The doctrine about infection. Pathogenicity and virulence of bacteria.

The doctrine about infection. Pathogenicity and virulence of bacteria.

An experimental infection. Methods of determination of DLM, LD50, DCL

 

 

The term infection (L. infectio to infect) signifies the sum of biological processes which take place in the macro-organism upon the penetration of pathogenic micro-organisms into it. independent of whether the penetration will entail the development of an obvious or a latent disease or whether the macro-organism will only become a temporary carrier of the causative agent.

The historically developed interaction of the susceptible human organism and the pathogenic micro-organism in certain conditions of the external and social environment which gives rise to an obvious or latent pathological process is called an infectious process.

From the biological point of view, the infectious process is a kind of parasitism in which two live organisms adapted to different environmental effects enter into combat.

Infectious disease designates one of the extreme degrees of manifestation of the infectious process. Infectious diseases are considered to be phenomena including biological and social factors. Thus, for example, the mechanisms of transmitting infectious diseases, their severity and outcome are provided for mainly by social conditions.

Infectious diseases differ from other diseases in that they are caused by live causative agents of a plant and animal origin and are characterized by contagiousness, the presence of a latent period, specific reactions of the body to the causative agent and production of immunity.

With the development of genetics the conception of the infectious agent has now become considerably extended. In many species of pathogenic agents the infectious properties are posses sed by high-molecular DNA containing cytoplasmic structures (plasmids) as well as by the nucleic acids of tumour (oncogenic) viruses which are not organisms but are capable of accomp lishing genetic information inherent in the corresponding viruses. It has thus been proved that besides diseases in which the infectious process is caused by living agents, there are infections occurring on a molecular level which are characterized by the ability to be transmitted not only through the external environment, but from the parents to the offspring.

Doctrine of Ineffective Revocation is a common law principle of trusts and estates law. It makes a revocation of a former will ineffective if the testator made the revocation through execution of a new will, and that newly executed will is determined invalid. This happens when a testator revokes the earlier on the mistaken belief that the new will is valid. The doctrine undoes only the revocation; it does not always accomplish the testator’s intent or validate an otherwise invalid will.

Doctrine of Ineffective Revocation is also called dependent-relative-revocation doctrine; conditional revocation or mistakenly induced revocation

 

Main Features of Pathogenic Microorganisms. Pathogenicity. This is the potential capacity of certain species of microbes to cause an infectious process. Pathogenicity is characterized by a complex of pathogenic properties in the microbe formed in the process of the historical development of the struggle for existence and , adaptation to parasitic life in plant, animal and human organisms. Pathogenicity is a specific character of pathogenic microbes. Pathogenic microbes, for the most part, are characterized by a specific action. Each species is capable of giving rise to a definite infectious process.

The specificity of the infectious process is quite an important feature which becomes evident in the localization of the causative agent, selectivity of tissue and organ affection, clinical picture of the disease, mechanisms of isolating microbes from the organism and in production of immunity. The peculiarities of each causative agent as an extreme stimulant are taken into account when devising methods of clinical and laboratory diagnosis, of therapy and prevention of infectious diseases.

Historically developed ecological factors play an essential role in the development of the specificity of pathogenic micro-organisms and their ability to cause diseases in certain species of hosts. These factors ensure a definite and regular nature of the transmission of the causative agent from one individual to another.

Virulence. Virulence signifies the degree of pathogenicity of the given culture (strain) Virulence, therefore, is an index of the qualitative individual nature of the pathogenic micro-organism. Virulence in pathogenic microbes changes under the influence of natural conditions.

It can be increased by a sequence of passages through susceptible laboratory animals, and also by transformation, transduction and lysogenic conversion. Virulence can be weakened by the action of different factors on the micro-organism, e. g. the defense forces of the organism, antimicrobial preparations, high temperatures, immune sera, disinfectants, seeding from one nutrient medium to another, etc. Artificial reduction of the virulence of pathogenic microbes is widely used in the preparation of live vaccines, applied for the specific prophylaxis of a number of infectious diseases.

The virulence of microorganisms is closely linked with the genetic function, the auxotrophic property in particular; in the presence of a deficit in two growth factors in mutant strains virulence is lost and cannot be restored while the immunogenic property is maintained Infection and infectious process.

In characterizing pathogenic microbes a unit of virulence has been established — Dlm (Dosis letalis minima), representing the minimum amount of live microbes which in a certain period of time bring about death of the corresponding laboratory animals. Since animals have an individual sensitivity to a pathogenic microbe, then the absolute lethal dose Dcl (Dosis certa letalis) which will kill 100 per cent of the experimental animals has been established. This provides for a more accurate characteristic. At present LD₅₀ (the dose which is lethal to one half of the infected animals) is considered to be the most suitable, the use of which allows for a minimal correction in evaluating the virulence in pathogenic bacteria, and may serve as an objective criterion for comparison with other units of virulence. That number of pathogenic bacteria which is capable of giving rise to an infectious disease is known as the infectious dose of a pathogenic micro-organism.

In tests on volunteers it was established, for instance, that the infectious dose is 10⁸ cells for enteropathogenic 0124 E. coli, 10⁵-10¹⁰ for Salmonella organisms, 10⁵ for Salmonella typhi (a dose of 10⁷ caused the disease in 50 per cent of infected volunteers, a dose of 10⁸-10¹⁰ in 89 to 95 per cent), 10⁶ -10¹¹ for the El Tor cholerae vibrio, and 10 to 100 bacterial cells for Shigella dysenteriae. The action of small and large doses of microbes is of great significance in the development of the infectious process, in the length of the incubation (latent) period, and in the severity and outcome of the disease.

Under favourable conditions one microbial cell with a cell division rate of 20 minutes can give a progeny of 250000 individuals in six hours, and in several hours the amount of microbes may attain many thousand millions which create a large physiological burden on the tissues and organs of the infected organism. The virulence of pathogenic micro-organisms is associated with toxin production, invasiveness, capsule production, aggressiveness and other factors.

 

 

 

 

 

 

 

 

 

 

 

The virulence of pathogenic microorganisms is associated with adherence, invasiveness, capsule production, toxin production, aggressiveness and other factors.

 

 

 

 

Adherence of bacteria to host cells

 

               

                      Adherence of vibrio cholera on the mucose

 

Microbial toxins. According to the nature of production, microbial toxins are subdivided into exotoxins and endotoxins. Exotoxins include toxins produced by the causative agents of botulism, tetanus, anaerobic infections, diphtheria, and by some species of Shigella and haemolytic streptococci, and staphylococci, by the causative agents of diphtheria, whooping cough, plague, cholera, anthrax, by the parahaemolytic vibrio, etc.

The mechanism of toxin production has been recently studied in more detail It was established that the genes of toxigenicity (tox⁺ genes) are located in the temperate phage DNA in some bacterial species (Cor. diphtheriae, S. aureus, etc.), in the plasmids in others (E. coli etc.); in Cl. histolyticum and Cl. novyi toxin production is associated with genes located in the DNA of the temperate phage and with genes responsible for sporulation.

Toxigenycity is not a compulsory species property since all known toxigenic bacteria may exist without producing a toxin. The activity of the tox⁺ genes is controlled by repressers of the bacterial cell itself, the production of which is induced by certain substances contained in the nutrient medium.

The activity of exotoxins is stipulated by certain parts of the protein molecule, active centres which are amine groups of toxins; the blocking of these groups with formaldehyde results in the loss of toxicity.

More than 50 protein exotoxins of bacteria are known to date. They are subdivided into three classes.

 

Class A includes exotoxins secreted into the external environment:  cholerogen (V. cholerae); haemolysin (V. parahaemolyticus); alpha, beta, delta and gamma haemolysins (S. aureus); histotoxin, dermonecrotoxin and haemolysin (Cor. diphtheriae), alpha– and delta–haemolysin and beta and epsilon toxin (C/. perfringens); oedema and lethal toxin (Bac. anthracis), and others.

Class B includes exotoxins which are partly secreted, partly bound with the microbial cell: labile toxin {Bord. pertussis); alpha toxin (C/. novyi); tetanospasmin (Cl. tetani); neurotoxin (C/. botulinum).

Class C consists of exotoxins bound with the microbial cell; Sh. dysenteriae exotoxin; Y. pestis mouse toxin; Cl. perfringens enterotoxin; Bord, pertussis histamine-sensitizing and lymphocytosis-stimulating factors.

 

Exotoxins easily diffuse from the cell into the surrounding nutrient medium. They are characterized by a markedly distinct toxicity, and act on the susceptible organism in very small doses. Exotoxins have the properties of enzymes hydrolysing vitally important components of the cells of tissues and organs.

 

In chemical structure exotoxins belong to substances of a protein nature. They are weakly stable to the action of light, oxygen, and temperature (they decompose at 60-80°C within 10-60 minutes, and on boiling they break down immediately). In a dried condition they are more stable to high temperature, light, and oxygen. The addition of saccharose to the toxins also increases their resistance to heat.

 

Under the effect of 0.3-0.4 per cent formalin and 38-40°C temperature, diphtheria toxin within 30 days loses its toxic properties and changes into an anatoxin.

 

Some exotoxins (diphtheria, tetanus and anaerobic infections) break Infection and infectious process down under the influence of digestive enzymes as a result of which they become harmless when administered orally. Other exotoxins (of Clostridium botulinum, Clostridium perfringens and pathogenic staphylococci) do not break down in the stomach and intestine and cause intoxication of the organism during oral administration.

The potency of toxins is determined on sensitive laboratory animals according to Dlm and LD₅₀. For example. 1 Dlm of the diphtheria toxin represents the minimal amount which during subcutaneous injection into 250 g guinea pigs kills them on the fourth day.

The minimal lethal dose of the native diphtheria toxin for the guinea pig is within the range of 0.002 ml, tetanus toxin for white mouse — 0.000005 ml, and botulinus toxin for the guinea pig — from 0.00001 to 0.000001 ml.

In recent years pure tetanus, botulinus and diphtheria toxins have been obtained. They are purified by different methods: coagulation at the isoelectric point, repeated precipitation by trichloroacetic acid at a low temperature and a pH of 4.0, salting out by ammonium sulphate and adsorption by various substances.

Purified toxins have a characteristically high toxicity for sensitive laboratory animals. Thus, for example, 1 mg of the diphtheria toxin contains 40 000 000 Dim for the guinea pig, and 1 mg of botulinus toxin contains 1000000000 Dim for the white mouse. Crystalline toxins are even more toxic.

The causative agents of enteric fever, paratyphoids, dysentery, gonorrhoea, meningitis and many other Gram-negative bacteria do not  produce exotoxins they contain endotoxins. Endotoxins are more  firmly bound with the body of the bacterial cell, are less toxic and act on the organism in large doses; their latent period is usually estimated in hours, and the selective action is poorly expressed. According to chemical structure, endotoxins are related to glucoside-lipid and polysaccharide compounds or phospholipid-protein complexes. They are thermostable. Some endotoxins withstand boiling and autoclaving at 120°C for 30 minutes. Under the effect of formalin and a high temperature they are partially rendered harmless.

 

Comparative Characteristics of Toxins

 

The majority of protein bacterial toxins catalyse certain chemical processes, break down vitally important compounds, are active in extremely small doses, have a latent period and inhibit the defensive functions of tissues. Some bacterial toxins have the properties of lecithinase. Thus, for example, Clostridium perfringens produces exotoxin (lecithinase C) which is able to cleave lecithin into phosphorylcholine and a diglyceride.

