first group – diseases, caused by insufficiency of the immune system (primary, second, transitory immunodeficiencies);

– the second group – diseases, caused by the excessive reaction of the immune system (autoimmune, allergic);

– the third group – infections of the immune system with localization of the causative agent in the immunocompetent cells;

– the fourth group – tumors of the immune system;

– the fifth group – diseases of the immune complexes.

Structure and principles of functioning of the immune system of men

The immune system is a specialized system of cells, tissues and organs, basic components of which are central (thymus and bone marrow) and peripheral lymphoid organs (spleen, lymph nodes, group of lymphatic follicles of the bowels – Peyer’s patches, tonsils), blood and lymphatic vessels. In the morphological aspect the immune system is considered as the totality of lymphocytes, macrophages, and a number of cells similar to the macrophages, including dendritic cells and epithelial cells of Langerhans (white dendritic epidermocytes).

 

Figure. I.1.  Thymus and bone marrow are primary lymphoid organs. They are sites of maturation for T and B cells respectively. Cellular and humeral immune responses occur in the secondary (peripheral) lymphoid organs and tissues. Secondary lymphoid organs can be classified according to the body regions which they defend. The spleen responds predominantly to blood-borne antigens. Lymph nodes mount immune responses to antigens circulating in the lymph, entering through the skin (subcutaneous lymph nodes) or through mucosal surfaces (visceral lymph nodes). Tonsils, Peyer’s patches and other mucosa-associated lymphoid tissues (blue boxes) react to antigens which have entered via the surface mucosal barriers. (I.Roitt et al., Immunology, 2001).

The chief characteristics of the immune system: prevalence in the organism, constant recirculation of the corresponding immune cells and their ability to produce specific molecules – antibodies regarding each antigen.

Basic tasks:

1. Protection of the organism from the penetration of genetically foreign external agents;

2. Control of constancy of the internal medium of the organism:

– destruction of the proper organism cells, which were subjected to genetic mutations;

– utilization of the proper organism cells destructed as a result of apoptosis;

– utilization of the cells (fragments, proteins), damaged as a result of inflammatory – destructive processes in the organism (injury, inflammation).

Acquired and congenital immunity

The immune response consists in recognition of the foreign genetic material and development of the responses directed at its elimination.

The immune reaction can be nonspecific (nonspecific immunity) and specific (specific immunity).

Nonspecific resistance is achieved by the cellular and humoral factors, which tightly interact in the achievement of the final effect – catabolism of the foreign substance: by macrophages, neutrophils, complement, lysozyme, properdin, IgA as well as by other cells and dissoluble factors. The factors of nonspecific resistance are also the skin and mucous membranes of the organism – physiological barriers and the secretion exerting the bactericidal effect. Saliva, gastric juice and digestive enzymes suppress growth and multiplication of the microbes.

Nonspecific immunity provides the first line of protection from the foreign particles and organisms, and it is accomplished by several types of cells. Phagocytes – the blood monocytes and tissue macrophages – absorb and destroy a lot of foreign particles. Polymorphonuclear leukocytes together with the mast cells participate in the protection from the microorganisms, being the most important components of the reaction of acute inflammation. Natural killers provide the first line of protection of the organism affecting its own cells infected with viruses or neoplastic process. The so-called proteins of the acute phase of inflammation and the complement system play an important part in the formation of responses of the nonspecific immunity.

The main characteristics of the acquired immunity (specific immunity) are the formation of specific antibodies, presence of immunological memory, possible participation of T-killers and learning ability. Instruction and memory are accomplished by the laws of clonal selection, i.e. genesis of the cell clones ready to the subsequent encounter with the identical (specific) antigen. Clonal selection is a proliferation of cells, caused by the specific antigen. The second immune response in comparison with the primary one is always more rapid and stronger (phenomenon of instruction at the level of the cellular populations).

Any immune response has two basic phases:

• Recognition of the antigen;

• Reactions, directed at its elimination.

The immune system has a number of mechanisms for destroying the pathogenic microbes, and each of them corresponds to this type of infection and concrete stage of the life cycle of the agent.

Figure I.2 The response to an initial infection occurs in three phases. The effector mechanisms that remove the infectious agent (eg phagocytes, NK cells, complement) are similar or identical in each phase but the recognition mechanisms differ. Adaptive immunity occurs late, because rare antigen-specific cells must undergo clonal expansion before they can differentiate into effector cells. After an adaptive immune response to a pathogen, the response to re-infection is much more rapid; pre-formed antibodies and effector cells act immediately on the pathogen, and immunological memory speeds a renewed adaptive response. (Charles A. Janeway et al., Immunobiology, 1999).

 

 

Figure I.3. Development of the immune response. The details of this response will he discussed in suhsequent chapters. When an antigen is introduced into a host, antigen-presenting cells (macrophages) process and present the antigen to CD4⁺ T-cells (usually consid­ered T_h-cells). This process and the release of interleukin-1 (IL-1), a helper monokine, activate the CD4⁺ T-cells. Activated CD4⁺ T cells help themselves and T -cells (usually considered T_h-cells) by releasing IL-2. Activated T-cells respond in cell-mediated immunity reactions by their di­rect participation as effector cells. Antigen-activated B-cells, af­ter processing and presenting anlijicn to activated B-cells, convert to plasma cells with help through direct interaction with T-cells and by cytokines like IL-4 and IL-5.The plasma cells se­crete specific antibodies against the inducing agent and thereby provide humoral immunity. Some cells become memory cells and react more rapidly to the next ex­posure. The T-cell receptors (TCR) of CD4⁺ T-cells recog­nize antigen only when associ­ated with class II or I molecules HLA, respectively. (Klaus D. Elgert, Immunology, 1996).

 

 

 

Phases of the immune response

(Charles A. Janeway et al., Immunobiology, 1999 with change)

 

 

Neutralization The antibodies are bound with the specific agent to oppose its vital activity. Antibodies to the external proteins of the capsid of some rhino-viruses prevent binding of the viral particles with the cells of organism and their infection.

Phagocytosis The antibodies realize their effect by activating complement or acting as the opsonins, which intensify absorption of the microbes by phagocytes and their processing.

Cytotoxic reactions and apoptosis The cytotoxic reactions are the effector immune mechanisms directed at intact cells, usually at those, which are too large for phagocytosis. This target cell is recognized either by the specific antibodies, which interact with the components of its surface or by T-cells by means of the antigenspecific receptors. The granules of the cytotoxic T-cells contain the compounds, called perforines, which are capable of creating canals in the external membrane of the target cells. Some cytotoxic cells are also capable of including the program of self-destruction of the target cell by their signal – the process of apoptosis.

In the normal condition the immune system cells are scattered all over the body tissues, but when there is a focus of infection, these cells and their byproducts are concentrated precisely in it. This process is called the inflammatory reaction.

Figure I.4. Phagocytosis.  Cells of the mononuclear phagocyte sys­tem are attracted to the site of infection by factors released by pathogens, damaged host cells, and other blood components. The phago­cytes engulf the pathogens, using pseudopodia. and internalize them as membrane-bound organelles (phago­somes) within the phagocyte. The phagosomes fuse with other organ­elles (lysosomes) containing hydro-lytic enzymes. These organelles arc then called phagolysosomes. Inside the phagolysosome, azurophile and specific granules discharge two groups of toxic substances into the organ­elle: (1) oxygen-dependent products formed by reactive oxygen metabo­lites and (2) oxygen-independent re-actants. such as proteases, lactoferrin. and phospholipase A₂. Organisms are killed by the action of superoxide ions, hypochlorite, and hydrogen per­oxide. The phagocyte’s activity is en­hanced by other parts of the immune system (Klaus D. Elgert, Immunology, 1996).

 

Chemotaxis and migration of the cells Having penetrated into the tissue, the cells migrate towards the infection focus under the effect of the chemical attraction, called chemotaxis. Active migration of the specific chemotaxic compounds by the concentration gradient is inherent to phagocytes. Especially intense chemotaxis is caused by a fragment of one of the complement components, C5a, attracting neutrophils and monocytes.

Figure I.5. The adaptive immune system modulates inflammatory processes via the complement system. Antigens (e.g. from, microorganisms) stimulate B-cells to produce antibodies including IgE, which binds to mast cells, while IgG and IgM activate complement Complement can also be activated directly via the alternative pathway. When triggered by antigen, the sensitized mast cells release their granule-associated mediators and eicosanoids (products of arachidonic acid metabolism, including prostaglandins and leukotrienes). In association with complement (which can also trigger mast cells via C3a and C5a) the mediators induce local inflammation, facilitating the arrival of leucocytes and more plasma enzyme system molecules. (I.Roitt et al., Immunology, 2001)

 

The major complex of histocompatibility

The major histocompatibility complex (MHC) is a group of genes and antigens of the cellular surface coded by them, which play the most important role in the recognition of the foreign substance and development of the immune response.

          The major molecules of histocompatibility are a family of glycoproteins coded by the genes, which compose the major histocompatibility complex. There are the genes within MHC, which control main transplantation antigens and genes, which determine the intensity of the immune response to this or that concrete antigen, the so-called Ir (immune response) – genes. The molecules of MHC in man are called HLA (human leucocyte-associated), since they were initially discovered on the leukocytes.

Antigens HLA (sometimes they are called transplantation antigens) are glycoproteins, which are located on the cell surface and coded by the group of the tightly linked genes of the 6th chromosome.

           The names genes and antigens of HLA consist of one or several letters and numbers, for example, A3, B45, DR15, DQ4. The letter marks the gene, and number means the allele of this gene, and digital designations are given in discovery of new alleles. There are more than 100 antigens of HLA today.

     There are 3 classes of HLA antigens.

            The class I includes the antigens A, B and C. The antigens of the class I are present on the surface of all nucleus-containing cells and thrombocytes. They are necessary for the recognition of the transformed cells by the cytotoxic T-lymphocytes. The cytotoxic T-lymphocytes (T- killers) recognize the target cells only in presence of the HLA antigens of the class I of their own genotype on their surface.

           Class II comprises antigens DR, DP and DQ. The antigens of the class II are present on the surface of B-lymphocytes, activated T-lymphocytes, monocytes, macrophages and dendritic cells, i.e., on the cells, which participate in the immune responses (lymphocytes, macrophages).

           The most important function of the HLA antigens of the class II is provision of interaction between the T-lymphocytes and macrophages in the process of the immune response. T- helpers recognize foreign antigen only after its processing by macrophages, combining with the HLA antigens of the class II and appearance of this complex on the macrophage surface.

            The genes of HLA of the class III control some components of the complement: C4 and C2, the tumor necrosis factors (TNF- alpha and TNF- beta), i.e., control the synthesis of the proteins, part of which participates in the immune processes. However, in contrast to the molecules HLA of the class I and the class II they do not participate in control of the immune response.

  Figure I.6. T_h1 and T_h2-cells recognize antigen presented by MHC class II molecules. Th1 and Th2-cells both recognize peptides bound to MHC class II molecules. On recognition of their spe­cific antigen on infected macrophages, Th1 cells activate the macrophage, lead­ing to the destruction of the intracellular bacteria (left panel). When T_h2-cells recognize antigen on B-cells, helper T-cells activate these cells to proliferate and differentiate into antibody-producing plasma cells (right panel) (Charles A. Janeway et al., Immunobiology, 1999).

