Physiology of leukocytes.
Leukocyte formula.
Blood types.
White blood cells, or leukocytes, are classified into two main groups: granulocytes and nongranulocytes (also known as agranulocytes).
General Structure and Function
1. protection from microbes, parasites, toxins, cancer
2. 1% of blood volume; 4-11,000 per cubic mm blood
3 diapedesis – can “slip between” capillary wall
4. amoeboid motion – movement through the body
5. chemotaxis – moving in direction of a chemical
6. leukocytosis – increased “white blood cell count” in response to bacterial/viral infection
The granulocytes, which include neutrophils, eosinophils, and basophils, have granules in their cell cytoplasm. Neutrophils, eosinophils, and basophils also have a multilobed nucleus. As a result they are also called polymorphonuclear leukocytes or “polys.” The nuclei of neutrophils also appear to be segmented, so they may also be called segmented neutrophils or “segs.”
The nongranuloctye white blood cells, lymphocytes and monocytes, do not have granules and have nonlobular nuclei. They are sometimes referred to as mononuclear leukocytes.
The lifespan of white blood cells ranges from 13 to 20 days, after which time they are destroyed in the lymphatic system. When immature WBCs are first released from the bone marrow into the peripheral blood, they are called “bands” or “stabs.” Leukocytes fight infection through a process known as phagocytosis. During phagocytosis, the leukocytes surround and destroy foreign organisms. White blood cells also produce, transport, and distribute antibodies as part of the body’s immune response.
1. Neutrophils – destroy and ingest bacteria & fungi (polymorphonuclear leuks.; “polys”)
a. most numerous WBC
b. defensins – antibiotic-like proteins (granules)
c. polymorphonuclear – many-lobed nuclei
d. causes lysis of infecting bacteria/fungi
e. HIGH poly count –> likely infection
2. eosinophils – lead attack against parasitic worms
a. only 1-4% of all leukocytes
b. two-lobed, purplish nucleus
c. acidophilic (red) granules with digest enzymes
d. phagocytose antigens & antigen/antibody complex
e. inactivate chemicals released during allergies
3. basophils – releases Histamine which causes inflammation, vasodilation, attraction of WBCs
a.RAREST of all leukocytes (0.5%)
b.deep purple U or S shaped nucleus
c.basophilic (blue) granules with HISTAMINE
d.related to “mast cells” of connective tissue
e.BOTH release Histamine with “IgE” signal
f.antihistamine – blocks the action of Histamine in response to infection or allergic antigen
Agranulocytes – WBCs without granules in cytoplasm
1. lymphocytes – two types of lymphocytes
a. T lymphocytes – (thymus) respond against virus infected cells and tumor cells.
b. B lymphocytes – (bone) differentiate into different “plasma cells” which each produce antibodies against different antigens
c. lymphocytes primarily in lymphoid tissues
d. very large basophilic (purple) nucleus
e. small lymphocytes in blood (5-8 microns)
f. larger lymphocytes in lymph organs (10-17 mic)
2. monocytes – differentiate to become macrophages; serious appetites for infectious microbes
a. largest of all leukocytes (18 microns)
b. dark purple, kidney shaped nucleus
Leukopoiesis and Colony Stimulating Factors (CSFs)
1. leukopoiesis – the production, differentiation, and development of white blood cells all cells derived from hemocytoblast
2. colony stimulating factors (CSF) – hematopoietic hormones that promote leukopoiesis
a. produced by Macrophages and T lymphocytes
i. macrophage-monocyte CSF (M-CSF)
ii. granulocyte CSF (G-CSF)
iii. granulocyte-macrophage CSF (GM-CSF)
iv. multi CSF (multiple lymphocyte action)
v. interleukin 3 (IL-3) (general lymphocytes)
Two measurements of white blood cells are commonly done in a CBC:
-the total number of white blood cells in a microliter (1×10–
-The percentage of each of the five types of white blood cells. This test is known as a differential and is reported in percentages.