Necrosis of the muscular tissue is caused as a result of the combined action of lecithinase, collagenase and mucinase (hyaluronidase). Collagenase and mucinase decompose the connective tissue of the muscles, and lecithinase dissolves the lecithin of the membrane of muscle fibres. Haemolysis during anaerobic infections takes place due to lysis of lecithin of the stroma in erythrocytes. Bacterial toxins are characterized by organotropy (monotropy and polytropy) due to which the toxigenic micro-organisms bring about tissue necrosis in localized foci of the causative agent.  The necrotic manifestation of toxins has a great adaptive significance for the causative agent. Firstly, the toxins change live and reactive tissue into a substrate harmless for the pathogenic microbes. Secondly, the necrotic tissue protects the parasite from the effects of the defense reactions of the macro-organism.

Toxins are regarded as enzymatic poisons, which are capable of arresting metabolic processes. This view point is considered to be the most probable. It has been suggested that in the process of development of saprophytic bacteria entering into long symbiosis with animal organisms, the ability to produce enzymes, facilitating symbiosis with tissues, and increasing their life activities at the expense of the host, was gradually raised. Eventually, due to the establishment of the parasitic mode of life, the enzymes of these bacteria became more specialized as a result of which adaptive enzymes transformed into enzymatic toxins — exotoxins. Therefore, it can be considered, that toxic infections were formed in a later period, preceded by a simple parasitism and a disease of a septic type.

Exotoxins are capable of causing potentiation when, under the effect of a mixture of toxins, their action in the organism is more marked. An especially distinct potential action is shown by toxins of organisms responsible for anaerobic infections and tetanus and of staphylococci, and diphtheria bacilli.

Some protein toxins cause haemolysis of erythrocytes (staphylococci, streptococci, etc.). The streptococcal haemolysin is freed from the cells during autolysis. Intravenous injection into guinea pigs is lethal. During subcutaneous injection in small doses it brings about the production of antibodies in the organism. The toxin is inactivated by cholesterol, pepsin, papain and trypsin. The haemolytic activity of the streptococcal  toxin is determined by the degree of haemolysis of a 1 per cent suspension of erythrocytes.

Microbes producing alpha-haemolysin cause the production of green or dark-green colonies of microbes on blood agar, as a result of the haematometamorphosis of the iron in erythrocytes. Beta-haemolysin dissolves erythrocytes, and upon the cultivation of bacteria producing beta-haemolysin, transparent zones of haemolysis are formed around the colonies.

Besides, a number of pathogenic microbes produce gamma-haemolysin which causes the haemolysis of erythrocytes in rabbits, humans, guinea pigs and is characterized by poor resistance to heating. Also, delta-haemolysin has been discovered which destroys the erythrocytes of man and some animals. It is excreted, for example, by pathogenic strains of staphylococci. Staphylococci and streptococci produce leucocidins which destroy pleomorphonuclear leucocytes.

 

 

Action of the hemolysin on red blood cells

 

Pathogenic strains of bacteria produce coagulase which causes coagulation of human, horse and rabbit plasma. Coagulase does not clot the plasma of guinea pigs, rats and chickens. Some bacterial enzymes have toxic properties.

 

 

 

 

Coagulase activity of bacteria

 

Thus, for example, more than 200 species of microbes (bacteria of pneumonia, ozaena and rhinoscleroma, Proteus, continental strains of causative agents of plague, etc.) produce urease, which proved to be a toxic enzyme. Many amino acid decarboxylases produced by causative agents of anaerobic infections and other microbes have toxic properties.

Lecithinases subdivided into A, B and C are typical enzymatic toxins. Lecithinase A is found in snake, bee and scorpion venom, lecithinase B — in plants, and lecithinase C — in many pathogenic microbes, especially in some causative agents of anaerobic infections.

Clostridium perfringens produces a typical alpha-toxin, which is considered to be a specific bacterial enzyme.

The enzyme neuraminidase (a protein) splits the alpha-ketoside bond formed by neuraminic acid in oligosaccharides, polysaccharides, and carbohydrate components of complex proteins. Neuraminidase is produced by cholera vibrios, the causative agents of anaerobic infection, streptococci, and Corynebacterium diphtheriae, and is found in the membranes of the influenza virus. It splits sialic acids from the surface of the cells, which results in changes in the three-dimensional configurations of the surfaces, diminished firmness of their structure, and reduced resistance.

Some microbes produce toxic substances: methylamine, dimethyl-amine, histamine, choline, neurine etc. Toxic amines are products of the decomposition of bacterial protein, and may accumulate in spoiled foodstuffs and serve as a source of food poisonings.

A number of micro-organisms produce ammonia and cause ammonia intoxication (Clostridium histolyticum, etc.). Ammonia is produced by deamination of amino acids.                                  

Rickettsial toxins are relatively labile substances, closely bound with the cells of the rickettsiae themselves. They comparatively quickly disintegrate after the death of rickettsiae from the action of formalin or heating at 56-60 C for 30 minutes.

Viral toxins are thermolabile, sensitive to the action of formalin and other substances. Viral toxins are easily neutralized by specific immune sera.

Human pathogenic viruses also contain toxic components. They have been found in causative agents of influenza, parotitis, etc. The influenza virus, for instance, contains five protein compounds: transcriptase, nucleoid protein, neuraminidase, haemagglutinin, and protein of the inner membrane; they are all heterogenous in relation to the human organism and, therefore, toxic.

Toxins cause distortions in metabolism, causing changes in the adrenalin and ascorbic acid levels. Under the influence of toxins, profound inhibition of such an important ‘ink in metabolism as the Krebs  oxidation cycle of tricarboxylic acids occurs.

Local as well as general manifestations of intoxications are accompanied by morphological changes in the formed elements of the blood. in the composition of proteins, enzymes, in the serological (production of antibodies), general clinical (temperature and neuropsychic) reactions, in disturbances in the respiratory organs, cardiovascular system, etc. Anatomical changes are characterized by inflammatory processes in the lymph nodes or by affection of certain organs and tissues.

 

 

Invasive properties of pathogenic bacteria. Virulent microbes are characterized by the ability to penetrate tissues of the infected organism. By chemical analysis it has been established that the greater part of the main substance of connective tissue contains polysaccharide – hyaluronic acid which is capable of resisting penetration into the tissue of different foreign Substances, including pathogenic microbes.

This protective barrier of connective tissue can be overcome due to the disintegration of hyaluronic acid by toxic substances of animal, plant and microbial origin. In 1928 F. Duran-Reynals established that upon infection of a rabbit with the vaccinia virus (cowpox), the infectious process increased considerably, if together with the virus aqueous extracts of rabbit, guinea pig or mouse testes were injected intracu- taneously. Later on, it was established that factors capable of increasing the permeability of tissues are found in some bacteria, snake venom and in different tissues and organs of animals. Substances causing this change in the permeability of tissues are known as spreading factors.

A certain enzyme was isolated from different tissues which hydrolysed hyaluronic acid. Some spreading factors are similar to this enzyme which is known as hyaluronidase.

Spreading factors are characterized by an extremely high activity. They act in very small doses, disintegrate at 60°C within 30 minutes, and have enzymatic properties. Spreading factors are not confined to hyaluronidase. They include substances differing iature. They include fibrinolysin produced by haemolytic streptococci of the A group, pathogenic staphylococci, and Infection and infectious process organisms involved in anaerobic infections, etc. Spreading factors increase the local primary action of pathogenic microbes affecting the connective tissue, and enhance the development of a general infection. They were found in many pathogenic micro-organisms (staphylococci, streptococci, causative agents of anaerobic infections, tetanus, diphtheria, etc.)

The effect of spreading factors on the course of infectious diseases is related to the virulence of the causative agent. In weakly virulent microbes such as staphylococci, colibacilli, and Proteus, the spreading factor increases the infectious process, if a large quantity of the microbes are injected. In highly virulent causative agents (Mycobacterium tuberculosis, type I S. pneumoniae) spreading factors increase the infectious disease with a minimal number of bacteria, sometimes with only several individuals.

 

The invasion of cells by bacteria

 

 

The role of capsular material in bacterial virulence. Some pathogenic micro-organisms (bacilli of anthrax, Clostridium perfringens, S. pneumoniae, causative agents of plague and tularaemia) are capable of producing a capsule in animal and human bodies. Certain micro-organisms produce capsules in the organism as well as iutrient media (causative agents of rhinoscleroma, ozaena, pneumonia).

Capsule production makes the microbes resistant to phagocytosis and antibodies, and increases their invasive properties. Thus, for example, capsular anthrax bacilli are not subject to phagocytosis, while noncapsular variants are easily phagocytized.

 

 

The high virulence of capsular microbes is associated with the toxic substances contained in the capsule. In chemical composition, the capsular material in some microbes consists of complex polysaccharides, and in others it consists of proteins. It can be different in separate strains of the same species, and on the other hand may be similar in different bacteria. There are nitrogenous and nitrogen free compounds in the capsular polysaccharides. They give the micro-organisms type specificity. In types II and III S. pneumoniae, the capsule is a glucoside of cellobiuronic acid in a highly polymerized state. In types I and IV the capsule contains highly polymerized compounds of aminosaccharides and organic substances. In some bacilli the capsule consists of polypeptides of d-glutamic acid, while in bacteria of Friedlander’s pneumonia it is a polymer carbohydrate, and in anthrax bacilli it consists of glycoprotein.

Bacterial aggressins. Besides toxigenicity, invasiveness and capsule production, pathogenic microbes are capable of producing substances which inhibit the defense mechanisms of the organism, and increase the pathogenic action of many causative agents of infectious diseases. 0. Bail named them aggressins. They were found in peritoneal and pleural exudates of laboratory animals infected with anthrax bacilli, S. pneumoniae, and other microbes. Aggressins alone, separated from bacteria and exudate cells by filtration, upon injection into the animal are harmless, but upon their addition to a non-lethal dose of the microbes, a severe         infectious process develops, often ending in death of the animal.

Aggressins were found in causative agents of enteric fever, paratyphoids, cholera, anthrax, diphtheria, plague, tuberculosis and pyogenic diseases. Aggressins are probably not one substance but several different substances occurring in the processes of vital activity of pathogenic microorganisms (some compounds of the surface structures of the microbial cells, products of DNA and RNA splitting).

 

Virulence is a dynamic property which is controlled by the mutation process occurring constantly both in the causative agent and in the host and which provides the continuity of the selection of changes advantageous for both. The ranges of virulence vary from absolute parasitism to the level of saprophytism. Pathogenic species may coexist with the macro-organism as latent forms for long periods of time.

 

Role of the Macro-Organism, Environment and Social Conditions in the Origin and Development of the Infectious Process.    The origin of the infectious disease depends on the reactivity of the human body, the quality and quantity of the causative agent, and the influence of the external environment and social conditions. Depending on the relationship of these factors, the infectious process may terminate in the death of the causative agent, the death of the host or the establishment of mutual adaptation between the host and the parasite.

The penetration of the causative agent into the body does not always entail disease but in many cases it is limited by a short-term infection without any manifestation of the disease or by a comparatively long carrying state (streptococci, adenoviruses, enteroviruses, herpes virus, malarial plasmodium, Entamoeba histolytica).

The reactivity of the human body with its immunobiological readiness to render the pathogenic micro-organism harmless is closely related with the environment, with conditions of life, nature of work and nutrition, hygienic and ageneral cultural level and many other factors.

The condition of the macro-organism and its resistance have a decisive significance in the origin, course and outcome of the infectious disease.

Susceptibility depends to a certain extent on age and sex due to certain physiological peculiarities. For example, during menstruation, pregnancy and labour the female organism becomes more sensitive, particularly to streptococcal diseases.