 

           The expression of antigens HLA regulates cytokine- interferon- gamma and factor of the tumor necrosis – powerful inductors of the HLA expression by the cells of many types. The majority of the HLA genes are highly polymorphous, i.e., there may be many alleles in the population in the specific locus of HLA. The inheritance of the HLA- genes occurs by the codominant sign, in which in descendents the HLA- alleles obtained from each of the parents are expressed in the same degree.

The expression of MHC molecules differs between tissues

(Charles A.Janeway et al., Immunobiology, 1999 with cange)

Cellular immunity

Figure. I.7. All the cells shown arise from the haemopoietic stem cell. Platelets produced by megakaryocytes are released into the circulation. Granulocytes and monocytes pass from the circulation into the tissues. Mast cells are identifiable in all tissues. B cells mature in the fetal liver and bone marrow in mammals, whereas Tcells mature in the thymus. The origin of the large granular lymphocytes with NK activity is probably the bone marrow Lymphocytes recirculate through secondary lymphoid tissues. Interdigitating cells and dendritic cells act as antigen-presenting cells in secondary lymphoid tissues. (I.Roitt et al., Immunology, 2001).

 

Antigen-processing cell (Antigen-presenting cell). For an antigen to be recognised by a T-lymphocyte, it must be first processed and presented in a form the antigen can recognise. This is the function of an antigen-processing cell. The process by which this takes place is the following: an antigen-processing cell engulfs an antigen → enzymes in the antigen-processing cell break down the antigen into smaller fragments → these fragments are transported to the surface of the antigen-processing cell (APC), bound with class II MHC molecules → a T-cell receptor caow recognize the antigen linked with the MHC and thus binds to it. Antigen-processing cells include: macrophages; dendritic cell; follicular dendritic cell; Langerhans cell; Langerhans cells are dendritic cells specific to the skin.

 

 

Figure I.8.  Mononuclear phagocytes (1), B-cells (2) and dendritic ceils (3) can all present antigen to MHC class ll-restricted T-helper (Th) cells. Macrophages take up bacteria or particulate antigen via non-specific receptors or as immune complexes, process it and return fragments to the cell surface in association with class II molecules. Activated B-cells can take up antigen via their surface immunoglobulin and present it to T-ceils associated with their class II molecules. Dendritic cells constitutively express class II MHC molecules and take up antigen by macropinocytosis. (I.Roitt et al., Immunology, 2001).

 

 

Figure I.9. Dendritic cells mature through at least two definable stages to become potent antigen-presenting cells in lymphoid tissue. Dendritic cells arise from bone marrow progenitors and migrate via the blood to peripheral tissues and organs, where they are highly phago­cytic via receptors such as DEC 205 but do not express co-stimulatory molecules (top panel). When they pick up antigen in the peripheral tissues, they are induced to migrate via the afferent lymphatic ves­sels to the regional lymph node. Here they express high levels of T-cell activating potential but are no longer phagocytic. Dendritic cells in lym­phoid tissue express B7.1, B7.2, and high levels of MHC class I and class II mol­ecules, as well as high levels of the adhesion molecules ICAM-1. ICAM-2, LFA-1, and LFA-3 (center panel). The photograph shows a mature dendritic cell (Charles A. Janeway et al., Immunobiology, 1999).

 

Dendritic cell. The interdigital dendritic cells are most important for the presentation of antigen to the T-cells. Dendritic cells are the principle antigen-processing cell involved in primary immune responses. Their major function is to obtain antigen in tissues, migrate to lymphoid organs and activate T-cells. Cell required for, but not actually mediating, a specific immune response.

Leukocyte White Blood Cell (WBC) White blood cells are the principal components of the immune system and function by destroying “foreign” substances such as bacteria and viruses. Like all blood cells, they are produced in the bone marrow. There are 5 main types of white blood cell, subdivided between 2 main groups:

Polymorphonuclear leukocytes (granulocytes): neutrophils; eosinophils; basophils

Mononuclear leukocytes: monocytes; lymphocytes.

Normal values range from 4100/ml to 10900/ml but can be altered greatly by factors such as exercise, stress and disease. A low WBC may indicate viral infection or toxic reactions. A high WBC count may indicate infection, leukemia, or tissue damage. An increased risk of infection occurs once the WBC drops below 100/ml.

Mononuclear phagocytes. The most important group capable of phagocytosis and long-life cells is a population of mononuclear phagocytes, proceeding from the stem cells of the bone marrow, having the function of capture of the particles, including those of the infectious agents, with their absorption and destruction. Each macrophage contains packets of chemicals and enzymes, which digest the ingested antigen or microbe. Phagocytes are strategically located in those tissues of the organism where the penetration of such particles is possible. For example, the Kupffer cells line the blood sinusoidal capillaries of the liver, and the synovial A- cells cover the joint cavities. The mononuclear phagocytes, which circulate with the blood, are called monocytes. They migrate from the blood in the tissues where they are converted into the tissue macrophages, capable of presenting antigens to T-lymphocytes very effectively.

 

Figure I.10. Maturation of immune system cells. Myeloid and lymphoid cells develop in adults from pluripotenf (many different potentials) stem cells in the bone marrow.This development is driven by colony-stimulating factors. Nonlymphoid stem cells give rise to elements of the peripheral blood, such as erythrocytes, platelets, granulocytes (basophils, eosinophils, or neutrophils), and mono­cytes (precursor cells for macrophages). Lymphoid stem cells can develop along two pathways. If these stem cells migrate through the thymus, they become T lymphocytes or T cells, represented by CD4⁺T and CD8⁺ T cells. If the lymphoid stem cells mature in the bone marrow, the cells become a population of lymphocytes, called B lymphocytes or B cells (Klaus D. Elgert, Immunology, 1996).

 

Polymorphonuclear neutrophils. It is the second significant group of the phagocytes. Neutrophils form a primary defense against bacterial infection. These are polymorphonuclear neutrophilic granulocytes, frequently called simply neutrophils or PMN. Neutrophils compose majority among the leukocytes of the blood and originate from the same early precursor cells as monocytes and macrophages. Similar to monocytes neutrophils migrate in the tissue reacting to the specific stimuli, but in contrast to the monocytes they are related to the short-lived cells, which perish after absorbing foreign material and destroying it.

 

Resident tissue macrophage populations (I.Roitt et al., Immunology, 2001)

 

The eosinophils are formed in the bone marrow, and all stages of differentiation to the mature cell occur there. The number of the eosinophils, which circulate in the blood flow, does not exceed 1% of the total number of these cells in the organism; they circulate for 10 hours in the blood, then they migrate in the tissue where they live for 48 hours and perish after degranulation. The eosinophils control release of the histamine and other biologically active materials, neutralizing their excessive quantity. This function is very important for the process of the organism protection in helminthiases and in development of the allergic reaction.

 

Causes of eosinophilia

 

The basophils take an active part in development of the allergic reactions of the immediate type. Having penetrated into tissues basophils are converted into the mast cells, which contain the large number of histamine – biologically active material, which stimulates the development of allergy. Because of the basophils the poisons of insects or animals are immediately blocked in the tissues and are not spread throughout the whole body. Also basophils regulate the ability of the blood coagulation with the aid of heparin.

The lymphocytes are the cells having the ability to react only to a limited group of structurally similar antigens, i.e., being committed regarding these antigens. This ability is determined by the presence of the corresponding membrane receptors in the lymphocyte, specific for the determinants of this or that antigen. Each lymphocyte possesses a population of receptors with the identical antigen-binding centers, and a separate group, or clone; the lymphocytes differ from another clone by the structure of the antigen-binding center of the receptors, capable of reacting only to the specific set of antigens.

The specific immunological recognition of the pathogenic organisms is completely the function of the lymphocytes; therefore it is they that initiate the reactions of the acquired immunity. The lymphocytes are distinguished between themselves not only by the specificity of their receptors, but also by their functional properties. The main lymphocyte sub-types are: B-cells (special B cells produce specific antibodies, proteins that help destroy foreign substances); T-cells (T-cells attack virus-infected cells, foreign tissue, and cancer cells. They also produce a number of substances that regulate the immune response); NK cells (among other functions, natural killer cells destroy cancer cells and virus-infected cells through phagocytosis and by producing substances that can kill such cells); Null cells (an early population of lymphocytes bearing neither T-cell nor B-cell differentiation antigens).

Basic classes of the T-lymphocytes: T-helpers, T-suppressors (cytotoxic T-lymphocytes), T-killers, T-cells of the immunological memory.

     

    Figure I.11. The sequence for cytotoxic T-cell-mediated ly­sis (Klaus D. Elgert, Immunology, 1996).

 

T-lymphocytes fulfill two functions in the organism: effector and regulatory. Specific cytotoxicity regarding the foreign cells is the basic effector function of the T-lymphocytes; they attack their organism cells, infected with a virus or another foreign agent.

T- killers destroy cells in the direct contact with the target (affected cell), the ejection of the oxidizing ferments occurs, which leads to lysis (dissolution) of the target cell and its destruction.  T – killer (cytoxic T-cells) – cells that kill target cells bearing appropriate antigen within the groove of an MHC class I molecule that is identical to that of the T-cell.

Figure I.12. The importance of CD4* T_h lymphocytes in the immune response. These cells are directly or indirectly responsible for many aspects of the immune re­sponse and for nonlymphoid cell functions (Klaus D. Elgert, Immunology, 1996).

 

The regulatory function of the T-lymphocytes consists in the regulation of the specific immune response. The recognition of the antigen by T-cells (T- helpers of the 1st type) occurs only under the condition of presentation in the association with the molecules of MNS (the main complex of histocompatibility). The T-lymphocytes accomplish their functions of effect on other cells by the release of the dissoluble proteins – cytokines, which transfer signals to other cells or via straight intercellular contacts. The T- helpers of the 2nd type stimulate the differentiation of the antibody-forming cells (B-lymphocytes) in response to the antigenic stimulus. The effect of the T- helpers is accomplished due to the direct interaction of the T- helpers with the B cells, or due to the activation of the B cells as a result of formation of the cytokines (lymphokines), in particular interleukin -2 by the T- helpers.

Figure I.13. There are three classes of effector T-cell, specialized to deal with three classes of pathogen. CD8 cytotoxic cells (left panels) kill target cells that display antigenic fragments of cytosolic pathogens, most notably viruses, bound to MHC class I molecules at the cell surface. T_H1 cells (middle panels) and Th2 cells (right panels) both express the CD4 co-receptor and recognize frag­ments of antigens degraded within intra­cellular vesicles, displayed at the cell surface by MHC class II molecules. The Th1 cells, upon activation, activate macrophages, allowing them to destroy intracellular microorganisms more efficiently; they can also activate В-cells to produce strongly opsonizing antibodies belonging to certain IgG subclasses (lgG1 and lgG3 in humans, and their homologs lgG2a and lgG2b in the mouse). Тн2 cells, on the other hand, drive В-cells to differentiate and produce immunoglobulins of all other types, and are responsible for initiating B-cell responses by activating naive В cells to proliferate and secrete IgM. The various types of immunoglobulin together make up the effector molecules of the humoral immune response. (Charles A. Janeway et al., Immunobiology, 1999).

T – suppressors (cytotoxic T-lymphocytes) are capable of suppressing the immune response. Activation of the T -suppressors passes a number of phases (the T- helpers participate in them) and it can be associated with the foreign antigen (specific) or cannot be associated (nonspecific). The basic concept of suppressor T-cells (cytotoxic T-lymphocytes) is a cell-type that specifically suppresses the action of other cells in the immune system, notably B-cells and T-cells, thereby preventing the establishment of an immune response. Most suppressor T-cells are CD8 positive – as are cytotoxic T-cells.