Normal values for total WBC and differential in adult males and females are:
· Total WBC: 4-9 *10 G/l
· Bands or stabs: 3 – 5 %
· Granulocytes (or polymorphonuclears)
o Neutrophils : 50 – 70% relative value (2500-7000 absolute value)
o Eosinophils: 1 – 3% relative value (100-300 absolute value)
o Basophils: 0.4% – 1% relative value (40-100 absolute value)
· Agranulocytes (or mononuclears)
o Lymphocytes: 25 – 35% relative value (1700-3500 absolute value)
o Moncytes: 4 – 6% relative value (200-600 absolute value)
Each differential always adds up to 100%. To make an accurate assessment, consider both relative and absolute values. For example a relative value of 70% neutrophils may seem withiormal limits; however, if the total WBC is 20,000, the absolute value (70% x 20,000) would be an abnormally high count of 14,000.
The numbers of leukocytes changes with age and during pregnancy.
· On the day of birth, a newborn has a high white blood cell count, ranging from 9,000 to 30,000 leukocytes. This number falls to adult levels within two weeks.
· The percentage of neutrophils is high for the first few weeks after birth, but then lymphocyte predominance is seen.
· Until about 8 years of age, lymphocytes are more predominant thaeutrophils.
· In the elderly, the total WBC decreases slightly.
· Pregnancy results in a leukocytosis, primarily due to an increase ieutrophils with a slight increase in lymphocytes.
Leukocytosis, a WBC above 10,000, is usually due to an increase in one of the five types of white blood cells and is given the name of the cell that shows the primary increase.
· Neutrophilic leukocytosis = neutrophilia
· Lymphocytic leukocytosis = lymphocytosis
· Eosinophilic leukocytosis = eosinophilia
· Monocytic leukocytosis = monocytosis
· Basophilic leukocytosis = basophilia
In response to an acute infection, trauma, or inflammation, white blood cells release a substance called colony-stimulating factor (CSF). CSF stimulates the bone marrow to increase white blood cell production. In a person with normally functioning bone marrow, the numbers of white blood cells can double within hours if needed. An increase in the number of circulating leukocytes is rarely due to an increase in all five types of leukocytes. When this occurs, it is most often due to dehydration and hemoconcentration. In some diseases, such as measles, pertussis and sepsis, the increase in white blood cells is so dramatic that the picture resembles leukemia. Leukemoid reaction, leukocytosis of a temporary nature, must be differentiated from leukemia, where the leukocytosis is both permanent and progressive. Therapy with steroids modifies the leukocytosis response. When corticosteroids are given to healthy persons, the WBC count rises. However, when corticosteroids are given to a person with a severe infection, the infection can spread significantly without producing an expected WBC rise. An important concept to remember is that, leukocytosis as a sign of infection can be masked in a patient taking corticosteroids.
Leukopenia occurs when the WBC falls below 4,000. Viral infections, overwhelming bacterial infections, and bone marrow disorders can all cause leukopenia. Patients with severe leukopenia should be protected from anything that interrupts skin integrity, placing them at risk for an infection that they do not have enough white blood cells to fight. For example, leukopenic patients should not have intramuscular injections, rectal temperatures or enemas.
· A WBC of less than 500 places the patient at risk for a fatal infection.
· A WBC over 30,000 indicates massive infection or a serious disease such as leukemia.
The Complement System
The cell-killing effects of innate and acquired immunity are mediated in part by a system of plasma enzymes originally named the complement system because they “complemented” the effects of antibodies. Nomenclature for the over 30 proteins in the system is confusing because it is a mixture of letters and numbers: examples include C1q, C3, and C3b. Three different pathways or enzyme cascades activate the system: the classic pathway, triggered by immune complexes; the mannose-binding lectin pathway, triggered when this lectin binds mannose groups in bacteria; and the alternative or properdin pathway, triggered by contact with various viruses, bacteria, fungi, and tumor cells. The proteins that are produced have three functions: They help kill invading organisms by opsonization, chemotaxis, and eventual lysis of the cells; they serve in part as a bridge from innate to acquired immunity by activating B cells and aiding immune memory; and they help dispose of waste products after apoptosis. Cell lysis, one of the principal ways the complement system kills cells, is brought about by inserting proteins called perforins into their cell membranes. The holes produced in this fashion permit free flow of ions, with disruption of membrane polarity.