Children are more susceptible to some infectious diseases, and less susceptible to others than adults. Resistance to many infectious diseases in children up to the age of 6 months is associated with a poorly developed central nervous system, and also with the presence of maternal immunity. Besides, it has been established that in relation to some diseases (dysentery, staphylococcal and streptococcal diseases, colienteritis and infections caused by Coxsackie virus) children are more susceptible than adults. The varied age resistance to infectious diseases depends on the nature of metabolism, the function of the organs of internal secretion, and on peculiarities of immunity.

 Such factors as nature of nutrition (general starvation, deficiency of proteins, fats, carbohydrates, vitamins, and trace elements), overstrain, Infection and infectious process cooling, sanitary-hygienic conditions of work and life, also various somatic diseases, chronic poisonings and disturbances in the normal activity of the central nervous system have the effect of increasing the susceptibility to infectious diseases.

        General starvation is accompanied by an aggravation of tuberculosis, dysentery, furunculosis and other diseases. As a result of starvatioot only individual, but specific immunity is lost. For example, during starvation, pigeons become susceptible to anthrax to which they are resistant in a normal state. Lowering of the resistance in animals is not only due to general starvation, but also due to a deficiency of individual components of food, e.g., proteins, fats, carbohydrates. Starvation is accompanied by a disturbance in the protein metabolism, which leads to a decrease in the synthesis of immune globulins (antibodies), and a lowering of the activity of phagocytes.

Vitamin deficiencies have a great influence on the susceptibility to infectious diseases. A deficiency of vitamin A provides for the appearance of catarrhs of the mucous membranes of the eye and leads to xerophthalmia, enhances the development of skin affections, bronchopneumonia, influenza and acute catarrhs of the upper respiratory tract. A deficiency of vitamin b) , causes an increased susceptibility to leprosy and to a number of pathogenic and conditionally pathogenic microbes Vitamin C deficiency causes a decline in the resistance to tuberculosis, diphtheria, streptococcus, staphylococcus, pneumococcus and other diseases.

Quite important is the fact that during many infectious diseases as a result of the lethal action of drugs on the normal intestinal microflora which supply the organism with vitamins of the B group, vitamin deficiencies develop.

In the past years great heed has been paid to the problems of the study of mineral metabolism. A deficiency of iron, calcium, magnesium, copper, zinc, iodine, manganese, boron, cobalt and molybdenum leads to a disturbance in metabolism, a decrease in the resistance of the organism and an increase in the susceptibility to infectious diseases. Small amounts of trace elements are capable of increasing the defence mechanisms of the macro-organism, in particular, the phagocytic activity of leukocytes. They restore the previously impaired biochemical functions.

Physical and mental overstrain associated with an irregular organization of working hours and a disturbance of conditions of life causes a weakening of the defence mechanisms to many infectious diseases. Cooling lowers the resistance of the organism in relation to pathogenic and conditionally pathogenic microbes, enhances the development of pneumonia, catarrhs of the upper respiratory tract and other diseases. Pasteur proved that cooling in chickens causes a disturbance of specific immunity to anthrax. When the environmental temperature increases, penguins die from auto-infections caused by aspergilli.

Cooling as well as overheating of the body of animals leads to disturbances in biocatalytic  reactions, a weakening of the organism and lowering of immunity to infectious diseases. It is known, for example, that acute catarrhs are observed in the autumn-winter period, while colienteritis and infections caused by Coxsackie and ECHO viruses develop in the summer.

The effect of ultraviolet rays and sunlight on the organism depends on the wave length, intensity and duration of application. Observations have shown that sunlight has a favourable effect on the organism, and to a certain degree increases the resistance to infectious diseases. However, in a number of cases, lengthy and intense irradiation is accompanied by a decrease in the resistance of the human organism to a number of pathogenic microbes. For example, spring relapses of malaria are observed in people infected by plasmodia and exposed to intense solar radiation.

Of great theoretical and practical importance is the action of ionising radiation. As has been established small doses of X-rays increase the resistance of animals to various diseases, while increased doses lower it and enhance the activity of normal microflora and development of bacteremia and septicaemia. At the same time the permeability of mucous membranes is disturbed, their barrier capacity is reduced, and the function of the reticuloendothelial system and defence properties of the blood are sharply lowered. Especially dangerous to man are increasing doses of ionising radiation as a result of the testing of nuclear weapons. Radioactive strontium accumulates in the atmosphere. It causes deep changes in the hemopoietic function of the bone marrow, the formation of tumours and impairs reproductive ability.

Poor sanitary hygienic conditions of work and life have an unfavourable effect on the human body. Despite the fact that there are 2.5 million tons of air per each person, due to its pollution in large cities and industrial centres the incidence of respiratory diseases among the people is growing lately. This results in the spread of chronic diseases (cancer of the lungs, emphysema, asthma, etc.). The polluted air has a detrimental effect on animals and the vegetable kingdom. A deficiency of oxygen in the building, an excess of carbon dioxide and other harmful gases cause chronic toxicosis and are favourable for the development of tuberculosis. The presence in the air of dust containing a large amount of silicates disturbs the integrity of the mucous membranes of the respiratory tract, increases the possibility of infection by different micro-organisms, and leads to such diseases as tuberculosis, actinomycosis, aspergillosis, etc. Limited insolation also causes various disturbances in the activity of the body and enhances the development of diseases. Besides these harmful external factors, a great influence on the susceptibility to infectious diseases is caused by various somatic diseases (diabetes and other disturbances of the endocrine organs, diseases of the cardiovascular system, liver, kidneys, chronic poisonings by alcohol, nicotine and other poisons).

The hypophysis-adrenal system is of great importance in maintaining stability of the internal medium of the organism. The system is stimulated by the action of different stimulants, e. g. mechanical traumas, cold, heat, ultraviolet and ionising radiation, micro-organisms, etc. As a result of an excess deficiency or abnormal combination of hormones such as STH (somatotrophic hormone), ACTH (adrenocorticotrophic hormone), various disturbances in the functions of the organism may occur. Thus, for example, cortisone inhibits the inflammatory reaction, and therefore enhances the development of the infectious process. The somatotrophic hormone, on the other hand, activates the inflammatory process and causes an anti-infectious action.

Disturbances of the normal activity of the central nervous system deserve special attention. As is known, the causative agents of infectious diseases are extraordinary biological stimulants. With experimental infections, principally by neurotropic stimulants, it had been observed long ago that the injection of the infected material into the brain is accompanied by the greatest number of deaths.

Mental disturbances also lower the regulating function of the central nervous system. The mental patients in psychiatric hospitals more often contract infectious diseases.

Under the influence of various national disasters (hunger, war, earthquakes, floods) infectious diseases attain a mass distribution and are accompanied by a high death-rate and disability.

Thus, the infectious process reveals itself in the unity of biological and social factors. The disease incidence, severity of the clinical course and death-rate depend closely on the activity of the main economic laws of social formations.

 

BIOLOGICAL EXAMINATION. Biological study consists of infecting animals for the purpose of isolating the culture of the causative agents and their subsequent examination for pathogenicity and virulence.

Choice of experimental animals depends on the aim of the study. Most frequently used are rabbits, guinea pigs, albino mice, and albino rats. This is explained by the fact that they are susceptible to the causative agents of various infections diseases in man, easy to handle, and propagate readily. Hamsters, polecats, cotton rats, monkeys, birds, etc. may also be occasionally infected.

Specialized, particularly virological, laboratories, make use of genetically standardized, so-called inbred animals (mice, rabbits, guinea pigs, and others).

Working with experimental animals, one should keep it in mind that they may have spontaneous bacterial and viral diseases and latent infections activated as a result of additional artificial in­oculation. This hinders the isolation of pure culture of the causative agent and determination of its aetiological role. Gnotobiotes (without microflora) and animals free of pathogenic microorganisms have no such drawback. Currently they include chickens, rats, mice, guinea pigs, pigs, etc.

Laboratory animals are distinguished by their species, age, and individual sensitivity toward microorganisms. Thus, in selecting animals for study it is necessary to take into account their species and age. For instance, sensitivity in mongrel animals may show con­siderable individual variations. The use of inbred animals with a definite constant susceptibility toward microorganisms excludes individual variations in sensitivity and allows for reproducible re­sults.

Animals are infected for .isolating pure culture of the causative agent in cases where it is impossible to obtain it by any other method (for example, in contamination of the studied objects by extra­neous microflora which inhibits growth of the causative agent and in case of insignificant amounts of microorganisms or their trans­formation into filtering forms). Thus, in studying decayed corpses of rodents for the presence of plague causative agents, one inoculates (with suspension of the organs or blood) guinea pigs which die 3-7 days later with manifestations of septicaemia. Pure culture of the causative agent is readily isolated from the blood of internal organs.

Contamination of susceptible animals for reproducing the infec­tious process is used in diseases caused by Rickettsia and viruses.

Injection to mice of material from a patient with tickborne enceph­alitis brings about paralysis and death in these animals. To de­termine pathogenicity and virulence of the causative agents of plague, tularaemia, botulism, anthrax, and some viral diseases, cultures obtained from patients arc inoculated into albino mice, guinea pigs. rats, or suckling mice.

 

Methods of inoculation. In experimental inoculation of animals the studied material is administered via different routes: epicutaneously, subcutaneously, intracutaneously, intramusculariy, intravenously, per os, and in various organs and tissues such as the brain, mucosa, respiratory tract, etc. The method of material inoculation depends on affinity of the causative agent to definite tissues of the organism (tropism), while the volume of inoculum depends on the method of its administration and the species of animals .

 

Examination OF MICROORGANISM VIRULENCE. In studying characteristics of pathogenic microorganisms, it may be occasionally necessary to determine their virulence (degree of pathogenicity). It may be done for the purpose of characterizing the infectious causal organisms isolated from patients, carriers or the environment, for estimating the residual virulence of live vaccines, and for determining the immunity tension in animals, etc.

Virulence is defined in Dlm (Dosis letalis minima), i.e., the minimal amount of microorganisms causing death of the animal of a definite species, and LD₅₀, i.e., the minimal dose inducing death of50 per cent of the infected animals. The LD₅₀ is a more reliable indicator of virulence, being less dependent on individual sensitivity of animals.

In determining the LD₅₀, one should strictly standardize such variables as a species, sex, and weight of animals and conditions of their keeping and feeding. Ten-fold dilutions are prepared from aculture of bacteria, viruses or toxin, each dilution being injected to 4-6 animals. After a definite period of time count the number of animals that 4 have died and calculate the LD₅₀ using the method proposed by Herd and Muench. The method  is based on a logical postulation that the tested animals, that have sussumbed following inoculation with some dilution of the infective material studied would have died following inoculation with any lower dilution The data presented in Table 2 demonstrate that 50 per cent of the dose (LD₅₀) is between 10^–5 and 10^–6 dilutions of the bacteria containing material.

 

Determination of the LD₅₀ by the Reed and Muench Technique

 

For its accurate expression, one should determine the value X which is added to the logarithm of the dilution below 50 per cent of the dose (5 in our example):

     A – 50

X = ———- ,

     A – B

where A is the proportion of dead animals receiving the dilution below 50 per cent of the dose (80 per cent in our case); B is the percentage of dead animals receiving the dilution above 50 per cent of the dose (20 per cent in our example). Putting actual values in the formula, one will have:

    80 – 50

X = ———- .

   80 – 20

Hence, in the aforementioned case the LD₅₀ corresponds to 10^-5,5 dilution of the bacterial suspension Since in obtaining serial ten–fold dilutions 0 1 ml of the infective material is transferred sequentially (adding it to 0 9 ml of solvent), 1 ml of the initial bacterial suspension contains 10^6,5 of the LD₅₀.

