by as. O.V. Pokryshko

Medical biology, microbiology,

virology, immunology department

1. Immunity and its classification

2. Factors of nonspecific (innate) defenses of organism: skin and mucouse membranes

humoral factors of defenses

complement system

phagocytosis

natural killer cells

Plan of lecture

Host defenses are composed of two complementary, frequently interacting systems:

(1) innate (nonspecific) defenses, which protect against microorganisms in general,

(2) acquired (specific) immunity, which protects against a particular microorganism.

Innate defenses is due to genetic and constitutional make-up of an individual.

It can be classified into three major categories:

(1) mechanical barriers, such as intact skin and mucous membranes;

(2) proteins, such as complement, lysozyme, and interferon;

(3) phagocytic cells, such as neutrophils, macrophages, and natural killer cells.

Intact skin is the first line of defense against many organisms. In addition to the physical barrier presented by skin,

Mechanical barriers and surface secretion

the fatty acids secreted by sebaceous glands in the skin have antibacterial and antifungal activity.

(mucociliary escalator) enabling it to be swallowed or coughed out.Nasal mucus is bactericidal for many microbes and viruses of influenza, herpes, poliomyelitis, etc.

A second important defense is the mucous membrane of the respiratory tract, which is lined with cilia and covered with mucus. Mucus traps dust. Ciliated cells moveCilia in the trachea

rhythmically beating.

the dust-laden mucus up and out of respiratory tract

The mouth is constantly bathed in saliva.

The intestinal mucosa is covered by lace-like network of mucus. Particles get enmeshed in the mucus and form small masses which are propelled by peristalsis.

Swallowed microorganisms are destroyed in the stomach’s highly acidic environment and digestive juiced.

Whenever the dust hits the conjunctiva we blink and tears are produced. The conjunctiva is freed of foreign particles by the flushing action of lachrymal secretions. The eyes become susceptible to infection when lachrymal secretions are absent.

Humoral defence mechanisms

Many microbicidal substances are present in the tissues and body fluids. These are non-specific. There is no specific recognition of the microorganism and the response is not enhanced by re-exposure to the same antigen.

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. Microbes which have penetrated into the mucous membranes are continuously destroyed by the action of lysozyme.

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.

Of a certain significance in physiological immunity is hyaluronic acid which inhibits the penetration of microbes into tissues and organs.

A substance which has bactericidal properties with regard to a number of microorganisms (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.

Leukines, thermostable substances freed of leucocytes render harmless Gram-positive as well as Gram-negative bacteria.

Iron-binding proteins. Some chemicals within the body bind iron (lactoferrin and transferrin), thereby withholding this essential growth element from pathogenic microorganisms. Transferrin transports iron from the small intestine, where the iron is absorbed, to the tissues, where the iron is used. Transferrin and lactoferrin bind iron, limiting the growth of pathogens in the blood.Lactoferrin is present in tears, semen, breast milk, bile, and nasopharyngeal, bronchial, cervical, and intestinal mucosal secretions. Transferrin is present in serum and the intercellular spaces of many tissues and organs. The concentration of free iron in the blood and other tissues is normally over a billion times lower than iron concentration required for growth by most microorganisms. Conversely, when the iron supply is more abundant, infection is more likely

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 in small amounts in normal human serum.

Interferons are a heterogeneous group of glycoproteins produced by human and other animal cells after viral infection (or after exposure to other inducers). Interferons are divided into three groups based on the cell of origin, namely, leukocyte, fibroblast, and lymphocyte. They are also known as alpha, beta, and gamma interferons, respectively.

Alpha and beta interferons are induced by viruses, whereas gamma (T cell, immune) interferon is induced by antigens and is one of the effectors of cell mediated immunity. Interferons are cytokines that inhibit the growth of certain cancer cells, bacteria, and protozoa, but the focus here will be on their inhibitory effect on viral growth.

Interferons have no direct effect on extracellular virus particles.

Interferons act by inducing the synthesis of three cell-encoded proteins that inhibit the translation of viral mRNA without affecting the translation of cellular mRNA.

Interferons are produced within a few hours of the initiation of viral replication, they may act in the early phase of viral diseases to limit the spread of virus. In contrast, antibody begins to appear in the blood several days after infection.

Complement, or the complement system, refers to a set of more than 20 large regulatory proteins produced by the liver that circulate in plasma in an inactive form. They account for about 10 per cent (by weight) of all plasma proteins.

The general functions of the complement system are

to enhance phagocytosis,produce inflammation, directly lyse microorganisms.

These functions are nonspecific: When the complement system is activated, complement proteins participate in a cascade of reactions that trigger an inflammatory response.

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, is entirely independent of antigen-antibody reactions.

Classic Pathway of Complement Activation. 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.

In the classic pathway, antigen-antibody complexes activate C1 to form a protease, which cleaves C2 and C4 to form a C4b2a complex (C3 convertase). It cleaves C3 molecules into two fragments, C3a and C3b. C3b forms a complex with C4b2a, producing a new enzyme, C5 convertase (C4b2a3b), which cleaves C5 to form C5a and C5b. C5b binds to C6 and C7 to form a complex that interacts with C8 and C9 to produce the membrane attack complex (C3b6789), which cause cytolysis.