Innate Immunity
The cells that mediate innate immunity include neutrophils, macrophages, and natural killer (NK) cells, large lymphocytes that are not T cells but are cytotoxic. All these cells respond to lipid and carbohydrate sequences unique to bacterial cell walls and to other substances characteristic of tumor and transplant cells. They exert their effects by way of the complement and other systems, with the cells they attack frequently dying by osmotic lysis or apoptosis. Their cytokines also activate cells of the acquired immune system.
Acquired Immunity
the key to acquired immunity is the ability of lymphocytes to produce antibodies that are specific for one of the many millions of foreign agents that may invade the body. The antigens stimulating antibody production are usually proteins and polypeptides, but antibodies can also be formed against nucleic acids and lipids if these are presented as nucleoproteins and lipoproteins, and antibodies to smaller molecules can be produced experimentally when the molecules are bound to protein. Acquired immunity has two components: humoral immunity and cellular immunity.
Humoral immunity is mediated by circulating immunoglobulin antibodies in the γ-globulin fraction of the plasma proteins. Immunoglobulins are produced by B lymphocytes, and they activate the complement system and attack and neutralize antigens. Humoral immunity is a major defense against bacterial infections.
Cellular immunity is mediated by T lymphocytes. It is responsible for delayed allergic reactions and rejection of transplants of foreign tissue. Cytotoxic T cells attack and destroy cells that have the antigen which activated them. They kill by inserting perforins (see above) and by initiating apoptosis. Cellular immunity constitutes a major defense against infections due to viruses, fungi, and a few bacteria such as the tubercle bacillus. It also helps defend against tumors.
Development of the Immune System
During fetal development, lymphocyte precursors come from the bone marrow. Those that populate the thymus become transformed by the environment in this organ into the lymphocytes responsible for cellular immunity (T lymphocytes).
humoral immunity (B lymphocytes the transformation to B lymphocytes occurs in bursal equivalents, ie, the fetal liver and, after birth, the bone marrow. After residence in the thymus or liver, many of the T and B lymphocytes migrate to the lymph nodes and bone marrow. Most of the processing occurs during fetal and neonatal life. However, there is also a slow, continuous production of new lymphocytes from stem cells in adults.
T and B lymphocytes are morphologically indistinguishable but can be identified by markers on their cell membranes. B cells differentiate into plasma cells and memory B cells. There are three major types of T cells: cytotoxic T cells, helper T cells, and memory T cells.
There are two subtypes of helper T cells: T helper 1 (TH1) cells secrete IL-2 and γ-interferon and are concerned primarily with cellular immunity; T helper 2 (TH2) cells secrete IL-4 and IL-5 and interact primarily with B cells in relation to humoral immunity. Cytotoxic T cells destroy transplanted and other foreign cells, with their development aided and directed by helper T cells. Markers on the surface of lymphocytes are assigned CD (clusters of differentiation) numbers on the basis of their reactions to a panel of monoclonal antibodies. Most cytotoxic T cells display the glycoprotein CD8, and helper T cells display the glycoprotein CD4. These proteins are closely associated with the T cell receptors and may function as coreceptors.
Natural killer cells are also cytotoxic lymphocytes, though they are not T cells.
Memory B Cells & T Cells
After exposure to a given antigen, a small number of activated B and T cells persist as memory B and T cells. These cells are readily converted to effector cells by a later encounter with the same antigen. This ability to produce an accelerated response to a second exposure to an antigen is a key characteristic of acquired immunity. The ability persists for long periods of time, and in some instances (eg, immunity to measles) it can be lifelong.