 

The dynamics of the development of the infectious process consists of the incubation and prodromal periods, the height of the disease and period of recovery      ( convalescence). A certain period of time elapses from the moment of penetration of the pathogenic microbe to the onset of the first sings of the disease, which has beeamed the incubation period of the disease. It varies from several hours ( in cholera, toxinfections and plague) to several months and years ( in leishmaniasis, leprosy).

The duration of the incubation period depends on the degree of the general and specific immunity of the human body, its reactivity, sensitization (increased sensitivity), influence of harmful environmental factors and social conditions of life, and on the dose and virulence of the causative agent .

Incubation Periods of Diseases

 

 

One of the forms of interrelationship which occurs between the pathogenic micro-organism and a human or animal body without manifesting an obvious disease is carrier state. The ability of the causative agent to carry infectious diseases has been confirmed only in a relatively immune organism. Regarding specificity of action, carrier state has much in common with the infectious process. In some infectious diseases an intense and prolonged post-infectious immunity is produced which excludes carrier state (measles, smallpox, chickenpox, etc.). In other diseases during the period of convalescence a carrier state may be prominent which is different in frequency and duration (cholera, enteric fever, paratyphoid, dysentery, amoebiasis, scarlet fever, diphtheria. meningitis, malaria, encephalitis, poliomyelitis, etc.).

Carrier state may be found in healthy persons who have come into contact with diphtheria, meningitis, enteric fever, cholera, amoebiasis, encephalitis and poliomyelitis patients. Carrier state with a duration of 3 months is considered acute, while carrier state for longer periods is Infection and infectious process considered chronic. Prolonged carrier state (years and decades) has been described in enteric fever.

When infection occurs not with one species of causative agent, but with two or more, one speaks of mixed infection (measles and scarlet fever, measles and tuberculosis). If the infectious process is caused by micro-organisms changed under the influence of one or several comembers of the parasite coenosis, then this state is known as parainfection.

In some cases infection causes a weakening of the body which then becomes susceptible to other diseases. Thus, for example, after influenza or measles pneumonia occurs. This is known as secondary infection.

There are also focal and generalized infections. For example, during infection with staphylococcus, the infectious process causes furunculosis, and if the causative agent penetrates into the blood sepsis will develop. An alternate occurrence of focal and generalized infections is observed during tuberculosis and syphilis.

Reinfection is a repeated infection by the same species of microbe responsible for the disease which terminated in convalescence (gonorrhoea, syphilis, etc.).

Superinfection is a fresh infection of the body in which the main disease has not ended. Superinfection occurs in many infectious diseases in their acute and chronic forms.

Relapse is a return of the symptoms of the same disease (relapsing fever, paratyphoid fevers, etc.). Of certain significance in the occurrences of relapses is the low level of immunolo gical activity of the organism during illness and convalescence.

 

TRANSMISSON   OF   INFECTIOUS   AGENTS. Infectious diseases that can be spread from one host to another are said to be communicable or contagious. The term communicable disease implies direct transmission from one person to another. Preventing such diseases often is accomplished by avoiding contact with infected individuals. Measures such as quarantine were devised to avoid exposure. Measles, German measles, influenza, gonorrhoea, and genital herpes are all highly communicable. This means that the pathogens causing these diseases are readily transmitted with high frequency from an infected individual to a susceptible host.

Some diseases are not caused by agents that are communicable from one human to another. Tetanus, rabies, and Lyme disease are examples of noncommunicable infectious diseases. This means that they are acquired from the environment and are not spread directly from one person to another. Some noncommunicable diseases are caused not by the effects of invading microorganisms on host tissues, but rather by the ingestion of toxins made by the invading microorganisms. Such diseases are called intoxications rather than infections. For example, staphylococcal food poisoning is an intoxication that results from the ingestion of enterotoxin rather than the growth of Staphylococcus bacteria in the body.

The source of an infectious agent is known as the reservoir. Humans are the principal reservoirs for microorganisms that cause human diseases. Individuals infected with a pathogen act as the source of infection for others. The pathogens that cause contagious diseases move from one infected individual to the next People who come in contact with someone suffering from a contagious disease are at risk of contracking that disease unless they are immune. If they are immune, their host defences protect them against that particular pathogen

In some cases, infected individuals do not develop disease symptoms Such individuals are called asymptomatic carriers or simply carriers. Although they do not become sick, carriers are important reservoirs of infectious agents. The classic case of disease transmission by such a carrier occurred in the earl 1900s when a cook, Mary Mallon, known as “Typhoid Mary,” spread typhoid fever from one community to another.

Some diseases can be transmitted to humans by direct contact with infected animals, by ingesting contaminated meat, or, more frequently, by vector. Vectors are organisms that carry the disease agent to the host. The vector need not develop disease It only transmits the disease agent from a reservoir to a susceptible individual. Arthropods, such as mosquitoes, are frequently the vectors of human disease

Pathogens also can be transmitted from infected mother to her fetus or infant. Syphilis and rubella can be transmitted across the placenta. Hepatitis, gonorrhoea, and chlamydial infection can be acquired as the newborn passes through the birth canal. Transmission between individuals can also be by sexual intercourse, touching, breathing aerosols (airborne, minute droplets of water that contain microorganisms), blood transfusions, or contaminated hypodermic needles.

The reservoirs of human pathogens can be nonliving sources such as soil and water For example,tetanus is generally acquired when spores of Clostridium tetani, which are widely distributed in soil, contaminate a wound. Often diseases acquired from such sources are noncommunicable. Such diseases are singular events and are not normally transmitted from one infected individual to the next

Thus, a reservoir is a source of an infectious agent, which may be air, water, soil, animals, or people.

Routes of diseases transmission. There are various modes which pathogens are transmitted from a source to a susceptible individual. 

Pathogenic microorganisms gain access to the body through a limited number of routes. These specific routes are known as portals of entry. The routes of entry are the respiratory tract, gastrotestinal tract, genitourinary tract, skin, and wounds. The invasive properties of specific pathogens permit them to penetrate the body’s defense mechanisms through a specific portal of entry. Most pathogenic microorganisms will cause disease only if they enter the body via this specific route. For example, depositing Clostridium tetani on the intact skin surface does not  result in disease, while deposition of C. tetani into deep wounds results in the deadly disease tetanus.

Portals of Entry for Some Specific Disease-causing Microorganisms

 

Portal of entry	Microorganism	Type of micro-organism	Disease
Skin	Staphylococcus aureus Papilloma virus Trichophyton and Epidermophyton species	Bacterium Virus Fungus	Impetigo Warts Athlete’s foot; tinea
Gastrointestinal tract	Salmonella typhi	Bacterium	Typhoid fever
	Poliovirus	Virus	Poliomyelitis
	Giardia lamblia	Protozoan	Giardiasis
Genitourinary tract	Treponema pallidum	Bacterium	Syphilis
	Herpes simplex virus	Virus	Genital herpes
Respiratory tract	Bordetella pertussis	Bacterium	Whooping cough
	Influenza virus	Virus	Influenza
	Histoplasma capsulatum	Fungus	Histoplasmosis
Wound	Clostrtdium perfringens	Bacterium	Gas gangrene
	Rabies virus	Virus	Rabies
	Sporotrix schenckii	Fungus	Rose’s disease

 

 

nosocomial (hospital-acquired) infections. Medical procedures are designed to cure disease Some procedures used in the treatment of diseases, however, can inadvertently introduce pathogenic microorganisms into the body and initiate an infectious process. Even a puncture wound with a sterile hypodermic syringe can pick up microorganisms from the vicinity of the puncture and carry them through the skin surface. The routine cleansing of wounds and use of topical antiseptics after minor skin punctures and abrasions are accepted prophylactic measures to prevent the establishment of infections

Patients in hospitals often are m a debilitated state of health. Their body defences are weak. They are therefore, susceptible to various infectious disease. The term nosocomial infections is used to describe hospital-acquired infection. Nosocomial infections include pneumonia acquired m hospital urinary tract infections that develop as a result of the insertion of a catheter, and infections of the genital tract that develop from gynaecological procedure. Nosocomial infections affect approximately 2 million  patients hospitalized annually m the United States alone. The numbers of nosocomial infections have been reduced in the United States during the past decade This has been accomplished by increasing epidemiological standards and procedures such as educational seminars, the infection control nurse,  and hospital committees designed to identify and break the routes of transmission of pathogens to patients.

Surgical procedures often expose deep body tissues to potentially pathogenic microorganisms. A surgical incision circumvents normal body defense mechanisms. Great care is taken in modern surgical practices, therefore, to minimize microbial contamination of exposed tissues. These practices include the use of clean operating rooms with minimal numbers of airborne microorganisms, sterile instruments, masks, and gowns. All of these prevent the spread of microorganisms from the surgical staff to the patient. The application of topical antiseptics before making incisions also prevents accidental contamination of the wound with the indigenous skin microbiota of the patient. After many surgical procedures, antibiotics are given for several days as a prophylactic measure.

Despite all of these precautions of maintaining aseptic practices, infections still sometimes occur after surgery. Infections after surgery can be serious because the patient is already in a debilitated state. The onset of such infections is generally marked by fever. A purulent lesion may develop around the wound. Serious complications may follow open heart surgery if the patient develops endocarditis, caused by Staphylococcus or Streptococcus species. In surgical procedures involving cutting the intestines, the normal gut microbiota may contaminate other body tissues unless great care is taken to minimize such contamination. Antibiotics are also used in such cases to prevent microbial growth. The specific microorganisms causing infections of surgical wounds and the specific tissues that may be involved depend on the nature of the surgery and the tissues that are exposed to potential contamination with pathogens.

Surgical practices use elaborate aseptic procedures to minimize potential infection, but nevertheless, infections sometimes occur after surgery, attesting to the vulnerability of the body to microbial infection when the skin barrier is disrupted and host defense mechanisms are impaired.

 

respiratory tract AND airborne transmission. We inhale 10,000 to 20,000 litres of air per day. This volume of air usually contains between 10,000 and 1,000,000 microorganisms. It should not be surprising, therefore, that the respiratory tract provides a portal of entry for many human pathogens. Potential pathogens freely enter the respiratory tract through the normal inhalation of air. Various viruses, bacteria, and fungi are able to multiply within the tissues of the respiratory tract. Sometimes they cause localized infections. At other times they enter the circulatory system through the numerous blood vessels associated with the respiratory tract and spread through the bloodstream to other sites in the body  (FIG 2). To establish an infection via the respiratory tract, a pathogen must overcome the natural defense mechanisms that are particularly extensive in the lower respiratory tract There are numerous phagocytic cells in this area. While the potential for respiratory infection is great, fortunately, the actual rate of disease is low.

Airborne transmission occurs when pathogenic microorganisms are transferred from an infected to a susceptible individual via the air Droplets regularly become airborne during normal breathing, but the coughing and sneezing associated with respiratory tract infections are primarily responsible for the spread of pathogens in aerosols and thus the airborne transmission of disease. Airborne pathogens often become suspended in aerosols. Aerosols are clouds of tiny water droplets suspended in air The incidence of these diseases can be reduced by covering one’s nose and mouth while coughing and sneezing and avoiding contact with contagious individuals. These are practices we are taught to follow at an early age.

 

Transmission through the air is undoubtedly the main route of transmission of pathogens that enter via the respiratory tract. In spite of conditions of dryness, extreme temperatures, and ultraviolet radiation that characterize the air and which prevent them from growing in that environment, microorganisms still reach new hosts through the air. Some bacteria, particularly Gram-positive bacteria, can survive for several months in dust particles. Bacterial and fungal spores and naked viruses can live even longer. The incidence of airborne infections has increased in recent years because so many new buildings are sealed and have self-contained recirculating air systems for temperature control.