In the alternative pathway, many unrelated substances, eg, bacterial lipopolysaccharides (endotoxin), fungal cells, and viral envelopes, can initiate the process by binding C3 and factor B. This complex is cleaved by a protease, factor D, to produce C3bBb. This acts as a C3 convertase to generate more C3b.

Activation of the complement

system

ACTIVATION OF COMPLEMENT

The general functionsThe general functions of the complement systemof the complement system

The bacteria of the normal flora of die skin, nasopharynx, colon, and vagina occupy these ecologic niches, preventing pathogens from multiplying in these sites. The importance of the normal, flora is appreciated in the occasional case when antimicrobial therapy suppresses these beneficial organisms, thereby allowing organisms such as Clostridium difficile and Candida albicans to cause diseases such as pseudomembranous colitis and vaginitis, respectively.

is provided by phagocytes and subpopulation of lymphocytes known as natural killer (NK) cells.As part of the inflammatory response, bacteria are engulfed (phagocytized) by polymorphonuclear neutrophils (PMNs) and macrophages. PMNs make up approximately 60% of the leukocytes in the blood, and their numbers increase significantly during infection (leukocytosis).

Cellular defence mechanism

Phagocytes are classified into microphages and macrophages. Microphages are polymorphonuclear leucocytes and macrophages consist of histiocytes which are the wandering amoeboid cell seen in tissues, fixed reticuloendothelial cells and monocytes of blood. In connective tissue they are known as histiocytes, in kidney as mesangial cells, in bones as osteoclast, in brain as microglial cells, in lungs as alveolar macrophages, in liver as Kupffer cells, and in the spleen, limph nodes and thymus as sinus lining macrophages.

Migration of PMNs to the site of the organisms is due to chemokines, such as interleukin-8, complement component C5a, and kallikrein, which – in addition to being chemotactic is the enzyme that catalyzes the formation of bradykinin.

The process of phagocytosis can be divided into three steps

migration, ingestion, killing.

Migration of polymorphonuclear neutrophils is called diapedesis and takes several minutes to form a vacuole (phagosome). This engulfment is enhanced by the binding of IgG antibodies (opsonins) to the surface of the bacteria, a process called opsonization. The C3b component of complement enhances opsonization.

(The outer cell membranes of both PMNs and macrophages have receptors both for the Fc portion of IgG and for C3b.)

Alveolar (Lung) Macrophage Attacking E. coli (SEM x10,000) 

Even in the absence of antibody, the C3b component of complement, which can be generated by the "alternative" pathway, can opsonize. This is particularly important for bacterial and fungal organisms whose polysaccharides activate the alternative pathway.

At the time of engulfment, a new metabolic pathway, known as the respiratory burst, is triggered; this results in the production of two microbicidal agents, the superoxide radical and hydrogen peroxide.

The killing of the organism within the phagosome is a two-step process that consists of degranulation followed by production of hypochlorite ions, which are probably the most important microbicidal agents.

The actual killing of the microorganisms

occurs by a variety of mechanisms, which fall into two categories:

oxygen-dependent oxygen-independent.

The most important oxygen-dependent mechanism is the production of the highly reactive hypochlorite ion by myeloperoxidase. Hypochlorite by itself damages cell walls but can also react with H2O2 to produce singlet oxygen,

which damages cells by reacting with double bonds in the fatty acids of membrane lipids.

The oxygen-independent mechanisms are important under anaerobic conditions. These mechanisms involve lactoferrin, which chelates iron from the bacteria; lysozyme, which degrades peptidoglycan in the bacterial cell wall; cationic proteins, which damage bacterial membranes and low pH.

Killing mechanisms of phagocytes

Natural killer (NK) cellsNatural killer (NK) cells are large granular lymphocytes that do not pass through the thymus, do not have an antigen receptor, and do not bear CD4 or CD8 proteins. Natural killer (NK) cells play an important role in the innate host defenses. They specialise in killing virus-infected cells and tumor cells by secreting cytotoxins (perforins 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 (ACC).

I. Nature of NK Cells Large granular lymphocytes

Lack T cell receptor, CD3 proteins, and surface IgM and IgD

Develop normally in absence of thymus

Normal numbers in SCID patients

Activity not enhanced by prior exposure

II. Function of NK Cells

Kill virus-infected cells and cancer cells

Killing is nonspecific, ie, not specific for viral or cancer antigens

Killing is not dependent on foreign antigen presentation by class I or II MHC proteins

Killing is activated by the failure of a cell to present self antigen in association with class I MHC proteins or by a reduction in the number of class I MHC proteins on the cell surface

Kill by producing performs and granzymes, which cause apoptosis of target cell

NK cells and their activation

Killing of opsonised target by by K cell

TWO TYPES OF IMMUNITY

Nonspecific (innate)•Physical and chemical agents•Lysozyme•Acute phase proteins•Complement system•Cytokines (chemokines)•Phagocytes (granulocytes, macrophages)•Natural killer (NK) cells•Dendritic cells•Toll-like receptors

Specific (adaptive)•Antibodies (B lymphocytes)•T lymphocytes