After activation in lymph nodes, lymphocytes disperse widely throughout the body and are especially plentiful in areas where invading organisms enter the body, eg, the mucosa of the respiratory and gastrointestinal tracts. This puts memory cells close to sites of reinfection and may account in part for the rapidity and strength of their response. Chemokines are involved in guiding activated lymphocytes to these locations.
It had been argued that the long life of memory cells involves their repeated exposure to small amounts of antigen. However, memory cells persist when infused into mice in which the ability to process the antigen to which they are sensitive has been abolished by gene knockout. It may be that they avoid apoptosis by taking up nerve growth factor in the peripheral tissues.
Antigen Recognition
The number of different antigens recognized by lymphocytes in the body is extremely large. The recognition ability is innate and develops without exposure to the antigen. Stem cells differentiate into many million different T and B lymphocytes, each with the ability to respond to a particular antigen. When the antigen first enters the body, it can bind directly to the appropriate receptors on B cells. However, a full antibody response requires that the B cells contact helper T cells. In the case of T cells, the antigen is taken up by an antigen-presenting cell and partially digested. A peptide fragment of it is presented to the appropriate receptors on T cells. In either case, the cells are stimulated to divide, forming clones of cells that respond to this antigen (clonal selection).
Antigen Presentation
Antigen-presenting cells (APCs) include specialized cells called dendritic cells in the lymph nodes and spleen and the Langerhans dendritic cells in the skin. Macrophages and B cells themselves can also function as APCs. In APCs, polypeptide products of antigen digestion are coupled to protein products of the major histocompatibility complex (MHC) genes and presented on the surface of the cell. The products of the MHC genes are called human leukocyte antigens (HLA).
The class I MHC proteins (MHC-I proteins) are coupled primarily to peptide fragments generated from proteins synthesized within cells. The peptides to which the host is not tolerant, eg, those from mutant or viral proteins, are recognized by T cells.
The class II MHC proteins (MHC-II proteins) are concerned primarily with peptide products of extracellular antigens, such as bacteria, that enter the cell by endocytosis and are digested in the late endosomes.
T Cell Receptors
The MHC protein-peptide complexes on the surface of the antigen-presenting cells bind to appropriate T cells. Therefore, receptors on the T cells must recognize a very wide variety of complexes.
CD8 occurs on the surface of cytotoxic T cells that bind MHC-I proteins.
CD4 occurs on the surface of helper T cells that bind MHC-II proteins
The CD8 and CD4 proteins facilitate the binding of the MHC proteins to the T cell receptors. The activated CD8 cytotoxic T cells kill their targets directly, whereas the activated CD4 helper T cells secrete cytokines that activate other lymphocytes.
B Cells
B cells can bind antigens directly, but they must contact helper T cells to produce full activation and antibody formation. It is the TH2 subtype that is mainly involved. Helper T cells are pushed along the TH2 line by the cytokine IL-4 .
On the other hand, IL-12 pushes helper T cells along the TH1 line. IL-2 acts in an autocrine fashion to cause activated T cells to proliferate.
The activated B cells proliferate and transform into memory B cells (see above) and plasma cells. The plasma cells secrete large quantities of antibodies into the general circulation. The antibodies circulate in the globulin fraction of the plasma (see below) and, like antibodies elsewhere, are called immunoglobulins. The immunoglobulins are actually the secreted form of antigen-binding receptors on the B cell membrane.
Immunoglobulins
Circulating antibodies protect their host by binding to and neutralizing some protein toxins, by blocking the attachment of some viruses and bacteria to cells, by opsonizing bacteria and by activating complement.
Five general types of immunoglobulin antibodies are produced by the lymphocyte-plasma cell system. The basic component of each is a symmetric unit containing four polypeptide chains The two long chains are called heavy chains, whereas the two short chains are called light chains. There are two types of light chains, κ and λ, and eight types of heavy chains. The chains are joined by disulfide bridges that permit mobility, and there are intrachain disulfide bridges as well. In addition, the heavy chains are flexible in a region called the hinge. Each heavy chain has a variable (V) segment in which the amino acid sequence is highly variable, a diversity (D) segment in which the amino acid segment is also highly variable, a joining (J) segment in which it is moderately variable, and a constant (C) segment in which the sequence is constant. Each light chain has a V, a J, and a C segment. The V segments form part of the antigen-binding sites (Fab portion of the molecule). The Fc portion of the molecule is the effector portion, which mediates the reactions initiated by antibodies.