 

gastrointestinal tract — water AND food borne transmission. Microorganisms routinely enter the gastrointestinal tract in association with ingested food and water Waterborne and foodborne pathogens can infect the digestive system and cause gastrointestinal symptoms The large resident microbiota that develops in the human intestinal tract after birth is important for the maintenance of good health This population is usually not involved m disease processes and is normally noninvasive Waterborne and foodborne transmission generally involves transmission of pathogens that enter via the mouth and exit via the anus Generally, the establishment of infection through the gastrointestinal tract requires a relatively large infectious dose This means that a relatively large number of pathogenic microorganisms are required to successfully overcome the inherent defense mechanisms of the gastrointestinal tract. High infectious doses often are encountered m waters contaminated by sewage or other sources of human fecal matter.

Genitourinary tract—sexual transmission. The genitourinary tract provides the portal of entry for pathogens that are directly transmitted during sexual intercourse. Such infections are known as venereal or sexually transmitted diseases. The physiological properties of the pathogens causing these diseases restrict their transmission, for the most part, to direct physical contact. They have very limited natural survival times outside infected tissues. The overall control of sexually transmitted diseases rests with breaking the network of transmission This necessitates public health practices that seek to identify and treat all sexual partners of any one diagnosed as having one of the sexually transmitted diseases.

superficial body tissues – direct contact transmission. In some cases the deposition of pathogenic microorganisms on the skin surface can lead to an infectious disease. Since they require direct contact between skin and microorganisms for transmission to occur, these diseases are called contact diseases. Some diseases transmitted in this manner are superficial skin infections. On others, the pathogens are able to enter the body and spread systemically. Relatively few microorganisms possess the enzymatic capability to establish infections through the skin surface. Some microorganisms, however, are able to enter the subcutaneous layers through the channels provided by hair follicles. Transmission of some contact diseases may follow minor abrasions that allow the pathogens to circumvent the normal skin barrier.

parenteral route.  Punctures, injections, bites, cuts, wounds, surgical incisions, and cracking skin due to swelling or drying establish portals of entry to a host for a potential pathogen. Such access is called the parenteral route (from Greek para [beside] and enterik [intestinal tract]. Microorganisms thus gain entry to the body by being deposited directly into the tissues beneath the skin or into the mucous membrane.

Animal Bites and Disease Transmission. Many animals are carriers of microorganisms and transmit pathogens to humans through bites. Animals that transmit pathogens are called vectors. In some cases the nonhuman animal also suffers disease, but in many cases such animals are only carriers. Animal bites simultaneously disrupt the skin barrier and inoculate the wound with microorganisms whose pathogens potentially may be life threatening. Arthropods, particularly insects, commonly act as vectors of some very dangerous human pathogens. Their bites can establish serious infections.

Many human infections that are transmitted via animal bites involve animals that act as reservoirs for the pathogens. The animal populations that maintain the pathogen and act as reservoirs may themselves suffer from a disease caused by that pathogen. These diseases, termed zoonoses, are defined as infectious diseases of nonhuman animals transmissible to humans.

 

 

 

 

Nonspecific host defenses. Immunity. Types and  forms of immunity.

 

The term immunity (L. immunis freed from) usually means resistance of the body to pathogenic microbes, their toxins or to other kinds of foreign substances.

Immunity is a complex of physiological defence reactions which determine the relative constancy of the internal medium of the macroorganism, hinder the development of the infectious process or intoxication, and are capable of restoring the impaired functions of the organism.

In the process of evolution, organisms have developed the property of distinguishing ‘self and ‘non-self very accurately, which is just what protects them from being penetrated by foreign proteins, including pathogenic micro-organisms and heterogenic transplants. The ‘non-self is detected by the lymphocyte receptors.

Insusceptibility to infectious diseases depends on many factors grouped under the names of resistance and immunity. Resistance is the insusceptibility of the body to the effect of pathogenic factors. Resistance embraces a wider group of phenomena of insusceptibility than immunity. Non-specific resistance is the insusceptibility of the body to injury by pathogenic factors: mechanical (traumas, rocking), physical (barometric pressure, cooling, overheating, radiation energy, ionising radiation), chemical (oxygen deficiency, excess of carbon dioxide, action of poisonous substances, drugs, poisons of a chemical and bacterial origin), and biological (pathogenic protozoa, fungi, bacteria, rickettsiae and viruses).

There may be resistance of the entire body and of its separate systems, although mutual dependence of both exists. Resistance is associated with the anatomical-physiological characteristics of the body, development of the central nervous system, and endocrine glands. It depends on the phylogenetic development of the animal, the individual and functional state of the body, and in man it depends also on social factors. Mental traumas predispose to somatic and infectious diseases; chronic hunger and vitamin deficiencies lead to a decline in resistance; intoxication by alcohol, opium, cocaine and other narcotics has a negative effect on human resistance.

In the traditions and life of ancient people considerable allowance has been made for preventive measures including vaccines against various diseases. Thus, for example, the inhabitants of East Africa from time immemorial have successfully used vaccinations against the bites of poisonous snakes. For vaccines they used snake venom, contained in a paste from plants. The paste, applied to cross-like scarifications on the skin of the person being vaccinated, caused a prolonged inflammation, and after being absorbed gradually helped in producing immunity to the lethal bites of poisonous snakes. Repeated vaccinations were made over a period of several years. Africans produced an artificial immunity to tick-borne relapsing fever by natural immunization. They carried on their body ticks which had contained a virus for a long time.

 

Types and Forms of Immunity

Modem classification subdivides immunity into two types according to origin: (1) species inherited and (2) acquired.

Species immunity is insusceptibility of certain species of animals to diseases which attack other species. It is transmitted by heredity from one generation to the next. An example of species immunity is insusceptibility of man to cattle plague, chicken cholera and infectious horse anaemia. On the other hand, animals are not infected by many human infections such as enteric fever, scarlet fever, syphilis, measles, etc.

Species immunity is the result of a long evolution of interrelations between the macro-organism and pathogenic micro-organism. It depends on those biological peculiarities of a given species of organism, which were formed during historical development in the course of natural selection, variation and adaptation to the environmental conditions.

The underlying factors of the mechanisms of species immunity (hereditary resistance) to infectious diseases are the absence in the organism’s cells of receptors and substrates necessary for the adsorption (attachment) and reproduction of the causative agent, the presence of substances which block the reproduction of pathogenic agents, and the ability of the macro-organism to synthesize various inhibitors in response to the penetration of the pathogenic microbes.

Acquired immunity is subdivided into natural and artificial. Natural immunity in turn is divided into (1) active, that is acquired following an obvious (postinfection) or latent disease or repeated infection without clinical manifestations, (2) passive immunity of the newborn (maternal, placental), i e. immunity iewly born children, acquired from the mother in the period of intrauterine development, through the placenta in the process of ontogenesis. The duration of immunity of the newborn is short. After about six months this immune state disappears and children become susceptible to many infections (measles, diphtheria, scarlet fever, etc.). Artificial immunity is reproduced by active or passive immunization.

 

 

Immunity is manifested on the cell, molecular, and organism levels. The organism’s immune  system is a sum total of lymphoid organs consisting of central (the thymus, bone marrow) and peripheral (lymph nodes, spleen, lymphocytes of peripheral blood) organs.

The thymus is the central organ of cell immunity in which the differentiation of stem cells into immunologically competent T-lymphocytes occurs. The function of these lymphocytes is discussed in the corresponding sections.

The systems of T-lymphocytes determined cell immunity in tuberculosis, leprosy, brucellosis, tularaemia and other diseases. The Fabricius’ pouch in birds (its analogue in mammals are thePeyer’s patches) is the central organ of humoral immunity.

The system of B-lymphocytes is responsible for humoral immunity against most bacterial infections and intoxications.

 

 

 

 

Non-specific Resistance

This form of defence which includes defensive properties is associated with phagocytosis, barrier function of the skin, mucous membranes, lymph nodes and other tissues and organs.

Phagocytosis. The most ancient form of immunity is phagocytosis, a defence adaptation which entails the seizure and digestion of foreign particles, including bacteria and remains of disintegrated cells, by phagocytes. The phenomenon of phagocytosis is of great importance in defence reactions of heritable and acquired immunity. I. Metchnikoff established that amoeboid cells of the mesoderm in transparent marine animals are capable of swallowing and digesting various foreign particles.

During early embryonic development amoeboid (mesenchymal) cells are produced between the epithelial cells, which do not take part in building up organs, from which all types of motile erythrocyte cells and various species of migrating leukocytes originate. They are contained in the thymus, bone marrow, spleen, lymph nodes, tonsils, appendix, and interstitial tissues of parenchymatous organs.

For more than a quarter of a century, I. Metchnikoff and his pupils accumulated facts confirming the defence role of phagocytosis during infection of vertebrate animals with pathogenic microbes. This provided for the possibility of establishing in the evolution and phylogenesis of cells the relation between digestion and phagocytosis. I. Metchnikoff subdivided those cells able to carry out phagocytosis into microphages and macrophages.

Microphages include granular leucocytes, neutrophils, eosinophils and basophils, of which only neutrophils have quite a marked ability for phagocytosis. Eosinophils and basophils are characterized by a weak phagocytic activity, although this problem has not yet been studied sufficiently.

Phagocytosis takes place with the help of macrophages which may be motile (monocytes of the blood, cells of the lymph nodes and spleen, polyblasts, histiocytes, etc.) or non-motile (reticular cells of the spleen, cells of the lymphatic tissue, endothelium of the blood vessels, etc.).

Much importance in the mechanism of the phagocytic reaction is attached to lysosomes (oval three-layer structures) which possess bactericidal properties in relation to different bacterial species and are capable of destroying foreign substances.

The complex of lysosome enzymes and the permeability of the lysosome membranes are determined genetically. Phagocytosis differs in the accomplishment of its defence function depending on the completeness of the set of the lysosome enzymes and the function of the lysosome membranes.

 

The process of phagocytosis consists of four phases. The first phase involves the approach of the phagocyte to the microbe by means of a positive chemotaxis. Under the influence of the productsof the life activities of microbes excitation of the phagocytes occurs, which leads to a change in the surface tension of the cytoplasm, and gives the phagocytes amoeboid motility.

In the second phase adsorption of the micro-organism on the surface of the phagocyte takes place. This process is completed under the influence of an electrolyte which alters the electrical potential of the phagocytized object (microbe).

The third phase is characterized by submergence of the microbe into the cytoplasm of the phagocyte, which seizes minute objects quite rapidly and large ones (some protozoa, actinomycetes, etc.) are engulfed in pieces.

The phagocytosed bacteria perish under the bactericidal effect of the heightened hydrogen ion concentration due to an increase of lactic acid in the cytoplasm of the phagocytes.

In the fourth phase intracellular digestion of the engulfed microbes by the phagocytes takes place.

 

In the process of phagocytosis various changes in the microbes can be observed, e. g. the production of granules in cholera vibrios, swelling of enteric fever bacteria, fragmentation of diphtheria bacilli, destruction of anthrax bacilli and swelling of cocci. Eventually, the phagocytized microbes become completely disintegrated.

Factors which speed up phagocytosis include calcium and magnesium salts, the presence of electrolytes and antibodies (opsonins and bacteriotropins), histamine, pyrogenic substances capable of raising the temperature of the tissues and the entire organism. Phagocytosis proceeds more vigorously in the immune than in the non-immune organism.