Thus, interaction of the T-helpers – T -suppressors controls the intensity of development of the specific response of the immune system to a foreigner. The relationship of T-helpers/T-suppressors is called the index of immunoregulation or immunoregulatory index.

The B-lymphocytes originate from the precursor cells of the bone marrow. The B-lymphocytes are the antibody-forming cells. There are receptors for the antigen on the membranes of the mature B – cell. The cell is activated in binding of the antigen with these receptors. It is necessary for this activation that the B – cell simultaneously with binding of the antigen begins to interact with the specific helper T-cell or so that the latter would synthesize the dissoluble factors of growth and differentiation.

 The cells of the immune system are identified by the surface markers CD – molecules (Cluster of differentiation) with the aid of the monoclonal antibodies to the given markers. Each cell expresses the specific CD – molecules on its surface, having determining them we can identify this cell. The process of identification of the cells of the immune system is called immunophenotyping in the international literature.

Cellular expression of the most frequently diagnosed CD of the molecules

 

Humoral immunity

A number of molecules – mediators including antibodies and cytokines as well as different proteins of serum released by the lymphocytes participate in development of the immune response. They belong to the humoral factors of nonspecific resistance: leukines are the substances obtained from the neutrophils, which manifest bactericidal effect regarding a number of bacteria; erythrite is a substance, obtained from the erythrocytes, having the bactericidal effect regarding the diphtheria bacillus; lysozyme is an enzyme produced by monocytes, macrophages, and lyses bacteria; properdin is a protein, which provides the bactericidal, virus-neutralizing properties of the blood serum; beta – lysines are bactericidal factors of the blood serum released by the thrombocytes.

Special attention is paid to cytokines among different endogenous mechanisms of the immunoregulation; they provide the interrelation of the immunocompetent and other cells, mediated by the molecules secreted by them. By the chemical nature the cytokines are proteins, polypeptides or glycoproteins. They are the biologically activated molecules, capable of influencing the processes of the cellular proliferation, differentiation and functional activity of the cells. Each cytokine serves as the inductor of the expression of the cascade of other cytokines and/or their receptors. Cytokines are soluble glycoproteins released by cells of the immune system, which act nonenzymatically through specific receptors to regulate immune responses. Cytokines resemble hormones in that they act at low concentrations bound with high affinity to a specific receptor. Common cytokines include: interleukins; lymphokines; interferons; colony stimulating factor; platelet-activating factor; tumor necrosis factor.

Interleukins (IL) Glycoproteins secreted by a variety of leukocytes which have effects on other leukocytes (Interleukin = between leukocytes ). This is a large group of cytokines (about hundred), synthesized mainly by the T-cells as well as mononuclear phagocytes or other tissue cells. Interleukins possess diverse functions, but most of them stimulate other cells for division or differentiation. Each interleukin acts on the separate, limited group of the cells, which express receptors specific to this IL.

Lymphokine   Soluble cytokines secreted by lymphocytes, which have a variety of effects on lymphocytes and other cell types.

Interferons (IFN) A group of cytokine proteins with antiviral properties, capable of enhancing and modifying the immune response. Interferon is released to coat uninfected cells so that they don’t become infected. Some interferons induce antiviral activity, others enhance the immune response. There are three main classes of interferon: alpha, beta and gamma. IFN_α is produced by virus-infected monocytes and lymphocytes. IFN_β is produced by virus-infected fibroflasts (and some other cell types). IFN_γ is produced by stimulated T and NK cells. IFN_γ increases MHC II expression, activates macrophages, neutrophils and NK cells as well as activating vascular endothelium, promoting T and B cell differentiation and increasing IL1 and IL2 synthesis. Also increases IgG2a and decreases IgE, G1, G2b and G3 (opposite of IL4). All IFNs induce cell growth, activate CTL and NK cells as well as increasing MHC I expression. Interferons contribute to antiviral stability by the cells uncontaminated by a virus and create the first line of protection against the majority of viruses.

Tumor Necrosis Factor (TNF) – is a protein (cytokine) which mediates tumor cell necrosis and destroys cancer cells. Found in two forms: TNFα (cachetin) and TNFβ (lymphotoxin). Both forms of TNF bind to the same receptors and therefore have the same activities. TNFα is produced by macrophages and some other cells. TNFβ is produced by T-cells. IL1 and TNF act alone or together to induce systemic inflammation (e.g., fever). LPS (an endotoxin) from bacteria stimulates production of TNFα. TNF is also chemotactic for neutrophils and monocytes, as well as increasing neutrophil activity. TNF causes the symptoms associated with bacterial infections (septic shock, fever, muscle ache, lethargy, headache, nausea and inflammation).

 

Figure I.15. TNFα has several functions in inflammation. It is prothrombotic and promotes leucocyte adhesion and migration (top). It has an important role in the regulation of macrophage activation and immune responses in tissues (centre) and also modulates haematopoiesis and lymphocyte development (bottom) (I.Roitt et al., Immunology, 2001)

 

Colony Stimulating Factor (CSF) Granulocyte-Colony Stimulating Factor (G-CSF), Macrophage-Colony stimulating factor (M-CSF), Granulocyte-Macrophage-Colony stimulating factor (GM-CSF). These cytokines participate in the division regulation and differentiation of the stem cells of the bone marrow and precursor cells of the blood leukocytes. Cytokine proteins that stimulate growth and reproduction of certain kinds of blood cells in the bone marrow. Also referred to as growth factors . The production of white blood cells is controlled by colony stimulating factors. The balance of different CSF is to a certain degree caused by the relationship between different types of the leukocytes formed in the bone marrow. Some CSFs stimulate further differentiation of the cells out of the bone marrow.

 

The main interleukins

 

The antibody-forming cells produce antibodies – the molecules of immunoglobulins (Ig), i.e., the proteins capable of interacting with the appropriate antigens. Each B – cell is programmed to produce, carry on itself and secrete the antibody only of one specificity.

Each immunoglobulin unit is made up of two heavy chains and two light chains and has two antigen-binding sites. Antibodies are diverse, with more than 1010 possible variations, yet each antibody is designed to recognize only a specfic antigen. Initially bound to B-cells, upon encountering its specific antigen, an antibody/antigen complex stimulates the B-cell to produce copies of the antibody with the aid of helper T-cells. The new antibodies, which are all designed to recognize the infecting antigen, are released into the intercellular fluid where they bind to the infecting antigen, identifying it for destruction by phagocytes and the complement system.

There are 5 classes of Ig – IgA, IgM, IgG, IgD, IgE, each of which possesses the specific effector functions.

Immunoglobulins A make 10-15% of all Ig of the blood serum; they are predominant Ig of secretions (mucous discharge of the respiratory tract, gastrointestinal tract, saliva, tears, colostrum and female milk). Secretory IgA is like a dimer, consisting of two molecules of IgA and is called secretory component. IgA, leaving the blood flow, penetrates through the epithelial layer and is bound with the secretory component (it is formed in the epithelial cells). The formed secretory IgA remains either on the surface of the epithelial cell or in the layer of the mucus above the epithelium and fulfills its basic nonspecific protective function. The newborns get the secretory IgA during the first days of life through the mother colostrum, protecting their bronchopulmonary and gastrointestinal tract until they form their own mechanisms of formation of the secretory IgA and their own microflora.

The immunoglobulins of the class G make 75% of all Ig of the blood serum of man. The molecular weight provides the possibility of penetration through the placenta from a mother to a fetus. The molecules of IgG are most long living of all immunoglobulins (23 days). There are four subclasses of Ig G in man: G1, G2, G3, G4, they are distinguished by the amino-acid composition. The immunoglobulins of the class M are evolutionarily the oldest Ig, their content in the blood serum makes 5-10% of the total number of Ig. Both classes of immunoglobulins provide specific immune protection from the majority of infectious agents.

Immunoglobulin E makes about 0,2% of all serum immunoglobulins. It is accumulated predominantly in the tissues, mucous and skin membranes, where it is gathered on the surface of the mast cells, basophils, eosinophils. Its life cycle is 2,5 days. Main function seems to be to protect the host against invading parasites. The antigen-specific IgE interacts with mast cells and eosinophils to protect the host against the invading parasite. IgE- antibodies play a basic role in the formation of the hypersensitivity response of the immediate type, i.e. anaphylaxis.

Figure I.16. Entry of virus at mucosal surfaces is inhibited by Ig A. Following the initial infection, the virus may spread to other tissues via the blood stream. Interferons produced by the innate (IFNα and IFNβ) and adaptive (IFNγ) immune responses make neighbouring cells resistant to infection by spreading virus. Antibodies are important in controlling free virus, whereas. T cells and NK cells are effective at killing infected cells (I.Roitt et al., Immunology, 2001).

 

Causes of raised IgE

 

Immunoglobulins D – represents about 0,25% of the total serum immunoglobulins and a half-life of 2,8 days.  Little is known on the humoral functions of this immunoglobulin. IgD is the major antigen receptor isotype on the surface of most peripheral B cells. Serum IgD was considered an early marker of B-cell activation. IgD can have a regulatory role, e.g., to enhance a protective antibody response of the IgM, IgG, or IgA isotype, or to interfere with viral replication. IgD is a potent inducer of tumor necrosis factor alpha (TNF-), IL-1, and IL-1 receptor antagonist. Monocytes seem to be the main producers of cytokines in vitro in the presence of IgD.

The proteins of the complement system serve as the mediators of phagocytosis, regulate the inflammatory reaction and, interacting with the antibodies, they participate in the immune protection of the organism. The complement system includes about a score of serum proteins, whose general function consists in the regulation of inflammation. The complement components interact between themselves and with other elements of the immune system. A number of microorganisms activate the complement system by the alternative way – a mechanism of the congenital, nonspecific immunity. As a result the complement components bind with the surface of microbes, which leads to the absorption of these agents by phagocytes. When the complement system participates in the reactions of the specific acquired immunity, they usually activate it by the classical way of the antibody having bound with the surface of microorganisms. The activation of complement is a cascade of reactions, in which each previous component acts on the following one.

The complement system is activated by the complex antigen-antibody in such sequence: C1, C4, C2 and C3, C5, C6, C7, C8, C9. The complement causes bacteriolysis, stimulates phagocytosis, and causes changes in the membrane, activation of the factors, which participate in the inflammatory reactions, leading to damage of the cells. The biological properties of the complement (C) include the following:

– C1-C4 – neutralization of viruses;

– C1-C5 – formation of the histamine-releasing factors (anaphylatoxins), intensification of the phagocytosis reactions;

– C5-C9 – participation in the reactions of cytolysis, bacteriolysis, in the reactions of the transplant rejection.

Figure I.17. The two complement activation pathways (Klaus D. Elgert, Immunology, 1996).

 

Lysozyme is a specific enzyme, which destroys the walls of bacteria. There is lysozyme in a large amount in the saliva; it explains its antibacterial properties.

Transferin is the protein, which competes with the bacteria for seizure of the specific substances (for example, iron), necessary for their development. As a result the growth and multiplication of the bacteria is slowed down.

C-reactive protein is activated similarly to the compliment on penetration into the blood of foreign structures. Addition of this protein to the bacteria makes them vulnerable for the cells of the immune system. C-reactive protein (CRP) is named for the ability to bind with the C- protein of pneumococci. Because of this binding phagocytes begin to absorb pneumococci more actively – the process is called opsonization. Antibodies and complement components mainly act as the opsonin, i.e. the opsonizing molecules.