Two of the classes of immunoglobulins contain additional polypeptide components :
IgMs, five of the basic immunoglobulin units join around a polypeptide called the J chain to form a pentamer.
IgAs, the secretory immunoglobulins, the immunoglobulin units form dimers and trimers around a J chain and a polypeptide that comes from epithelial cells, the secretory component (SC).
Recognition of Self
A key question is why T and B cells do not form antibodies against and destroy the cells and organs of the individual in which they develop. Current evidence indicates that self antigens are presented along with nonself antigens but are then eliminated during development (tolerance). Central tolerance occurs in the thymus for T cells and the bone marrow for B cells. This is supplemented by peripheral tolerance occurring in the lymph nodes and elsewhere in the body.
Autoimmunity
Sometimes the processes that eliminate antibodies against self antigens fail, and a variety of different autoimmune diseases are produced. These can be B cell- or T cell-mediated and can be organ-specific or systemic. They include type 1 diabetes mellitus (antibodies against pancreatic islet B cells), myasthenia gravis (antibodies against nicotinic cholinergic receptors), and multiple sclerosis (antibodies against myelin basic protein and several other components of myelin). In some instances, the antibodies are against receptors and are capable of activating receptors; for example, antibodies against TSH receptors increase thyroid activity and cause Graves’ disease . Other conditions are due to the production of antibodies against invading organisms that cross-react with normal body constituents . An example is rheumatic fever following a streptococcal infection; a portion of cardiac myosin resembles a portion of the streptococcal M protein, and antibodies induced by the latter attack the former and damage the heart. Some conditions may be due to bystander effects, in which inflammation sensitizes T cells in the neighborhood, causing them to become activated when otherwise they would not respond. However, much is still uncertain about the pathogenesis of autoimmune disease.
Tissue Transplantation
The T lymphocyte system is responsible for the rejection of transplanted tissue. When tissues such as skin and kidneys are transplanted from a donor to a recipient of the same species, the transplants “take” and function for a while but then become necrotic and are “rejected” because the recipient develops an immune response to the transplanted tissue. This is generally true even if the donor and recipient are close relatives, and the only transplants that are never rejected are those from an identical twin.
A number of treatments have been developed to overcome the rejection of transplanted organs in humans. The goal of treatment is to stop rejection without leaving the patient vulnerable to massive infections. One approach is to kill T lymphocytes by killing all rapidly dividing cells with drugs such as azathioprine, a purine antimetabolite, but this makes patients susceptible to infections and cancer. Another is to administer glucocorticoids, which inhibit cytotoxic T cell proliferation by inhibiting production of IL-2, but these cause osteoporosis, mental changes, and the other stigmas of Cushing’s syndrome . A third is treatment with cyclosporine .
ABO Blood Types. History
The most well known and medically important blood types are in the AB0 group. They were discovered in 1900 and 1901 at the University of Vienna by Karl Landsteiner in the process of trying to learn why blood transfusions sometimes cause death and at other times save a patient. In 1930, he belatedly received the Nobel Prize for this discovery.
All humans and many other primates can be typed for the AB0 blood group. There are four principal types: A, B, AB, and 0. There are two antigens and two antibodies that are mostly responsible for the AB0 types. The specific combination of these four components determines an individual’s type in most cases. The table below shows the possible permutations of antigens and antibodies with the corresponding AB0 type (“yes” indicates the presence of a component and “no” indicates its absence in the blood of an individual).