 

 

 

 

 

 

 

 

 

 

Toxins of bacteria, leucocidin, capsular material of bacteria, cholesterol, quinine, alkaloids and also a block of the reticuloendothelial system inhibit phagocytosis.

Besides complete phagocytosis incomplete phagocytosis is observed in certain diseases (gonorrhoea, leishmaniasis, tuberculosis, leprosy) in which micro-organisms are absorbed by phagocytes, but do not perish, are not digested, and in some cases reproduce.

Viruses are also digested in the macrophages of immune animals under the effect of the acid content of the vacuoles and the enzymes of the phagocytes though, unlike bacteria, viruses are intracellular parasites and are capable, to a great degree, of resisting phagocytosis.

 

 

 

 

 

 

 

The skin, mucous membranes and lymph nodes. The data published by I. Metchnikoff on phagocytosis created wide interest and called for the necessity of carrying out numerous experimental investigations, as a result of which the defence mechanism of the skin, mucous membranes, lymph nodes and cells of many tissues and organs was established.

In a normal, uninjured state, the skiot only is a true mechanical protective barrier, but a bactericidal factor. It has been established that the clean skin of a healthy person has a lethal action on a number of microbes (haemolytic streptococcus, salmonellae of enteric fever and paratyphoid fever, colibacillus, etc.). Investigations confirmed that washing the hands not only aids in mechanically removing microbes from the surface of the skin, but also in increasing its bactencidal properties.

The mucous membranes of the eyes, nose, mouth, stomach and other organs have defence adaptations. Like the skin barrier, the mucous membranes perform antimicrobial function as a result of their impermeability to different microbes and the bactericidal action of their secretions. In lacrimal fluid, sputum, saliva, blood, milk, tissues and organs lysozyme is found. It is found in some bacterial cells.

Due to the establishment of this defence mechanism the biological role of lacrimal fluid, saliva, nasal mucus and sputum becomes apparent. A lack of lysozyme in the tears affects the cornea. When animals lick their wounds they transfer lysozyme into them. Microbes which have penetrated into the mucous membranes are continuously destroyed by the action of lysozyme. Nasal mucus is bactericidal for many microbes and viruses of influenza, herpes, poliomyelitis, etc.

Bactericidal properties are not limited to the action of lysozyme. There are other antibiotics produced by the organs and tissues, which are capable of inhibiting microbes. A special substance inhibin has been found in the saliva, and the antibiotic erythrin — in the erythrocytes Both preparations have a bacteriostatic action on diphtheria bacilli Interferon is one of the powerful inhibitors of viruses.

Of a certain significance in physiological immunity is hyaluronic acid which inhibits the penetration of microbes into tissues and organs. Gastric juice has quite marked bactericidal properties in relation to many causative agents, especially those of the Salmonella group and organisms responsible for food poisonings.

Besides the defence adaptations of the skin and mucous membranes, a large role is played iatural immunity by the lymph nodes in which the pathogenic microbes penetrating through the injured skin and mucous membranes are localized and rendered harmless. Inflammation develops in the lymph nodes.

The inflammatory reaction is characterized by the liberation from the tissues of a number of substances (leucotoxin, leucopenic factor, histamine, serotonin, etc.) under the influence of which changes in the leucocytes occur. As a result they become sticky and adhere to the capillary wall, where they enter into the tissues. They enhance (induce) proliferation of adjacent cells. Leucocytes accumulated in the inflammatory zone produce a protective barrier which hinders the spreading of microbes into the tissues, blood and organs. Phagocytosis plays a great role in the blocking and destruction of micro-organisms in the inflammation focus.

In the inflammatory focus the temperature rises and acidosis and hypoxia develop, which cause a fatal effect on the bacteria and viruses. The reproduction of bacteria and viruses diminishes in the acid medium and virus adsorption by the susceptible cells reduces.

An increase in the body temperature of the macro-organism also suppresses the activity of bacteria and viruses. Fever is considered an important factor in the recovery from a virus infection.

 

The excretory functions of the kidneys and saliva, the secretions of the respiratory passages, intestine, and mammary and sweat glands are a powerful factor of non-specific immunity.

 

 

Insusceptibility associated with the bactericidal properties of the blood is a later form of defence inherent in vertebrates.

A substance which has bactericidal properties with regard to a number of micro-organisms (causative agents of anthrax, tetanus, botulism, gas gangrene, and diphtheria, and staphylococci, pneumococci, bovine brucellae, etc ) is beta-lysin which is a substance of a complex nature, a thermostable fraction of normal serum, decomposing at temperatures of 63-70°C or under the action of ultraviolet rays. From human serum a fraction was isolated which is characterized by a bactericidal action in relation to diphtheria bacilli, and is not identical to beta-lysin.

From the blood of people with an elevated temperature a component X-lysin was isolated which dissolved mainly Gram-negative micro-organisms (meningococci, paratyphoid bacteria) and to a lesser degree –  Gram-positive organisms X-lysin acts without the participation of complement, and is thermostable (withstands a temperature of 68-100 °C).

Leukines,  thermostable substances freed of leucocytes, pertain to bactericidal substances They disintegrate at a temperature of 75-80 °C. Leukines render harmless Gram-positive as well as Gram-negative bacteria.

C-reactive protein (the name is associated with C-polysaccharide of the type II St pneumoniae) having immunological properties was discovered m 1930 in the serum of patients with pneumococcal diseases. C-reactive protein is considered to be conjugated with reactive, defensive, non-specific natural processes It has also been found in the serum of patients with typhus fever, tuberculosis and other infections The component parts of urine, prostatic fluid, extracts from the liver, brain, spleen and other tissues and organs are characterized by bactericidal properties.

 Interferons are glycoproteins that block virus replication and exert many immunomodulating functions. Alpha interferon (from leukocytes) and beta interferon (from fibroblasts) are induced by viruses (or double-stranded RNA) and have antiviral activity. Gamma interferon is a lymphokine produced primarily by the Th-1 subset of helper T cells. It is one of the most potent activators of the phagocytic activity of macrophages, NK cells, and neutrophils, thereby enhancing their ability to kill microorganisms and tumor cells. For example, it greatly increases the killing of intracellular bacteria; such as M tuberculosis, by macrophages. It also increases the synthesis of class I and II MHC proteins in a variety of cell types. This enhances antigen presentation by these cells.    

Under the influence of the virus the cells of affected tissues excrete interferon which does not have a specific action, but renders viruses harmless. Interferon is present m small amounts iormal human serum.

 

 

 

 

A tuberculostatic factor has been discovered in human blood which is characterized by the ability to kill tubercle bacilli In 1954 L Pillemer established that after treating serum with zymozan (obtained from yeasts) it loses its bactericidal activity A precipitate is formed m the serum After treating the precipitate a substance was isolated, which, on addition to serum, restored the lost bactericidal activity This substance was named properdin (L. perdin destroy). Properdin is a serum protein, an euglobulm, which plays an important part in immunity The greatest amount is found in the blood of rats then in  a decreasing order in the blood of mice, cows, pigs, humans, rabbits,  sheep and guinea pigs. It is possible that properdin is composed of a group of antigens related to thermolabile class M immunoglobulins.

The synthesis of complement, properdin, lysozyme, interferon, and other natural inhibitors is determined genetically, inherited, and belongs to the factors of species immunity.

A great role in humoral activity is played by antibodies (immunoglobulins), the origin and accumulation of which occur under the influence of antigens.

Investigations have confirmed that microbes which have penetrated into the blood are rendered harmless by substances in the plasma. J. Fodor, G. Nuttall and others established the bactericidal action of  blood, exudates and other fluids of animals and humans. G Buchner showed that serum has a lethal effect on microbes, but on heating it defensive forces considerably weaken. The bactericidal matter of fresh normal serum at first was named alexin (Gk alexin to ward off) then complement (L. complementum anything which completes) Since the complement dissolves some species of bacteria and cells, it is sometimes called lysin (alpha-lysin).

It was observed too that antiserum could exert two entirely different effects on gram-negative bacteria or red blood cells (RBCs). In one case, when fresh antiserum was used, such cells were lysed. On the other hand, if the antiserum was heated to 56°C for 30 minutes or aged about 1 week, it could no longer cause lysis but instead would agglutinate the bacteria or RBCs. However, when fresh normal serum, such as guinea pig serum, was added to the heated or aged antiserum, the ability to cause cell lysis was regained. The lytic effect, therefore, requires two factors: (1) specific antibody, and (2) a labile component present iormal serum. This latter substance has been given the name complement. Subsequent research has revealed that complement is a multicomponent system composed of many different proteins.

The activation of the complement proteins proceeds by two triggered enzyme systems, wherein a series of inactive proteolytic enzymes (zymogens) are converted into biologically active proteases, each possessing an extremely fine specificity for its substrate. There are two pathways for the activation of the system, each initiated by a different sequence of events. The classic pathway of complement activation is set in motion by antigen-antibody complexes, whereas the alternate pathway, which is phylogenetically much older, is entirely independent of antigen-antibody reactions. Instead, certain components are activated by the presence of a series of foreign substances, not the least of which are infecting bacteria and viruses. The two pathways, however, have much in common, particularly the fact that their final membrane-attack components are identical.

Biologic Function of the Complement System. Before the details of the classic pathway of complement activation are outlined, the biologic function of this system should be considered. This function can best be appreciated by noticing that after antibody has reacted with its antigen, it can do little more. In other words, antibody might precipitate an antigen or, if the antigen is cellular, might cause agglutination but, with the exception of the neutralization of toxins or of virus infectivity, antibody alone is an ineffective means of protection against infection. Thus, for practical purposes, the major function of an antibody is to recognize a foreign antigen and bind to it. By doing so, it provides a site for phagocyte interaction and for the initiation of the reactions of the complement system. It is the activation of this system that (1) leads to the lysis of foreign cells, (2) further enhances phagocytosis of invading microorganisms, and (3) causes local inflammation, stimulating the chemotactic activity of the host’s leukocytes.

In addition, after activation of the system, interaction of one or more complement components with specific receptors on cell surfaces can result in (1) enhancement of antibody-dependent cellular cytotoxicity (ADCC); (2) increased oxidative metabolism; (3) secretion of vasoactive amines and leukotrienes;  (4) secretion of monokines; (5) stimulation of prostaglandin and thromboxane pathways; (6) modulation of lymphocyte activation and antibody responses; and (7) mobilization of leukocytes from the bone marrow. The following sections are concerned with a step-by-step dissection of the component parts and reactions of this system and the role that it plays in the destruction of foreign cells.

 

 

Classic Pathway of Complement Activation. The operation of the complement system consists of anumber of reactions, each of which activates the next reaction in the series. A primary event must occur, however, to initiate the reactions that eventually involve the many components of the complement system.

In the case of the classic pathway, the initiating event occurs when the first component of complement reacts with antigen-antibody complexes in which the antibody is either IgM or IgG. IgA, IgD, and IgE are not effective in activating complement.

Once initiated, the activation of the complement system may have various effects, depending on the type of foreign cell involved in the antigen-antibody reaction. In the case of a gram-negative bacterium, the integrity of the cell membrane is destroyed, permitting the lysozyme-mediated lysis and death of the cell. Gram-positive organisms are not lysed, but the activation of complement by a gram-positive cell and antibody results in the release of fragments of complement components that aid in phagocytosis by binding to the antigen, providing a receptor for the host leukocyte. In addition, many eukaryotic cells, such as RBCs or virus-infected cells, are lysed by complement. The complex reactions that produce all these effects can be divided into three series of reactions involving the complement system: (1) the activation of the recognition unit, (2) the assembly of the activation unit, and (3) the assembly of the attack unit.