 

The components of the immune system

Principles of innate and adaptive immunity

The recognition and effector mechanisms of adaptive immunity

Immunology is a relatively new science. Its origin is usually attributed to Edward Jenner, who discovered  in 1796 that cowpox, or vaccinia, induced protection against human smallpox, an often fatal disease. Jenner called his  procedure vaccination, and this term is still used to describe the inoculation of healthy individuals with weakened or  attenuated strains of disease-causing agents to provide protection from disease. Although Jenner’s bold experiment  was successful, it took almost two centuries for smallpox vaccination to become universal, an advance that enabled  the World Health Organization to announce in 1979 that smallpox had been eradicated, arguably the  greatest triumph of modern medicine.   

The components of the immune system.

The cells of the immune system originate in the bone marrow, where many of them also mature. They then migrate to  guard the peripheral tissues, circulating in the blood and in a specialized system of vessels called the lymphatic  system.   

  Structure of the immune system.    

The white blood cells of the immune system derive from precursors in the bone marrow.

  All the cellular elements of blood, including the red blood cells that transport oxygen, the platelets that trigger blood  clotting in damaged tissues, and the white blood cells of the immune system, derive ultimately from the same  progenitor or precursor cells the hematopoietic stem cells in the bone marrow. As these stem cells can give rise to  all of the different types of blood cells, they are often known as pluripotent hematopoietic stem cells. Initially, they  give rise to stem cells of more limited potential, which are the immediate progenitors of red blood cells, platelets, and  the two main categories of white blood cells. The different types of blood cell and their lineage relationships are  summarized in Fig. 1.3. We shall be concerned here with all the cells derived from the common lymphoid progenitor  and the myeloid progenitor, apart from the megakaryocytes and red blood cells. 

 All the cellular elements of blood, including the lymphocytes of the adaptive immune system, arise  from hematopoietic stem cells in the bone marrow.

   These pluripotent cells divide to produce two more specialized  types of stem cells, a common lymphoid progenitor that gives rise to the T and B lymphocytes responsible for  adaptive immunity, and a common myeloid progenitor that gives rise to different types of leukocytes (white blood  cells), erythrocytes (red blood cells that carry oxygen), and the megakaryocytes that produce platelets that are  important in blood clotting. The existence of a common lymphoid progenitor for T and B lymphocytes is strongly  supported by current data. T and B lymphocytes are distinguished by their sites of differentiation T cells in the  thymus and B cells in the bone marrow and by their antigen receptors. Mature T and B lymphocytes circulate  between the blood and peripheral lymphoid tissues. After encounter with antigen, B cells differentiate into antibodysecreting  plasma cells, whereas T cells differentiate into effector T cells with a variety of functions. A third lineage of  lymphoid-like cells, the natural killer cells, derive from the same progenitor cell but lack the antigen-specificity that  is the hallmark of the adaptive immune response (not shown). The leukocytes that derive from the myeloid stem cell  are the monocytes, the dendritic cells, and the basophils, eosinophils, and neutrophils. The latter three are collectively  termed either granulocytes, because of the cytoplasmic granules whose characteristic staining gives them a distinctive  appearance in blood smears, or polymorphonuclear leukocytes, because of their irregularly shaped nuclei. They  circulate in the blood and enter the tissues only when recruited to sites of infection or inflammation where neutrophils  are recruited to phagocytose bacteria. Eosinophils and basophils are recruited to sites of allergic inflammation, and  appear to be involved in defending against parasites. Immature dendritic cells travel via the blood to enter peripheral  tissues, where they ingest antigens. When they encounter a pathogen, they mature and migrate to lymphoid tissues,  where they activate antigen-specific T lymphocytes. Monocytes enter tissues, where they differentiate into  macrophages; these are the main tissue-resident phagocytic cells of the innate immune system. Mast cells arise from  precursors in bone marrow but complete their maturation in tissues; they are important in allergic responses.  The myeloid progenitor is the precursor of the granulocytes, macrophages, dendritic cells, and mast cells of the  immune system. Macrophages are one of the three types of phagocyte in the immune system and are distributed  widely in the body tissues, where they play a critical part in innate immunity. They are the mature form of  monocytes, which circulate in the blood and differentiate continuously into macrophages upon migration into the  tissues. Dendritic cells are specialized to take up antigen and display it for recognition by lymphocytes. Immature  dendritic cells migrate from the blood to reside in the tissues and are both phagocytic and macropinocytic, ingesting  large amounts of the surrounding extracellular fluid. Upon encountering a pathogen, they rapidly mature and migrate  to lymph nodes.  Mast cells, whose blood-borne precursors are not well defined, also differentiate in the tissues. They mainly reside  near small blood vessels and, when activated, release substances that affect vascular permeability. Although best  known for their role in orchestrating allergic responses, they are believed to play a part in protecting mucosal surfaces  against pathogens.  The granulocytes are so called because they have densely staining granules in their cytoplasm; they are also  sometimes called polymorphonuclear leukocytes because of their oddly shaped nuclei. There are three types of  granulocyte, all of which are relatively short lived and are produced in increased numbers during immune responses,  when they leave the blood to migrate to sites of infection or inflammation. Neutrophils, which are the third  phagocytic cell of the immune system, are the most numerous and most important cellular component of the innate  immune response: hereditary deficiencies ieutrophil function lead to overwhelming bacterial infection, which is  fatal if untreated. Eosinophils are thought to be important chiefly in defense against parasitic infections, because their  numbers increase during a parasitic infection. The function of basophils is probably similar and complementary to  that of eosinophils and mast cells; we shall discuss the functions of these cells and their role in allergic  inflammation. The cells of the myeloid lineage are shown    

 

Natural killer (NK) cells.

  These are large granular lymphocyte-like cells with important functions in  innate immunity. Although lacking antigen-specific receptors, they can detect and attack certain virus-infected cells.  Photograph courtesy of N. Rooney and B. Smith.  1-2. Lymphocytes mature in the bone marrow or the thymus.  The lymphoid organs are organized tissues containing large numbers of lymphocytes in a framework of nonlymphoid  cells. In these organs, the interactions lymphocytes make with nonlymphoid cells are important either to lymphocyte  development, to the initiation of adaptive immune responses, or to the sustenance of lymphocytes. Lymphoid organs  can be divided broadly into central or primary lymphoid organs, where lymphocytes are generated, and peripheral or  secondary lymphoid organs, where adaptive immune responses are initiated and where lymphocytes are maintained.  The central lymphoid organs are the bone marrow and the thymus, a large organ in the upper chest; the location of the  thymus, and of the other lymphoid organs, is shown schematically. 

The distribution of lymphoid tissues in the body.

 Lymphocytes arise from stem cells in bone marrow,  and differentiate in the central lymphoid organs (yellow), B cells in bone marrow and T cells in the thymus. They  migrate from these tissues and are carried in the bloodstream to the peripheral or secondary lymphoid organs (blue),  the lymph nodes, the spleen, and lymphoid tissues associated with mucosa, like the gut-associated tonsils, Peyer’s  patches, and appendix. The peripheral lymphoid organs are the sites of lymphocyte activation by antigen, and  lymphocytes recirculate between the blood and these organs until they encounter antigen. Lymphatics drain  extracellular fluid from the peripheral tissues, through the lymph nodes and into the thoracic duct, which empties into  the left subclavian vein. This fluid, known as lymph, carries antigen to the lymph nodes and recirculating  lymphocytes from the lymph nodes back into the blood. Lymphoid tissue is also associated with other mucosa such  as the bronchial linings (not shown).  Both B and T lymphocytes originate in the bone marrow but only B lymphocytes mature there; T lymphocytes  migrate to the thymus to undergo their maturation. Thus B lymphocytes are so-called because they are bone marrow  derived, and T lymphocytes because they are thymus derived. Once they have completed their maturation, both types  of lymphocyte enter the bloodstream, from which they migrate to the peripheral lymphoid organs. 

The peripheral lymphoid organs are specialized to trap antigen, to allow the initiation of adaptive immune  responses, and to provide signals that sustain recirculating lymphocytes. 

Pathogens can enter the body by many routes and set up an infection anywhere, but antigen and lymphocytes will  eventually encounter each other in the peripheral lymphoid organs the lymph nodes, the spleen, and the mucosal  lymphoid tissues. Lymphocytes are continually recirculating through these tissues, to which antigen is  also carried from sites of infection, primarily within macrophages and dendritic cells. Within the lymphoid organs,  specialized cells such as mature dendritic cells display the antigen to lymphocytes.  The lymph nodes are highly organized lymphoid structures located at the points of convergence of vessels of the  lymphatic system, an extensive system of vessels that collects extracellular fluid from the tissues and returns it to the  blood. This extracellular fluid is produced continuously by filtration from the blood, and is called lymph. The vessels  are lymphatic vessels or lymphatics. Afferent lymphatic vessels drain fluid from the tissues and also  carry antigen-bearing cells and antigens from infected tissues to the lymph nodes, where they are trapped. In the  lymph nodes, B lymphocytes are localized in follicles, with T cells more diffusely distributed in surrounding  paracortical areas, also referred to as T-cell zones. Some of the B-cell follicles include germinal centers, where B  cells are undergoing intense proliferation after encountering their specific antigen and their cooperating T cells.   B and T lymphocytes are segregated in a similar fashion in the other peripheral lymphoid tissues, and we shall  see that this organization promotes the crucial interactions that occur between B and T cells upon encountering  antigen. 

 

Organization of a lymph node.

 

 As shown in the diagram on the left, a lymph node consists of an  outermost cortex and an inner medulla. The cortex is composed of an outer cortex of B cells organized into lymphoid  follicles, and deep, or paracortical, areas made up mainly of T cells and dendritic cells. When an immune response is  underway, some of the follicles contain central areas of intense B-cell proliferation called germinal centers and are  known as secondary lymphoid follicles. These reactions are very dramatic, but eventually die out as senescent  germinal centers. Lymph draining from the extracellular spaces of the body carries antigens in phagocytic dendritic  cells and macrophages from the tissues to the lymph node via the afferent lymphatics. Lymph leaves by the efferent  lymphatic in the medulla. The medulla consists of strings of macro-phages and antibody-secreting plasma cells  known as the medullary cords. Naive lymphocytes enter the node from the bloodstream through specialized  postcapillary venules (not shown) and leave with the lymph through the efferent lymphatic. The light micrograph  shows a section through a lymph node, with prominent follicles containing germinal centers.  The spleen is a fist-sized organ just behind the stomach  that collects antigen from the blood. It also  collects and disposes of senescent red blood cells. Its organization is shown schematically. The bulk of the  spleen is composed of red pulp, which is the site of red blood cell disposal. The lymphocytes surround the arterioles  entering the organ, forming areas of white pulp, the inner region of which is divided into a periarteriolar lymphoid  sheath (PALS), containing mainly T cells, and a flanking B-cell corona.   

Lymphocytes circulate between blood and lymph. 

Small B and T lymphocytes that have matured in the bone marrow and thymus but have not yet encountered antigen  are referred to as naive lymphocytes. These cells circulate continually from the blood into the peripheral lymphoid  tissues, which they enter by squeezing between the cells of capillary walls. They are then returned to the blood via the  lymphatic vessels  or, in the case of the spleen, return directly to the blood. In the event of an infection,  lymphocytes that recognize the infectious agent are arrested in the lymphoid tissue, where they proliferate and  differentiate into effector cells capable of combating the infection. 