For example, people with type A blood will have the A antigen on the surface of their red cells (as shown in the table below). As a result, anti-A antibodies will not be produced by them because they would cause the destruction of their own blood. However, if B type blood is injected into their systems, anti-B antibodies in their plasma will recognize it as alien and burst or agglutinate the introduced red cells in order to cleanse the blood of alien protein.
Individuals with type 0 blood do not produce AB0 antigens. Therefore, their blood normally will not be rejected when it is given to others with different AB0 types. As a result, type 0 people are universal donors for transfusions, but they can receive only type 0 blood themselves. Those who have type AB blood do not make any AB0 antibodies. Their blood does not discriminate against any other AB0 type. Consequently, they are universal receivers for transfusions, but their blood will be agglutinated when given to people with every other type because they produce both kinds of antigens.
It is easy and inexpensive to determine an individual’s ABO type from a few drops of blood. A serum containing anti-A antibodies is mixed with some of the blood. Another serum with anti-B antibodies is mixed with the remaining sample. Whether or not agglutination occurs in either sample indicates the ABO type. It is a simple process of elimination of the possibilities. For instance, if an individual’s blood sample is agglutinated by the anti-A antibody, but not the anti-B antibody, it means that the A antigen is present but not the B antigen. Therefore, the blood type is A.
Genetic Inheritance Patterns
AB0 blood types are inherited through genes on chromosome 9, and they do not change as a result of environmental influences during life. An individual’s AB0 type is determined by the inheritance of 1 of 3 alleles (A, B, or 0) from each parent. The possible outcomes are shown below:
Both A and B alleles are dominant over 0. As a result, individuals who have an A0 genotype will have an A phenotype. People who are type 0 have 00 genotypes. In other words, they inherited a recessive 0 allele from both parents. The A and B alleles are codominant. Therefore, if an A is inherited from one parent and a B from the other, the phenotype will be AB. Agglutination tests will show that these individuals have the characteristics of both type A and type B blood.
Despite the fact that the blood types of children are solely determined by inheritance from their parents, paternity in the U.S. and many other nations cao longer be legally established based on conventional blood typing. To do that, it is necessary to compare HLA types and/or DNA sequences. The use of DNA is more accurate in determining paternity, but it is also more expensive than HLA typing.
Antibodies to alien antigens in the AB0 group are usually present in our plasma prior to the first contact with blood of a different AB0 type. This may be partly explained by the fact that these antigens are also produced by certain bacteria and possibly some plants.0 When we come in contact with them, our bodies may develop long-term active immunity to their antigens and subsequently to the same antigens on the surface of red blood cells.0 This usually occurs to babies within the first six months following their birth.
Environmental Factors
While blood types are 100% genetically inherited, the environment potentially can determine which blood types in a population will be passed on more frequently to the next generation. It does this through natural selection. Specific ABO blood types are thought to be linked with increased or decreased susceptibility to particular diseases. For instance, individuals with type A blood are at a somewhat higher risk of contracting smallpox and developing cancer of the esophagus, pancreas, and stomach. People who are type O are at a higher risk for contracting cholera and plague as well as developing duodenal and peptic ulcers. Research suggests that they are also more tasty to mosquitoes. That could be a significant factor in contracting malaria.
If two different blood types are mixed together, the blood cells may begin to clump together in the blood vessels, causing a potentially fatal situation. Therefore, it is important that blood types be matched before blood transfusions take place. In an emergency, type O blood can be given because it is most likely to be accepted by all blood types. However, there is still a risk involved.
A person with type A blood can donate blood to a person with type A or type AB. A person with type B blood can donate blood to a person with type B or type AB. A person with type AB blood can donate blood to a person with type AB only. A person with type O blood can donate to anyone.
A person with type A blood can receive blood from a person with type A or type O. A person with type B blood can receive blood from a person with type B or type O. A person with type AB blood can receive blood from anyone. A person with type O blood can receive blood from a person with type O.
Because of these patterns, a person with type O blood is said to be a universal donor. A person with type AB blood is said to be a universal receiver. In general, however, it is still best to mix blood of matching types and Rh factors.