 

THE RECOGNITION UNIT. Of the nine known components of the classic pathway, only the first   component, C1, is involved in the recognition unit. This componentis composed of three different proteins—C1q, C1r, and C1s—which interact with each other on an antigen-antibody complex. It appears that antigen-antibody reactions bring numerous antibody molecules into aggregates that can be cross-linked by C1q. This clustering of antibody molecules can be mimicked by mild heating or chemically cross-linking antibodies to form an aggregate in the absence of antigen. Such artificial aggregates easily react with C1q, leading to complement activation.

The reaction begins when C1q, the recognition subunit of C1, binds to the constant region of the antibody complexes. The binding site for C1q is located in the fourth domain (C_H4) of IgM and in the second domain (C_H2) of IgG. Some variability exists in the capacity of the IgG subclasses to activate complement. The reason for this is unknown.

C1r and C1s are both present in the C1 complex as inactive proteases. Limited amino acid sequence studies have shown considerable homology between these two components, as well as some homology with other serine proteases. In the presence of Ca²⁺ they are bound to the collagen portion of C1q as a tetrameric complex consisting of two molecules each of C1r and C1s [(C1r₂C1s₂C1q]. In solution, C1q, C1r, and C1s exist in an easily dissociable complex, but after C1q cross-links antibody, this association becomes much more stable, presumably because of a conformational change in C1q. The actual binding to C1q appears to occur through C1r, because isolated C1r will bind to antibody-C1q, whereas C1s will not.

After binding to antibody-C1q, a conformational change occurs in C1r, which exposes an enzymatic site that catalyses its own hydrolysis, converting it into an active serine protease (C1r) whose only known substrate is C1s. (Complement components that have been modified to become enzymatically active are written with a bar over the complement designation, as in C1r). Once activated, C1r splits off a peptide from C1s, converting C1s also into a serine protease, C1s, which initiates the assembly of the activation unit.

ASSEMBLY OF THE ACTIVATION UNIT. The first step in the assembly of the activation unit occurs when C1s splits off a 77-amino acid vasoactive polypeptide (C4a) from the N-terminal chain of C4 to generate C4b. This exposes an intrachain thioester bond within C4b, which is the reactive site responsible for attachment to the cell membrane or to the cross-linked antibody molecules bound to C1q or to those immediately adjacent. Unbound molecules of C4b are rapidly inactivated. C1s also cleaves C2 into two components: C2a (a 70,000-dalton fragment) and C2b (a 30,000-dalton polypeptide).

At this point, C2a remains bound to the antibody-bound or membrane – bound C4b to form a new  active protease, C4b2a, which is called C3 convertase. The catalytic site of C3 convertase exists in the C2a fragment, and the role of C4b appears to be one of binding C2a, thus stabilizing the C3 convertase. The newly formed C3 convertase, C4b2a, cleaves C3 into two fragments, C3a and C3b, again exposing a highly reactive thioester bond in C3b. The C3b molecule reacts with residues on the cell surface, on the antigen-antibody complex, or on the C3 convertase itself, forming a new enzymatically active complex, C4b2a3b, called C5 convertase. Those C3b molecules that do not immediately react with sites nearby are inactivated by hydrolysis of this thioester bond. This C4b2a3b is the activation unit of the classic pathway, and its function is to split C5 into C5a and C5b, which initiates the formation of the membrane-attack complex. Interestingly, both C3 convertase and C5 convertase use the same catalytic site in the bound C2a. In the C5 convertase molecule, C3b acts as a binding site for the C5 substrate. It seems probable, however, that the C5 convertase continues to split C3 into C3a and C3b.

ASSEMBLY OF THE MEMBRANE-ATTACK COMPLEX. The splitting of C5 by C5 convertase is the last enzymatic reaction involved in the classic pathway of complement activation; the subsequent assembly of the remaining components of the attack complex are nonenzymatic, occurring spontaneously as follows:

1. C5b reacts with one molecule of C6, forming a relatively stable complex of C5b6.C5b alone is very unstable in serum, with a half-life measured in milliseconds; however, C5b is stabilized by reacting with C6 while still bound to the C3b of the C5 convertase.

2. In the fluid phase, C7 is added to C5b6 to create C5b67, the first component of the attack complex that has membrane-binding properties. C5b67 has a half-life of about 100 milliseconds in the fluid phase, but it is stabilized by binding directly to the membrane, independent of C3b.

3. C5b67 now expresses a C8-binding site, and when C8 binds to the complex, it is inserted into the lipid bilayer. At this point, the complex is capable of slow lysis of susceptible cells, which becomes more rapid with the addition of C9 in the next step. However, animals deficient in the C9 component are able to deal with most infections normally.

4. Aggregation of C5b-8 within the membrane then occurs, resulting in formation of a C9-binding site. Interaction of C9 with this site causes a conformational change in C9, exposing hydrophobic regions and facilitating its insertion into the membrane and its polymerization into a poly-C9 complex.

 

 

 

Structure of the C1 Molecule. The C1 protein is composed of three proteins: C1q, which binds to the Fc portion of the Ab molecule; C1s, which can enzymatically cleave the next complement component, C4; and C1r, which acts as a bridge connecting C1q to C1s.

 

 

 

 

 

OTHER MECHANISMS OF ACTIVATING THE CLASSIC PATHWAY. The classic pathway of complement activation is normally thought of as one initiated by antigen-antibody complexes but, actually, it involves C1, C2, and C4 to form the C3 convertase, because the alternate pathway of activation uses a different C3 convertase. With this definition, several other elements can be added that activate the classic pathway, namely, viral membranes, the lipid A portion of endotoxin, mitochondrial membranes, and miscellaneous polycations and polyanions such as heparin, protamine, and nucleic acids. The mechanism whereby these compounds initiate C1 activation is unknown, but it can be assumed that any substance possessing binding sites for C1q might initiate this pathway.

Interestingly, nonprimate retroviruses activate primate C1 directly. This activation is dependent on the C1s portion of C1. This may be the primary mode of defense against these viruses, because it is known that primates do not produce antibody to these nonprimate retroviruses. It may be that the viruses are cleared from the body by activating the classic pathway before antibody synthesis is induced.

 

Alternate Pathway of Complement Activation. The alternate pathway of complement activation (the properdin pathway) does not require the presence of antibodies for initiation and, as a result, provides a mechanism of nonspecific resistance to infection. Moreover, this pathway does not use C1, C4, or C2, which are the early reactants in the classic pathway of complement activation. Remember, however, that the overall result of this pathway is the same as that of the classic pathway: C3 is split into C3a and C3b, and C5 is cleaved to form C5a and C5b, thus permitting the spontaneous formation of the C5b-9 membrane-attack complex. The enzymes catalyzing these conversions are different from the C3 and C5 convertases described for the classic pathway of complement activation.

RECOGNITION AND ASSEMBLY. This pathway bypasses both the recognition unit and the  assembly of the activation unit as described for the classic pathway. Instead, there arc at least three  normal serum proteins that, when activated together with C3, form a functional C3 convertase and a C5 convertase. These are factor B, factor D, and properdin (P). To fully comprehend this pathway, the reader should keep the following facts in mind:

1. These are normal serum proteins, and the alternate pathway routinely undergoes activation in the absence of any stimulus.

2. In the absence of initiators (which is discussed later), the initial complexes of the alternate pathway are rapidly destroyed.

3. In the presence of these initiators, such complexes are stabilized, and complement is activated to form the identical membrane-attack complexes described for the classic pathway.

 

 

 

Reaction Sequence. The pathway appears to be initiated in the following steps:

1. The C3 molecule has an internal intrachain thioester bond formed by the reaction of the side chains of two amino acids: cysteine and glutamic acid. In the blood, this thioester bond reacts with a molecule of H₂O to form the unstable C3-H₂O complex. This unstable, yet active, complex then binds to serum factor B to form another unstable complex, C3B. The factor B portion of the C3B complex is split by factor D (a normal serine protease) into two fragments, Ba and Bb. Ba is a 33,000-dalton peptide that is released during the reaction, and Bb is a 60,000-dalton peptide that remains bound to C3 to form a C3Bb complex. The C3Bb acts, probably in the fluid phase, as an initial C3 convertase to split C3 into C3a and C3b.

2. C3b binds factor B, forming the transient intermediate C3bB, which is then subject to cleavage by factor D, to form C3bBb. This is the active C3 convertase of the alternate pathway.

Notice that there have been two C3 convertascs formed to this point (ie, C3Bb and C3bBb) that differ in the form of the C3 portion but that have the same enzymatic specificity. The C3bBb enzyme, however, is capable of reacting with surfaces (eg, the cell membrane) and thus can be stabilized.

3. As more C3b is generated by C3bBb, it continues to attach to the membrane. When an additional molecule of C3b becomes bound to the C3bBb, the specificity of the convertase is shifted to a C5 convertase (designated C3bBb3b).

4. Properdin enters the reaction sequence and binds to both the C3 and C5 convertase to protect the complex from the action of factor I (a normal component of serum that is capable of inactivating C3b). Properdin interaction, therefore, is the terminal event in the assembly of the activation unit for this pathway.

5. Once the C5 convertase is formed, C5 is cleaved to form C5a and C5b, and the spontaneous formation of the attack complex (C5b-9) quickly follows. The formation of this attack complex proceeds in the same manner as it does in the classic pathway discussed above.

 

 

 

The activation of either the classical complement pathway or the alternative pathway leads to the formation of C3 convertase (C4b2b or C3bBb). Cleavage of C3 and binding of C3b to C3 convertases results in the formation of C5-converting enzymes (C4b2bC3b or C3bBb3b). At the end, both pathways form MAC, which mediates lysis of target cells.

The Lytic Event. It is now well established that membrane-bound C5b-8 can cause slow lysis of a susceptible cell and that the addition of C9 greatly accelerates the cell lysis. However, no real consensus exists concerning exactly how lysis occurs. Aggregation of the C5b-8 complex in the membrane clearly forms a binding site for C9. Interaction of C9 with this site induces large conformational changes in C9, exposing hydrophobic regions responsible for insertion of C9 in the membrane and the formation ofpoly-C9. Poly-C9 is composed of 12 to 18 molecules ofC9 and is the ring-like structure seen in electron micrographs of cells treated with antibody and complement. The poly-C9 structure forms at high concentrations and is not required for efficient lysis. Lysis of the cell to which the membrane-attack complex is bound occurs primarily by extensive disruption of the lipid bilayer, an increase in membrane permeability, and marked changes in membrane potential, pH, and cytosoliccation concentrations, leading to complete loss of electrochemical gradients and rupture of the plasmamembrane.

Biological Consequences of Complement Activation

Complement activatioot only causes cell lysis, but other effects as well. Some of these include: contraction of smooth muscle, release of histamine from mast cells and platelets, enhanced phagocytosis, chemotaxis of phagocytes, and activation of lymphocytes and macrophages.

The main activity of C3a and C5a is anaphylaxis. These cause histamine release from mast cells and basophils, which can affect the activity of smooth muscle. Spasmogenicity, accounts for the ability of these molecules to induce an anaphylactic response in animals. In addition to spasmogenicity, histamine release induced by the anaphylatoxins as effects on inflammation. The cellular responses of neutrophils and monocytes to C5a include (1) degranulation and lysosomal enzyme release, (2) cell adherence, and (3) chemotactic migration.