Circulating lymphocytes encounter antigen in peripheral lymphoid organs.

Naive lymphocytes recirculate constantly through peripheral lymphoid tissue, here illustrated as a lymph node behind the knee, a  popliteal lymph node. Here, they may encounter their specific antigen, draining from an infected site in the foot.  These are called draining lymph nodes, and are the site at which lymphocytes may become activated by encountering  their specific ligand.  When an infection occurs in the periphery, for example, large amounts of antigen are taken up by dendritic cells  which then travel from the site of infection through the afferent lymphatic vessels into the draining lymph nodes   In the lymph nodes, these cells display the antigen to recirculating T lymphocytes, which they also help to  activate. B cells that encounter antigen as they migrate through the lymph node are also arrested and activated, with  the help of some of the activated T cells. Once the antigen-specific lymphocytes have undergone a period of  proliferation and differentiation, they leave the lymph nodes as effector cells through the efferent lymphatic vessel  Because they are involved in initiating adaptive immune responses, the peripheral lymphoid tissues are not static  structures but vary quite dramatically depending upon whether or not infection is present. The diffuse mucosal  lymphoid tissues may appear in response to infection and then disappear, whereas the architecture of the organized  tissues changes in a more defined way during an infection. For example, the B-cell follicles of the lymph nodes  expand as B lymphocytes proliferate to form germinal centers, and the entire lymph node enlarges, a  phenomenon familiarly known as swollen glands.

Principles of innate and adaptive immunity. 

The macrophages and neutrophils of the innate immune system provide a first line of defense against many common  microorganisms and are essential for the control of common bacterial infections. However, they cannot always  eliminate infectious organisms, and there are some pathogens that they cannot recognize. The lymphocytes of the  adaptive immune system have evolved to provide a more versatile means of defense which, in addition, provides  increased protection against subsequent reinfection with the same pathogen. The cells of the innate immune system,  however, play a crucial part in the initiation and subsequent direction of adaptive immune responses, as well as  participating in the removal of pathogens that have been targeted by an adaptive immune response. Moreover,  because there is a delay of 4-7 days before the initial adaptive immune response takes effect, the innate immune  response has a critical role in controlling infections during this period. 

Most infectious agents induce inflammatory responses by activating innate immunity. 

Microorganisms such as bacteria that penetrate the epithelial surfaces of the body for the first time are met  immediately by cells and molecules that can mount an innate immune response. Phagocytic macrophages conduct the  defense against bacteria by means of surface receptors that are able to recognize and bind common constituents of  many bacterial surfaces. Bacterial molecules binding to these receptors trigger the macrophage to engulf the  bacterium and also induce the secretion of biologically active molecules. Activated macrophages secrete cytokines,  which are defined as proteins released by cells that affect the behavior of other cells that bear receptors for them.  They also release proteins known as chemokines that attract cells with chemokine receptors such as neutrophils and  monocytes from the bloodstream.  The cytokines and chemokines released by macrophages in response to  bacterial constituents initiate the process known as inflammation. Local inflammation and the phagocytosis of  invading bacteria may also be triggered as a result of the activation of complement on the bacterial cell surface.  Complement is a system of plasma proteins that activates a cascade of proteolytic reactions on microbial surfaces but  not on host cells, coating these surfaces with fragments that are recognized and bound by phagocytic receptors on  macrophages. The cascade of reactions also releases small peptides that contribute to inflammation.   

Bacterial infection triggers an inflammatory response.

 

Macrophages encountering bacteria in the  tissues are triggered to release cytokines that increase the permeability of blood vessels, allowing fluid and proteins to  pass into the tissues. They also produce chemokines that direct the migration of neutrophils to the site of infection.  The stickiness of the endothelial cells of the blood vessels is also changed, so that cells adhere to the blood vessel  wall and are able to crawl through it; first neutrophils and then monocytes are shown entering the tissue from a blood  vessel. The accumulation of fluid and cells at the site of infection causes the redness, swelling, heat, and pain, known  collectively as inflammation. Neutrophils and macrophages are the principal inflammatory cells. Later in an immune  response, activated lymphocytes may also contribute to inflammation.  Inflammation is traditionally defined by the four Latin words calor, dolor, rubor, and tumor, meaning heat, pain,  redness, and swelling, all of which reflect the effects of cytokines and other inflammatory mediators on the local  blood vessels. Dilation and increased permeability of the blood vessels during inflammation lead to increased local  blood flow and the leakage of fluid, and account for the heat, redness, and swelling. Cytokines and complement  fragments also have important effects on the adhesive properties of the endothelium, causing circulating leukocytes to  stick to the endothelial cells of the blood vessel wall and migrate between them to the site of infection, to which they  are attracted by chemokines. The migration of cells into the tissue and their local actions account for the pain. The  main cell types seen in an inflammatory response in its initial phases are neutrophils, which are recruited into the  inflamed, infected tissue in large numbers. Like macrophages, they have surface receptors for common bacterial  constituents and complement, and they are the principal cells that engulf and destroy the invading micro-organisms.  The influx of neutrophils is followed a short time later by monocytes that rapidly differentiate into macrophages.  Macrophages and neutrophils are thus also known as inflammatory cells. Inflammatory responses later in an  infection also involve lymphocytes, which have meanwhile been activated by antigen that has drained from the site of  infection via the afferent lymphatics.  The innate immune response makes a crucial contribution to the activation of adaptive immunity. The inflammatory  response increases the flow of lymph containing antigen and antigen-bearing cells into lymphoid tissue, while  complement fragments on microbial surfaces and induced changes in cells that have taken up microorganisms  provide signals that synergize in activating lymphocytes whose receptors bind to specific microbial antigens.  Macrophages that have phagocytosed bacteria and become activated can also activate T lymphocytes. However, the  cells that specialize in presenting antigen to T lymphocytes and initiating adaptive immunity are the dendritic cells. 

Activation of specialized antigen-presenting cells is a necessary first step for induction of adaptive  immunity.  T

he induction of an adaptive immune response begins when a pathogen is ingested by an immature dendritic cell in  the infected tissue. These specialized phagocytic cells are resident in most tissues and are relatively long-lived,  turning over at a slow rate. They derive from the same bone marrow precursor as macrophages, and migrate from the  bone marrow to their peripheral stations, where their role is to survey the local environment for pathogens.  Eventually, all tissue-resident dendritic cells migrate through the lymph to the regional lymph nodes where they  interact with recirculating naive lymphocytes. If the dendritic cells fail to be activated, they induce tolerance to the  antigens of self that they bear.  The immature dendritic cell carries receptors on its surface that recognize common features of many pathogens, such  as bacterial cell wall proteoglycans. As with macrophages and neutrophils, binding of a bacterium to these receptors  stimulates the dendritic cell to engulf the pathogen and degrade it intracellularly. Immature dendritic cells are also  continually taking up extracellular material, including any virus particles or bacteria that may be present, by the  receptor-independent mechanism of macropinocytosis. The function of dendritic cells, however, is not primarily to  destroy pathogens but to carry pathogen antigens to peripheral lymphoid organs and there present them to T  lymphocytes. When a dendritic cell takes up a pathogen in infected tissue, it becomes activated, and travels to a  nearby lymph node. On activation, the dendritic cell matures into a highly effective antigen-presenting cell (APC)  and undergoes changes that enable it to activate pathogen-specific lymphocytes that it encounters in the lymph node  Activated dendritic cells secrete cytokines that influence both innate and adaptive immune responses,  making these cells essential gatekeepers that determine whether and how the immune system responds to the  presence of infectious agents.    

Dendritic cells initiate adaptive immune responses.

 

 Immature dendritic cells resident in infected  tissues take up pathogens and their antigens by macropinocytosis and receptor-mediated phagocytosis. They are  stimulated by recognition of the presence of pathogens to migrate via the lymphatics to regional lymph nodes, where  they arrive as fully mature nonphagocytic dendritic cells. Here the mature dendritic cell encounters and activates  antigen-specific naive T lymphocytes, which enter lymph nodes from the blood via a specialized vessel known from  its cuboidal endothelial cells as a high endothelial venule (HEV). 

Lymphocytes activated by antigen give rise to clones of antigen-specific cells that mediate adaptive  immunity. 

The defense systems of innate immunity are effective in combating many pathogens. They are constrained, however,  by relying on germline-encoded receptors to recognize microorganisms that can evolve more rapidly than the hosts  they infect. This explains why they can only recognize microorganisms bearing surface molecules that are common to  many pathogens and that have been conserved over the course of evolution. Not surprisingly, many pathogenic  bacteria have evolved a protective capsule that enables them to conceal these molecules and thereby avoid being  recognized and phagocytosed. Viruses carry no invariant molecules similar to those of bacteria and are rarely  recognized directly by macrophages. Viruses and encapsulated bacteria can, however, still be taken up by dendritic  cells through the nonreceptor-dependent process of macropinocytosis. Molecules that reveal their infectious nature  may then be unmasked, and the dendritic cell activated to present their antigens to lymphocytes. The recognition  mechanism used by the lymphocytes of the adaptive immune response has evolved to overcome the constraints faced  by the innate immune system, and enables recognition of an almost infinite diversity of antigens, so that each  different pathogen can be targeted specifically.  Instead of bearing several different receptors, each recognizing a different surface feature shared by many pathogens,  each naive lymphocyte entering the bloodstream bears antigen receptors of a single specificity. The specificity of  these receptors is determined by a unique genetic mechanism that operates during lymphocyte development in the  bone marrow and thymus to generate millions of different variants of the genes encoding the receptor molecules.  Thus, although an individual lymphocyte carries receptors of only one specificity, the specificity of each lymphocyte  is different. This ensures that the millions of lymphocytes in the body collectively carry millions of different antigen  receptor specificities the lymphocyte receptor repertoire of the individual. During a person’s lifetime these  lymphocytes undergo a process akin to natural selection; only those lymphocytes that encounter an antigen to which  their receptor binds will be activated to proliferate and differentiate into effector cells.  This selective mechanism was first proposed in the 1950s by Macfarlane Burnet to explain why antibodies, which  can be induced in response to virtually any antigen, are produced in each individual only to those antigens to which  he or she is exposed. He postulated the preexistence in the body of many different potential antibody-producing cells,  each having the ability to make antibody of a different specificity and displaying on its surface a membrane-bound  version of the antibody that served as a receptor for antigen. On binding antigen, the cell is activated to divide and  produce many identical progeny, known as a clone; these cells caow secrete clonotypic antibodies with a  specificity identical to that of the surface receptor that first triggered activation and clonal expansion.   Burnet called this the clonal selection theory.   

Clonal selection.

 

   Each lymphocyte progenitor gives rise to many lymphocytes, each bearing a distinct  antigen receptor. Lymphocytes with receptors that bind ubiquitous self antigens are eliminated before they become  fully mature, ensuring tolerance to such self antigens. When antigen interacts with the receptor on a mature naive  lymphocyte, that cell is activated and starts to divide. It gives rise to a clone of identical progeny, all of whose  receptors bind the same antigen. Antigen specificity is thus maintained as the progeny proliferate and differentiate  into effector cells. Once antigen has been eliminated by these effector cells, the immune response ceases. 

Clonal selection of lymphocytes is the central principle of adaptive immunity. 

Remarkably, at the time that Burnet formulated his theory, nothing was known of the antigen receptors of  lymphocytes; indeed the function of lymphocytes themselves was still obscure. Lymphocytes did not take center  stage until the early 1960s, when James Gowans discovered that removal of the small lymphocytes from rats resulted  in the loss of all known adaptive immune responses. These immune responses were restored when the small  lymphocytes were replaced. This led to the realization that lymphocytes must be the units of clonal selection, and  their biology became the focus of the new field of cellular immunology.  Clonal selection of lymphocytes with diverse receptors elegantly explained adaptive immunity but it raised one  significant intellectual problem. If the antigen receptors of lymphocytes are generated randomly during the lifetime of  an individual, how are lymphocytes prevented from recognizing antigens on the tissues of the body and attacking  them?

Ray Owen had shown in the late 1940s that genetically different twin calves with a common placenta were  immunologically tolerant of one another’s tissues, that is, they did not make an immune response against each other. 

Sir Peter Medawar then showed in 1953 that if exposed to foreign tissues during embryonic development, mice  become immunologically tolerant to these tissues.

Burnet proposed that developing lymphocytes that are potentially  self-reactive are removed before they can mature, a process known as clonal deletion. He has since been proved right  in this too, although the mechanisms of tolerance are still being worked out, as we shall see when we discuss the  development of lymphocytes.  Clonal selection of lymphocytes is the single most important principle in adaptive immunity. Its four basic postulates  are listed. The last of the problems posed by the clonal selection theory how the diversity of  lymphocyte antigen receptors is generated was solved in the 1970s when advances in molecular biology made it  possible to clone the genes encoding antibody molecules.   

The four basic principles of clonal selection.

 

 The structure of the antibody molecule illustrates the central puzzle of adaptive immunity. 

Antibodies, as discussed above, are the secreted form of the B-cell antigen receptor or BCR. Because they are  produced in very large quantities in response to antigen, they can be studied by traditional biochemical techniques;  indeed, their structure was understood long before recombinant DNA technology made it possible to study the  membrane-bound antigen receptors of lymphocytes. The startling feature that emerged from the biochemical studies  was that an antibody molecule is composed of two distinct regions. One is a constant region that can take one of only  four or five biochemically distinguishable forms; the other is a variable region that can take an apparently infinite  variety of subtly different forms that allow it to bind specifically to an equally vast variety of different antigens.  This division is illustrated in the simple schematic diagram, where the antibody is depicted as a Y-shaped  molecule, with the constant region shown in blue and the variable region in red. The two variable regions, which are  identical in any one antibody molecule, determine the antigen-binding specificity of the antibody; the constant region  determines how the antibody disposes of the pathogen once it is bound.  

 

 

Schematic structure of an antibody molecule.

 

   The two arms of the Y-shaped antibody molecule  contain the variable regions that form the two identical antigen-binding sites. The stem can take one of only a limited  number of forms and is known as the constant region. It is the region that engages the effector mechanisms that  antibodies activate to eliminate pathogens.  Each antibody molecule has a twofold axis of symmetry and is composed of two identical heavy chains and two  identical light chains. Heavy and light chains both have variable and constant regions; the variable regions  of a heavy and a light chain combine to form an antigen-binding site, so that both chains contribute to the antigenbinding  specificity of the antibody molecule. For the time being we are concerned only with the properties of immunoglobulin molecules as  antigen receptors, and how the diversity of the variable regions is generated.   

Antibodies are made up of four protein chains.

 

 There are two types of chain in an antibody molecule:  a larger chain called the heavy chain (green), and a smaller one called the light chain (yellow). Each chain has both a  variable and a constant region, and there are two identical light chains and two identical heavy chains in each  antibody molecule. 

Each developing lymphocyte generates a unique antigen receptor by rearranging its receptor genes. 

How are antigen receptors with an almost infinite range of specificities encoded by a finite number of genes? This  question was answered in 1976, when Susumu Tonegawa discovered that the genes for immunoglobulin variable  regions are inherited as sets of gene segments, each encoding a part of the variable region of one of the  immunoglobulin polypeptide chains. During B-cell development in the bone marrow, these gene segments  are irreversibly joined by DNA recombination to form a stretch of DNA encoding a complete variable region.  Because there are many different gene segments in each set, and different gene segments are joined together in  different cells, each cell generates unique genes for the variable regions of the heavy and light chains of the  immunoglobulin molecule. Once these recombination events have succeeded in producing a functional receptor,  further rearrangement is prohibited. Thus each lymphocyte expresses only one receptor specificity.   

The diversity of lymphocyte antigen receptors is generated by somatic gene rearrangements.

  Different parts of the variable regions of antigen receptors are encoded by sets of gene segments. During a  lymphocyte’s development, one member of each set of gene segments is joined randomly to the others by an  irreversible process of DNA recombination. The juxtaposed gene segments make up a complete gene that encodes the  variable part of one chain of the receptor, and is unique to that cell. This random rearrangement is repeated for the set  of gene segments encoding the other chain. The rearranged genes are expressed to produce the two types of  polypeptide chain. These come together to form a unique antigen receptor on the lymphocyte surface. Each  lymphocyte bears many copies of its unique receptor.  This mechanism has three important consequences. First, it enables a limited number of gene segments to generate a  vast number of different proteins. Second, because each cell assembles a different set of gene segments, each cell  expresses a unique receptor specificity. Third, because gene rearrangement involves an irreversible change in a cell’s  DNA, all the progeny of that cell will inherit genes encoding the same receptor specificity. This general scheme was  later also confirmed for the genes encoding the antigen receptor on T lymphocytes. The main distinctions between B-  and T-lymphocyte receptors are that the immunoglobulin that serves as the B-cell antigen receptor has two identical  antigen-recognition sites and can also be secreted, whereas the T-cell antigen receptor has a single antigenrecognition  site and is always a cell-surface molecule. We shall see later that these receptors also recognize antigen in  very different ways.  The potential diversity of lymphocyte receptors generated in this way is enormous. Just a few hundred different gene  segments can combine in different ways to generate thousands of different receptor chains. The diversity of  lymphocyte receptors is further amplified by junctional diversity, created by adding or subtracting nucleotides in the  process of joining the gene segments, and by the fact that each receptor is made by pairing two different variable  chains, each encoded in distinct sets of gene segments. A thousand different chains of each type could thus generate  106 distinct antigen receptors through this combinatorial diversity. Thus a small amount of genetic material can  encode a truly staggering diversity of receptors. Only a subset of these randomly generated receptor specificities  survive the selective processes that shape the peripheral lymphocyte repertoire; nevertheless, there are lymphocytes  of at least 108 different specificities in an individual at any one time. These provide the raw material on which clonal  selection acts.

Lymphocytes proliferate in response to antigen in peripheral lymphoid organs, generating effector cells  and immunological memory. 

The large diversity of lymphocyte receptors means that there will usually be at least a few that can bind to any given  foreign antigen. However, because each lymphocyte has a different receptor, the numbers of lymphocytes that can  bind and respond to any given antigen is very small. To generate sufficient antigen-specific effector lymphocytes to  fight an infection, a lymphocyte with an appropriate receptor specificity must be activated to proliferate before its  progeny finally differentiate into effector cells. This clonal expansion is a feature common to all adaptive immune  responses.  As we have seen, lymphocyte activation and proliferation is initiated in the draining lymphoid tissues, where naive  lymphocytes and activated antigen-presenting cells can come together. Antigens are thus presented to the naive  recirculating lymphocytes as they migrate through the lymphoid tissue before returning to the bloodstream via the  efferent lymph. On recognizing its specific antigen, a small lymphocyte stops migrating and enlarges. The chromatin  in its nucleus becomes less dense, nucleoli appear, the volume of both the nucleus and the cytoplasm increases, and  new RNAs and proteins are synthesized. Within a few hours, the cell looks completely different and is known as a  lymphoblast.  The lymphoblasts now begin to divide, normally duplicating themselves two to four times every 24 hours for 3 to 5  days, so that one naive lymphocyte gives rise to a clone of around 1000 daughter cells of identical specificity. These  then differentiate into effector cells. In the case of B cells, the differentiated effector cells, the plasma  cells, secrete antibody; in the case of T cells, the effector cells are able to destroy infected cells or activate other cells  of the immune system. These changes also affect the recirculation of antigen-specific lymphocytes. Changes in the  cell-adhesion molecules they express on their surface allow effector lymphocytes to migrate into sites of infection or  stay in the lymphoid organs to activate B cells.   

Transmission electron micrographs of lymphocytes at various stages of activation to effector function.

 

  Small resting lymphocytes (top panel) have not yet encountered antigen. Note the scanty cytoplasm, the  absence of rough endoplasmic reticulum, and the condensed chromatin, all indicative of an inactive cell. This could  be either a T cell or a B cell. Small circulating lymphocytes are trapped in lymph nodes when their receptors  encounter antigen on antigen-presenting cells. Stimulation by antigen induces the lymphocyte to become an active  lymphoblast (center panel). Note the large size, the nucleoli, the enlarged nucleus with diffuse chromatin, and the  active cytoplasm; again, T and B lymphoblasts are similar in appearance. This cell undergoes repeated division,  which is followed by differentiation to effector function. The bottom panels show effector T and B lymphocytes.  Note the large amount of cytoplasm, the nucleus with prominent nucleoli, abundant mitochondria, and the presence of  rough endoplasmic reticulum, all hallmarks of active cells. The rough endoplasmic reticulum is especially prominent  in plasma cells (effector B cells), which are synthesizing and secreting very large amounts of protein in the form of  antibody.   After a naive lymphocyte has been activated, it takes 4 to 5 days before clonal expansion is complete and the  lymphocytes have differentiated into effector cells. That is why adaptive immune responses occur only after a delay  of several days. Effector cells have only a limited life-span and, once antigen is removed, most of the antigen-specific  cells generated by the clonal expansion of small lymphocytes undergo apoptosis. However, some persist after the  antigen has been eliminated. These cells are known as memory cells and form the basis of immunological memory,  which ensures a more rapid and effective response on a second encounter with a pathogen and thereby provides  lasting protective immunity.  The characteristics of immunological memory are readily observed by comparing the antibody response of an  individual to a first or primary immunization with the response elicited in the same individual by a secondary or  booster immunization with the same antigen. As shown the secondary antibody response occurs after a  shorter lag phase, achieves a markedly higher level, and produces antibodies of higher affinity, or strength of binding,  for the antigen. We shall describe the mechanisms of these remarkable changes. The cellular  basis of immunological memory is the clonal expansion and clonal differentiation of cells specific for the eliciting  antigen, and it is therefore entirely antigen specific. 

The course of a typical antibody response.

 

 First encounter with an antigen produces a primary  response. Antigen A introduced at time zero encounters little specific antibody in the serum. After a lag phase,  antibody against antigen A (blue) appears; its concentration rises to a plateau, and then declines. When the serum is  tested for antibody against another antigen, B (yellow), there is none present, demonstrating the specificity of the  antibody response. When the animal is later challenged with a mixture of antigens A and B, a very rapid and intense  secondary response to A occurs. This illustrates immunological memory, the ability of the immune system to make a  second response to the same antigen more efficiently and effectively, providing the host with a specific defense  against infection. This is the main reason for giving booster injections after an initial vaccination. Note that the  response to B resembles the initial or primary response to A, as this is the first encounter of the animal with antigen  B.  It is immunological memory that enables successful vaccination and prevents reinfection with pathogens that have  been repelled successfully by an adaptive immune response. Immunological memory is the most important biological  consequence of the development of adaptive immunity, although its cellular and molecular basis is still not fully  understood.