CHEMOTAXIS. Any substance that attracts leukocytes to an area of inflammation is a chemotactic agent. Factors Ba (the split product from the alternate pathway) and C5a are both chemotactic for PMNs and macrophages, thus contributing to local inflammation. C5b67, the partially formed attack complex, also has been implicated as a chemotactic agent. Interestingly, although the removal of the terminal arginine from C5a by carboxypeptidase B completely eliminates all anaphylatoxic activity, its removal does not affect the chemotactic activity of C5a.

IMMUNE ADHERENCE. C3b is an effective opsonin, stimulating the phagocytosis of antigen-antibody aggregates, cells, and viruses. Its opsonic effectiveness stems from the presence of specific C3b receptors on PMNs, monocytes, macrophages, and mast cells. C3b also binds to antigen-antibody aggregates and to antibody- sensitized cells and viruses. Thus, C3b can act as a bridge to bring antibody-coated material into intimate contact with phagocytic cells, inducing their phagocytosis and destruction.

 

The acute inflammatory response is characterized by symptoms of redness, pain, swelling, and heat due to the action of C4a, C3a, C5a, and histamine. Inflammation’s primary goal is to set into motion a series of events that result in the elimination of foreign and damaged cells. This response can be mediated by the anaphylatoxins and their byproducts.

 

 

Interactions Between Complement and Other Protein Systems. As might be expected, a system as seemingly complex as the complement system does not operate in a void. Proteins of the complement system do interact with proteins of other complex systems, that is, the coagulation, fibrinolytic, and kinin systems. Examples of the interaction between components of the various systems are many. Plasmin has been shown to cleave C3 to give C3b- and C3a-like fragments, which possess chemotactic and anaphylatoxic activities. Protein S of the coagulation system circulates as a complex with C4-binding protein, although the biologic significance of this interaction is unknown. Kallikrein has been shown to cleave C5 with the release of a fragment that is chemotactic for PMNs.

Human platelets react directly with antigen-antibody complexes through cell-surface fg receptors. Experimental results suggest that activation of platelets to release vasoactive, granule-associated amines occurs by mechanisms requiring complement. This may occur because of platelet lysis via the classic pathway of complement activation or by nonlytic mechanisms that also involve complement.

 

 

NATURAL KILLER CELLS

Natural killer (NK) cells play an important role in the innate host defenses.

 

Important features of natural killer (NK) cells

 

They spe­cialise in killing virus-infected cells and tumor cells by secreting cytotoxins (performs and granzymes) similar to those of cytotoxic T lymphocytes and by participating in Fas-Fas ligand-mediated apoptosis. They are called “natural” killer cells because they are active without prior exposure to the virus, are not enhanced by exposure, and are not specific for any virus. They can kill without antibody, but antibody enhances their effectiveness, a process called antibody-dependent cellular cytotoxicity (ADCC). IL-12 and gamma interferon are potent activators of NK cells. From 5 to 10% of peripheral lymphocytes are NK cells.

 

 

 

 

 

NK cells are lymphocytes with some T cell markers, but they do not have to pass through the thymus in order to mature. They have no immunologic memory and, unlike cytotoxic T cells, have no T cell receptor; also, killing does not require-recognition of MHC proteins. In fact, NK cells have re­ceptors that detect the presence of class I MHC proteins on the cell surface. If a cell displays sufficient class I MHC proteins, that cell is not killed by the NK cell.

Many virus-infected cells and tu­mor cells display a significantly reduced amount of class I MHC proteins, and it is those cells that are recognized and killed by the NK cells.   

Summary of the Non-Specific Immune Response:

  By “chronic disease” Hahnemann did not mean exactly the same thing as is now generally understood by the phrase – a disease that lasts a long time and is incurable. To make his meaning clear, I caot do better than quote Hahnemann’s own definition of acute and chronic diseases, from paragraph 72 of his Organon :-

     “The diseases to which is liable are either rapid morbid processes of the abnormally deranged vital force, which have a tendency to finish their course more or less quickly, but always in a moderate time – these are termed acute diseases ; or they are diseases of such a character that, with small, often imperceptible beginnings, dynamically derange the living organism, each in its own peculiar manner, and cause it to deviate from the healthy condition in such a way that the automatic life energy, called vital force, whose office it is preserve the health, only opposes to them at the commencement and during their progress, imperfect, unsuitable, useless resistance, but must helplessly suffer (them to spread and ) itself to be more and more abnormally deranged, until at length the organism is destroyed ; these are termed chronic diseases. They are caused by infection from a chronic miasm.”

     By “miasm” Hahnemann means an infectious principle, which, when taken into the organism, may set up a specific disease. According to Hahnemann, there were not only miasms of acute disease, like the infectious principle of scarlatina, for example, but also of chronic diseases. Among the latter he recognised three-syphilis, sycosis and psora. The first is the lues venerea, which is recognised by all schools alike. The second is allied to this, but is distinguished by the production of characteristic warty growths. The third is a discovery of Hahnemann’s, about which there has been the greatest misconception.

     Before giving an account of what Hahnemann meant by “psora,” I will give a familiar instance of a chronic miasm – the disease set up by vaccination. Vaccinia or “Cow-pox,” as the late Dr. Matthews Duncan pointed out, is extremely analogous to syphilis in many of its characters, and not the least in the appearance of secondary disorders after the primary illness is over. The course of the disease is well known. The virus having been introduced through an abrasion of the skin, in about a week inflammation occurs at the spot. Then there appears first a vesicle, then a pustule, then a scab, and finally a scar when the scab drops off. During the time that this series of events is occurring, constitutional symptoms manifest themselves, chiefly in the form of fever and undefined malaise. When the healing has taken place, three may be nothing more occur. The organism may have reacted perfectly and discharged the miasm. But this is not often the case. The diminished susceptibility to small-pox infection shows a change of a deep constitutional character. This constitutional change has beeamed “vaccinosis” by Burnett, and, as I can attest, is the parent of much chronic illness. Often skin eruptions occur, lasting for years, or various other kinds of ill-health, lasting, it may be, as long as life lasts, and not seldom shortening life. When such a series of disorders occurs, it is not (according to Hahnemann’s doctrine, though he did not use this illustration) a succession of new diseases, but different evolutions of one and the same disease, the “miasm” of Vaccinia producing the chronic malady, vaccinosis.

     In the early years of his homњopathic practice Hahnemanoticed that in certain cases the remedies he gave only produced temporary benefit. In these cases he found that the homњopathically of the remedies given was not complete. There was some factor in the case which had not been matched. It became apparent to him, then, that he had not only to take account of the malady from also of previous and apparently different maladies. And he found the remedies which corresponded, in their action, to the whole course of the pathological life of a patient were needed for a cure ; and through his provings he discovered what these deeply acting remedies were.

     Many cases he met with in practice in which the ill-health dated from the suppression of a skin disease, probably years before. That skin disease, said Hahnemann, is really a part of the present disorder. To take a common example, asthma is often found to appear after the “cure” by external means of a skin disorder. The patient is not suffering from two diseases : there is, according to Hahnemann’s pathology, one chronic miasm at work producing the two effects.

     The large majority of chronic diseases Hahnemann traced to the chronic miasm he termed “psora,” and he maintained on the skin of the miasm was an eruption of itching vesicles, of which the itch vesicle was a type. It has of which the itch vesicle was a type. It has been started that Hahnemann ascribed to the itch the production of nine-tenths of chronic diseases, and he has been accused of ignorance iot knowing that itch was caused by an insect. But Hahnemanot only knew of the itch-insect, he actually figured it in one of his works. But he maintained that, in spite of the presence of the insect, this was not the whole of the disease – just as the tubercle bacillus is not the whole of pulmonary consumption. If it were, no doctors would escape consumption, since they inhale the bacillus constantly from their patients. “The itch,” Hahnemann maintained, “is chiefly an internal disease’. ‘Psora is an internal disease – a sort of internal itch – an may exist with or without an eruption upon the skin.’ ‘Psora forms the basis of the itch.’ To the reckless suppression of the chief external symptoms of psora Hahnemann ascribed the prevalence of chronic disorders.

     To put it in other words, the psora doctrine of Hahnemann is practically the same as the doctrine of certain French authorities who ascribe a great variety of chronic diseases to what they call a ‘herpetic diathesis’, that is to say, a morbid state of the organism liable to manifest itself on the skin by an itching vesicular eruption.

     The essential truth of Hahnemann’s doctrine may be seen by taking a glance at the history of individuals and families. The skin eruptions of childhood, the late development of bones and teeth, the anaemia of puberty, and the consumption which finally carries off the patient, are not so many different diseases, but different manifestations of one and the same disease, whether we call it ‘psora’ with Hahnemann, or ‘herpetic diathesis’ with the French. Then, again, take a family : one member has enlarged and inflamed glands, one ulceration of the eyes, one a chronic cough, one hysteria, one eczema. They are all children of the same parents, with the same elements of heredity, and their diseases are essentially one and the same, only manifesting itself differently in different individuals. This disease Hahnemann called a ‘chronic miasm’. The seat of its operations is the vital force, which can only be freed from it by dynamically acting homњopathic remedies.

 

 

     REFERENCES:

1.     K. Pyatkin,  Yu. Krivoshein. Microbiology, 1987, P. 175-210, 264-268.

2.     Hadbook on Microbiology. Laboratory diagnosis of Infectious Disease/ Ed by Yu.S. Krivoshein, 1989, P. 36-37 .

3.     Medical Microbiology and Immunology: Examination and Board Rewiew /W. Levinson, E. Jawetz.– 2003.– P.59-80, 353-362

4.     Review of Medical Microbiology /E. Jawetz, J. Melnick, E. A. Adelberg/ Lange Medical Publication, Los Altos, California, 2002. – P. 109-114, 144-175.

5.     Wesley A.Volk et al. Essentials of Medical Microbiology. Lippincott – Raven Publishers, Inc., Philadelphia–New York.–1995.–725 p.

 

Internet adresses:

http://www.online-medical-dictionary.org/Microbial+Antagonism.asp?q=Microbial+Antagonism

http://www.mansfield.ohio-state.edu/~sabedon/biol2035.htm

http://en.wikipedia.org/wiki/Antibiotic

http://www.nlm.nih.gov/medlineplus/antibiotics.html 

http://www.intmed.mcw.edu/AntibioticGuide.html

http://whyfiles.org/038badbugs/

http://www.niaid.nih.gov/factsheets/antimicro.htm

http://textbookofbacteriology.net/BSRP.html

http://www.bacteriamuseum.org/niches/pbacteria/pathogenicity.shtml

http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap20.html#Symbioses

http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap12.html#NSD_mechanisms

http://www2.hawaii.edu/~johnb/micro/m130/m130lect12.html

http://www.merck.com/mmpe/sec14/ch167/ch167b.html

http://en.wikipedia.org/wiki/Phagocytosis

http://en.wikipedia.org/wiki/Complementary

 http://www-micro.msb.le.ac.uk/MBChB/Merralls/Merralls.html               

http://en.wikipedia.org/wiki/Immune_system

http://www-micro.msb.le.ac.uk/MBChB/1b.html

http://www.answers.com/topic/lipopolysaccharide-1

http://www.answers.com/topic/flagellar-antigen

http://www.google.com/search?hl=en&q=K+antigen&btnG=Search

http://www.callutheran.edu/Academic_Programs/Departments/BioDev/omm/viral_antigens/molmast.htm

http://en.wikipedia.org/wiki/Major_histocompatibility_complex

http://www.cryst.bbk.ac.uk/pps97/assignments/projects/coadwell/MHCSTFU1.HTM

http://en.wikipedia.org/wiki/Antibody

http://www.fleshandbones.com/readingroom/pdf/291.pdf