 Interaction with other cells as well as with antigen is necessary for lymphocyte activation. 

Peripheral lymphoid tissues are specialized not only to trap phagocytic cells that have ingested antigen  but also to promote their interactions with lymphocytes that are needed to initiate an adaptive immune  response. The spleen and lymph nodes in particular are highly organized for the latter function.  All lymphocyte responses to antigen require not only the signal that results from antigen binding to their receptors,  but also a second signal, which is delivered by another cell. Naive T cells are generally activated by activated  dendritic cells but for B cells, the second signal is delivered by an armed  effector T cell. Because of their ability to deliver activating signals, these three cell types are known as professional  antigen-presenting cells, or often just antigen-presenting cells. Dendritic cells are  the most important antigenpresenting cell of the three, with a central role in the initiation of adaptive immune  responses. Macrophages can also mediate innate immune responses directly and make a crucial  contribution to the effector phase of the adaptive immune response. B cells contribute to adaptive immunity by  presenting peptides from antigens they have ingested and by secreting antibody.   

 

 

Two signals are required for lymphocyte activation.

 

 As well as receiving a signal through their  antigen receptor, mature naive lymphocytes must also receive a second signal to become activated. For T cells (left  panel) it is delivered by a professional antigen-presenting cell such as the dendritic cell shown here. For B cells (right  panel), the second signal is usually delivered by an activated T cell.   

The professional antigen-presenting cells.

 

 The three types of professional antigen-presenting cell are  shown in the form in which they will be depicted throughout this book (top row), as they appear in the light  microscope (second row; the relevant cell is indicated by an arrow), by transmission electron microscopy (third row)  and by scanning electron microscopy (bottom row). Mature dendritic cells are found in lymphoid tissues and are  derived from immature tissue dendritic cells that interact with many distinct types of pathogen. Macrophages are  specialized to internalize extracellular pathogens, especially after they have been coated with antibody, and to present  their antigens. B cells have antigen-specific receptors that enable them to internalize large amounts of specific  antigen, process it, and present it.   Thus, the final postulate of adaptive immunity is that it occurs on a cell that also presents the antigen. This appears to  be an absolute rule in vivo, although exceptions have been observed in in vitro systems. Nevertheless, what we are  attempting to define is what does happen, not what can happen. 

The recognition and effector mechanisms of adaptive immunity.

 Clonal selection describes the basic operating principle of the adaptive immune response but not how it defends the  body against infection. In the last part of this chapter, we outline the mechanisms by which pathogens are detected by  lymphocytes and are eventually destroyed in a successful adaptive immune response. The distinct lifestyles of  different pathogens require different response mechanisms, not only to ensure their destruction but also for their  detection and recognition. We have already seen that there are two different kinds of antigen receptor: the  surface immunoglobulin of B cells, and the smaller antigen receptor of T cells. These surface receptors are adapted to  recognize antigen in two different ways: B cells recognize antigen that is present outside the cells of the body, where,  for example, most bacteria are found; T cells, by contrast, can detect antigens generated inside infected cells, for  example those due to viruses.  

Antibodies can participate in host defense in three main ways.

 The left panels show antibodies  binding to and neutralizing a bacterial toxin, thus preventing it from interacting with host cells and causing pathology.  Unbound toxin can react with receptors on the host cell, whereas the toxin:antibody complex cannot. Antibodies also  neutralize complete virus particles and bacterial cells by binding to them and inactivating them.   The antigen:antibody  complex is eventually scavenged and degraded by macrophages. Antibodies coating an antigen render it recognizable  as foreign by phagocytes (macrophages and neutrophils), which then ingest and destroy it; this is called opsonization.  The middle panels show opsonization and phagocytosis of a bacterial cell. The right panels show activation of the  complement system by antibodies coating a bacterial cell. Bound antibodies form a receptor for the first protein of the  complement system, which eventually forms a protein complex on the surface of the bacterium that, in some cases,  can kill the bacterium directly. More generally, complement coating favors the uptake and destruction of the  bacterium by phagocytes. Thus, antibodies target pathogens and their toxic products for disposal by phagocytes. 

T cells are needed to control intracellular pathogens and to activate B-cell responses to most antigens. 

Pathogens are accessible to antibodies only in the blood and the extracellular spaces. However, some bacterial  pathogens and parasites, and all viruses, replicate inside cells where they cannot be detected by antibodies. The  destruction of these invaders is the function of the T lymphocytes, or T cells, which are responsible for the cellmediated  immune responses of adaptive immunity.  Cell-mediated reactions depend on direct interactions between T lymphocytes and cells bearing the antigen that the T  cells recognize. The actions of cytotoxic T cells are the most direct. These recognize any of the body’s cells that are  infected with viruses, which replicate inside cells, using the biosynthetic machinery of the cell itself. The replicating  virus eventually kills the cell, releasing new virus particles. Antigens derived from the replicating virus are, however,  displayed on the surface of infected cells, where they are recognized by cytotoxic T cells. These cells can then control  the infection by killing the infected cell before viral replication is complete. Cytotoxic T cells typically  express the molecule CD8 on their cell surfaces.   

Mechanism of host defense against intracellular infection by viruses.

 

Cells infected by viruses are  recognized by specialized T cells called cytotoxic T cells, which kill the infected cells directly.   The killing mechanism involves the activation of enzymes known as caspases, which cleave after aspartic acid. These in turn  activate a cytostolic nuclease in the infected cell, which cleaves host and viral DNA. Panel a is a transmission  electron micrograph showing the plasma membrane of a cultured CHO cell (the Chinese hamster ovary cell line)  infected with influenza virus. Many virus particles can be seen budding from the cell surface. Some of these have  been labeled with a monoclonal antibody that is specific for a viral protein and is coupled to gold particles, which  appear as the solid black dots in the micrograph. Panel b is a transmission electron micrograph of a virus-infected cell  (V) surrounded by cytotoxic T lymphocytes. Note the close apposition of the membranes of the virus-infected cell  and the T cell (T) in the upper left corner of the micrograph, and the clustering of the cytoplasmic organelles in the T  cell between the nucleus and the point of contact with the infected cell.   Other T lymphocytes that activate the cells they recognize are marked by the expression of the cell-surface molecule  CD4 instead of CD8. Such T cells are often generically called helper T, or TH cells, but this is a term that we will use  for a specific subset of CD4 T cells. CD4 T lymphocytes can be divided into two subsets, which carry out different  functions in defending the body, in particular from bacterial infections. The first subset of CD4 T lymphocytes is  important in the control of intracellular bacterial infections.   Some bacteria grow only in the intracellular membranebounded  vesicles of macrophages; important examples are Mycobacterium tuberculosis and M. leprae, the pathogens  that cause tuberculosis and leprosy. Bacteria phagocytosed by macrophages are usually destroyed in the lysosomes,  which contain a variety of enzymes and antimicrobial substances. Intracellular bacteria survive because the vesicles  they occupy do not fuse with the lysosomes. These infections can be controlled by a subset of CD4 T cells, known as  a TH1 cells, which activate macrophages, inducing the fusion of their lysosomes with the vesicles containing the  bacteria and at the same time stimulating other antibacterial mechanisms of the phagocyte. TH1 cells also  release cytokines and chemokines that attract macrophages to the site of infection.   

Mechanism of host defense against intracellular infection by mycobacteria.

 

 Mycobacteria are engulfed by macrophages but resist being destroyed by preventing the fusion of the intracellular vesicles in which  they reside with the lysosomes containing bactericidal agents; instead they persist and replicate in these vesicles.  However, when a specific TH1 cell recognizes an infected macrophage, it releases cytokines that activate the  macrophage and induce lysosomal fusion and macrophage bactericidal activity. The light micrographs (bottom row)  show resting (left) and activated (right) macrophages infected with mycobacteria. The cells have been stained with an  acid-fast red dye to reveal mycobacteria. These are prominent as red-staining rods in the resting macrophages but  have been eliminated from the activated macrophages..  T cells destroy intracellular pathogens by killing infected cells and by activating macrophages but they also have a  central role in the destruction of extracellular pathogens by activating B cells. This is the specialized role of the  second subset of CD4 T cells, called TH2 cells.   We shall see, when we discuss the humoral immune  response in detail, that only a few antigens with special properties can activate naive B lymphocytes on their own.  Most antigens require an accompanying signal from helper T cells before they can stimulate B cells to proliferate and  differentiate into cells secreting antibody. The ability of T cells to activate B cells was discovered long  before it was recognized that a functionally distinct class of T cells activates macrophages, and the term helper T cell  was originally coined to describe T cells that activate B cells. Although the designation ‘helper’ was later extended to  T cells that activate macrophages (hence the H in TH1), we consider this usage confusing and we will, in the  remainder of this book, reserve the term helper T cells for all T cells that activate B cells.

 T cells are specialized to recognize foreign antigens as peptide fragments bound to proteins of the major  histocompatibility complex. 

All the effects of T lymphocytes depend upon interactions with target cells containing foreign proteins. Cytotoxic T  cells and TH1 cells interact with antigens produced by pathogens that have have infected the target cell or that have  been ingested by it. Helper T cells, in contrast, recognize and interact with B cells that have bound and internalized  foreign antigen by means of their surface immunoglobulin. In all cases, T cells recognize their targets by detecting  peptide fragments derived from the foreign proteins, after these peptides have been captured by specialized molecules  in the host cell and displayed by them at the cell surface. The molecules that display peptide antigen to T cells are  membrane glycoproteins encoded in a cluster of genes bearing the cumbersome name major histocompatibility  complex, abbreviated to MHC.  The human MHC molecules were first discovered as the result of attempts to use skin grafts from donors to repair  badly burned pilots and bomb victims during World War II. The patients rejected the grafts, which were recognized  as being ‘foreign.’ It was soon appreciated from studies in mice that rejection was due to an immune response, and  eventually genetic experiments using inbred strains of mice showed that rapid rejection of skin grafts is caused by  differences in a single genetic region. Because they control the compatibility of tissue grafts, these genes became  known as ‘histocompatibility genes.’ Later, it was found that several closely linked, and highly polymorphic genes  specify histocompatibility, which led to the term major histocompatibility complex. The central role of the MHC in  antigen recognition by T cells, was discovered later still, revealing the true  physiological function of the proteins encoded by the MHC. This, in turn, led to an explanation for the major effect  on tissue compatibility for which they were named. We shall discuss these diverse functions of MHC molecules  

Two major types of T cell recognize peptides bound to proteins of two different classes of MHC molecule. 

There are two types of MHC molecule, called MHC class I and MHC class II. These differ in several subtle ways but  share most of their major structural features. The most important of these is formed by the two outer extracellular  domains of the molecule, which combine to create a long cleft in which a single peptide fragment is trapped during  the synthesis and assembly of the MHC molecule inside the cell. The MHC molecule bearing its cargo of peptide is  then transported to the cell surface, where it displays the peptide to T cells. The antigen receptors of T  lymphocytes are specialized to recognize a foreign antigenic peptide fragment bound to an MHC molecule. A T cell  with a receptor specific for the complex formed between that particular foreign peptide and MHC molecule can then  recognize and respond to the antigen-presenting cell.

  

 

MHC molecules on the cell surface display peptide fragments of antigens.

  MHC molecules are  membrane proteins whose outer extracellular domains form a cleft in which a peptide fragment is bound. These  fragments, which are derived from proteins degraded inside the cell, including foreign protein antigens, are bound by  the newly synthesized MHC molecule before it reaches the surface. There are two kinds of MHC molecule MHC  class I and MHC class II with related but distinct structures and functions.  The most important differences between the two classes of MHC molecule lie not in their structure but in the source  of the peptides that they trap and carry to the cell surface.

MHC class I molecules collect peptides derived from  proteins synthesized in the cytosol, and are thus able to display fragments of viral proteins on the cell surface.   

MHC class II molecules bind peptides derived from proteins in intracellular vesicles, and thus display  peptides derived from pathogens living in macrophage vesicles or internalized by phagocytic cells and B cells   We shall see exactly how peptides from these different sources are made accessible to the two  types of MHC molecule.   

MHC class I molecules present antigen derived from proteins in the cytosol.

  In cells infected with viruses, viral proteins are synthesized in the cytosol. Peptide fragments of viral proteins are transported into the  endoplasmic reticulum (ER) where they are bound by MHC class I molecules, which then deliver the peptides to the  cell surface.   

MHC class II molecules present antigen originating in intracellular vesicles.

 

 Some bacteria infect  cells and grow in intracellular vesicles. Peptides derived from such bacteria are bound by MHC class II molecules  and transported to the cell surface (top row). MHC class II molecules also bind and transport peptides derived from  antigen that has been bound and internalized by B-cell antigen receptor-mediated uptake into intracellular vesicles  (bottom row).  Having reached the cell surface with their peptide cargo, the two classes of MHC molecule are recognized by  different functional classes of T cell. MHC class I molecules bearing viral peptides are recognized by cytotoxic T  cells, which then kill the infected cell; MHC class II molecules bearing peptides derived from pathogens  taken up into vesicles are recognized by TH1 or TH2 cells.  

Cytotoxic T cells recognize antigen presented by MHC class I molecules and kill the cell.

 

 The peptide:MHC class I complex on virus-infected cells is detected by antigen-specific cytotoxic T cells. Cytotoxic T  cells are preprogrammed to kill the cells they recognize.   

 

TH1 and helper T cells recognize antigen presented by MHC class II molecules.

 

  On recognition of  their specific antigen on infected macrophages, TH1 cells activate the macrophage, leading to the destruction of the  intracellular bacteria (left panel). When helper T cells recognize antigen on B cells, they activate these cells to  proliferate and differentiate into antibody-producing plasma cells (right panel).  The antigen-specific activation of these effector T cells is aided by co-receptors that distinguish between the two  classes of MHC molecule; cytotoxic cells express the CD8 co-receptor that binds MHC class I molecules, whereas  TH1 and TH2 cells express the CD4 co-receptor with specificity for MHC class II molecules. However, even before T  cells have encountered the specific foreign antigen that activates them to differentiate into effector cells, they express  the appropriate co-receptor to match their receptor specificity. The maturation of T cells into either CD8 or CD4 T  cells reflects the testing of T-cell receptor specificity that occurs during development, and the selection of T cells that  can receive survival signals from self MHC molecules. Exactly how this selective process works, and how it  maximizes the usefulness of the T cell repertoire is a central question in immunology.  On recognizing their targets, the three types of T cell are stimulated to release different sets of effector molecules.  These can directly affect their target cells or help to recruit other effector cells in ways we shall discuss.  These effector molecules include many cytokines, which have a crucial role in the clonal expansion of lymphocytes  as well as in innate immune responses and in the effector actions of most immune cells; thus, understanding the  actions of cytokines is central to understanding the various behaviors of the immune system. 

Defects in the immune system result in increased susceptibility to infection. 

We tend to take for granted the ability of our immune systems to free our bodies of infection and prevent its  recurrence. In some people, however, parts of the immune system fail. In the most severe of these immunodeficiency  diseases, adaptive immunity is completely absent, and death occurs in infancy from overwhelming infection unless  heroic measures are taken. Other less catastrophic failures lead to recurrent infections with particular types of  pathogen, depending on the particular deficiency. Much has been learned about the functions of the different  components of the human immune system through the study of these immunodeficiencies.  More than twenty-five years ago, a devastating form of immunodeficiency appeared, the acquired immune deficiency  syndrome, or AIDS, which is itself caused by an infectious agent. This disease destroys the T cells that activate  macrophages, leading to infections caused by intracellular bacteria and other pathogens normally controlled by these  T cells. Such infections are the major cause of death from this increasingly prevalent immunodeficiency disease,  which is discussed fully together with inherited immunodeficiencies.  AIDS is caused by a virus, the human immunodeficiency virus, or HIV, that has evolved several strategies by which  it not only evades but also subverts the protective mechanisms of the adaptive immune response. Such strategies are  typical of many successful pathogens and we shall examine a variety of them. The conquest of many of  the world’s leading diseases, including malaria and diarrheal diseases (the leading killers of children) as well as the  more recent threat from AIDS, depends upon a better understanding of the pathogens that cause them and their  interactions with the cells of the immune system.

 Understanding adaptive immune responses is important for the control of allergies, autoimmune disease,  and organ graft rejection.

  Many medically important diseases are associated with a normal immune response directed against an inappropriate  antigen, often in the absence of infectious disease. Immune responses directed at noninfectious antigens occur in  allergy, where the antigen is an innocuous foreign substance, in autoimmune disease, where the response is to a self  antigen, and in graft rejection, where the antigen is borne by a transplanted foreign cell. What we call a successful  immune response or a failure, and whether the response is considered harmful or beneficial to the host, depends not  on the response itself but rather on the nature of the antigen.     

 

 

Immune responses can be beneficial or harmful depending on the nature of the antigen.

 

  Beneficial responses are shown in white, harmful responses in shaded boxes. Where the response is beneficial, its absence is  harmful.  Allergic diseases, which include asthma, are an increasingly common cause of disability in the developed world, and  many other important diseases are now recognized as autoimmune. An autoimmune response directed against  pancreatic в cells is the leading cause of diabetes in the young. In allergies and autoimmune diseases, the powerful  protective mechanisms of the adaptive immune response cause serious damage to the patient.  Immune responses to harmless antigens, to body tissues, or to organ grafts are, like all other immune responses,  highly specific. At present, the usual way to treat these responses is with immunosuppressive drugs, which inhibit all  immune responses, desirable or undesirable. If it were possible to suppress only those lymphocyte clones responsible  for the unwanted response, the disease could be cured or the grafted organ protected without impeding protective  immune responses. There is hope that this dream of antigenspecific immunoregulation to control unwanted immune  responses could become a reality, since antigen-specific suppression of immune responses can be induced  experimentally, although the molecular basis of this suppression is not fully understood. We shall see   how the mechanisms of immune regulation are beginning to emerge from a better understanding of the functional  subsets of lymphocytes and the cytokines that control them, and we shall discuss the present state of understanding of  allergies, autoimmune disease, graft rejection, and immunosuppressive drugs  

Vaccination is the most effective means of controlling infectious diseases. 

Although the specific suppression of immune responses must await advances in basic research on immune regulation  and its application, the deliberate stimulation of an immune response by immunization, or vaccination, has achieved  many successes in the two centuries since Jenner’s pioneering experiment.  Mass immunization programs have led to the virtual eradication of several diseases that used to be associated with  significant morbidity (illness) and mortality (Fig. 1.33). Immunization is considered so safe and so important that  most states in the United States require children to be immunized against up to seven common childhood diseases.  Impressive as these accomplishments are, there are still many diseases for which we lack effective vaccines. And  even where vaccines for diseases such as measles or polio can be used effectively in developed countries, technical  and economic problems can prevent their widespread use in developing countries, where mortality from these  diseases is still high. The tools of modern immunology and molecular biology are being applied to develop new  vaccines and improve old ones, and we shall discuss these advances. The prospect of controlling these  important diseases is tremendously exciting. The guarantee of good health is a critical step toward population control  and economic development. At a cost of pennies per person, great hardship and suffering can be alleviated.   

 

Successful vaccination campaigns.

 

 Diphtheria, polio, and measles and its consequences have been  virtually eliminated in the United States, as shown in these three graphs. SSPE stands for subacute sclerosing  panencephalitis, a brain disease that is a late consequence of measles infection in a few patients. When measles was  prevented, SSPE disappeared 15 20 years later. However, as these diseases have not been eradicated worldwide,  immunization must be maintained in a very high percentage of the population to prevent their reappearance. 

Summary. 

Lymphocytes have two distinct recognition systems specialized for detection of extracellular and intracellular  pathogens. B cells have cell-surface immunoglobulin molecules as receptors for antigen and, upon activation, secrete  the immunoglobulin as soluble antibody that provides defense against pathogens in the extracellular spaces of the  body. T cells have receptors that recognize peptide fragments of intracellular pathogens transported to the cell surface  by the glycoproteins of the major histocompatibility complex (MHC). Two classes of MHC molecule transport  peptides from different intracellular compartments to present them to distinct types of effector T cell: cytotoxic T  cells that kill infected target cells, and TH1 cells and helper T cells that mainly activate macrophages and B cells,  respectively. Thus, T cells are crucially important for both the humoral and cell-mediated responses of adaptive  immunity. The adaptive immune response seems to have engrafted specific antigen recognition by highly diversified  receptors onto innate defense systems, which have a central role in the effector actions of both B and T lymphocytes.  The vital role of adaptive immunity in fighting infection is illustrated by the immunodeficiency diseases and the  problems caused by pathogens that succeed in evading or subverting an adaptive immune response. The antigenspecific  suppression of adaptive immune responses is the goal of treatment for important human diseases involving  inappropriate activation of lymphocytes, whereas the specific stimulation of an adaptive immune response is the basis  of successful vaccination.  The immune system defends the host against infection. Innate immunity serves as a first line of defense but lacks the  ability to recognize certain pathogens and to provide the specific protective immunity that prevents reinfection.  Adaptive immunity is based on clonal selection from a repertoire of lymphocytes bearing highly diverse antigenspecific  receptors that enable the immune system to recognize any foreign antigen. In the adaptive immune response,  antigen-specific lymphocytes proliferate and differentiate into effector cells that eliminate pathogens. Host defense  requires different recognition systems and a wide variety of effector mechanisms to seek out and destroy the wide  variety of pathogens in their various habitats within the body and at its surface. Not only can the adaptive immune  response eliminate a pathogen but, in the process, it also generates increased numbers of differentiated memory  lymphocytes through clonal selection, and this allows a more rapid and effective response upon reinfection. The  regulation of immune responses, whether to suppress them when unwanted or to stimulate them in the prevention of  infectious disease, is the major medical goal of research in immunology.    

 References.

·         Davidson’s Principles and practice of medicine (21^st revised ed.) / by Colledge N.R., Walker B.R., and Ralston S.H., eds. – Churchill Livingstone, 2010. – 1376 p.

·         Harrison’s principles of internal medicine (18^th edition) / by Longo D.L., Kasper D.L., Jameson J.L. et al. (eds.). – McGraw-Hill Professional, 2012. – 4012 p.

·         The Merck Manual of Diagnosis and Therapy (nineteenth Edition) / Robert Berkow, Andrew J. Fletcher and others. – published by Merck Research Laboratories, 2011.

·         http://emedicine.medscape.com

·         http://meded.ucsd.edu/clinicalmed/introduction.htm