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Tag words: innate immunity, natural immunity, antimicrobial defense, individual resistance, cellular defense, lysozyme, complement, normal flora, inflammation, inflammatory exudate, phagocytosis, opsonization, neutrophils, macrophages, oxidative burst, mast cells.

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Immune Defense against Bacterial Pathogens: Innate Immunity (page 2)

(This chapter has 6 pages)

© Kenneth Todar, PhD

Innate Immunity

Innate Immunity is a form of non specific host defense against invading bacteria. It is natural or "innate" to the host, depending, in part, on genetics.  Innate defense mechanisms are contitutive to the host, meaning they are continually ready to respond to invasion and do not require a period of time for induction. The most important components of innate immunity are anatomical barriers, intact normal flora, tissue bactericides including complement, and ability to undergo inflammatory and phagocytic responses.

Innate immunity provides the first line of defense against invading bacteria. The skin and mucous membranes provide  physical and chemical barriers to infection. The normal bacterial flora antagonize colonization of body surfaces by nonindigenous bacteria. The internal tissues invariably contain bactericidal substances. The most noteworthy antibacterial substance is the enzyme lysozyme, which is present in mucus and all bodily tissues and secretions. If these barriers are penetrated, the body contains cells that respond rapidly to the presence of the invader. These cells include macrophages and neutrophils that engulf foreign organisms and kill them. Bacterial invasion is also challenged by the activation of complement in blood and tissues and the incitement of an inflammatory process which has the tendency to focus both the innate and adaptive immune defenses on the site of invasion.

Categories of Innate or Nonspecific Immunity

The first four categories are generally considered non cellular defenses. Inflammation and Phagocytosis are forms of cellular defense.

1. Differences in susceptibility to certain pathogens

2. Anatomical defense

3. Tissue bactericides, including complement

4. Microbial antagonism

5. Inflammation (ability to undergo an inflammatory response)

6. Phagocytosis

Differences in Susceptibility of Animal Hosts to Microbial Pathogens (Natural Immunity)

Natural immunity or resistance is based on the genetics of the host. There are two aspects: (1) resistance among all members of a species, called species resistance and (2) resistance within members of the same animal species, called individual resistance.

Species resistance

Certain animals are naturally resistant or non susceptible to certain pathogens. Certain pathogens infect only humans, not lower animals, e.g. syphilis, gonorrhea, measles, poliomyelitis. On the other hand, certain pathogens (e.g. canine distemper virus) do not infect humans. Shigella infects humans and baboons but not chimpanzees. Little information is available to explain these absolute differences in susceptibility to a pathogen but it could be due to:

Absence of specific tissue or cellular receptors for attachment (colonization) by the pathogen. For example, different strains of enterotoxigenic E. coli, defined by different fimbrial antigens, colonize human infants, calves and piglets by recognizing species-specific carbohydrate receptors on enterocytes in the gastrointestinal tract.

Temperature of the host and ability of pathogen to grow. For example, birds do not normally become infected with mammalian strains of Mycobacterium tuberculosis because these strains cannot grow at the high body temperature of birds. The anthrax bacillus (Bacillus anthracis) will not grow in the cold-blooded frog (unless the frog is maintained at 37o).

Lack of the exact nutritional requirements to support the growth of the pathogen. Naturally-requiring purine-dependent strains of Salmonella typhi grow only in hosts supplying purines. Mice and rats lack this growth factor in blood and pur- strains are avirulent. By injecting purines into these animals, such that the growth factor requirement for the bacterium is satisfied, the organisms prove virulent.

Lack of a target site for a microbial toxin. Most toxins produced by bacterial cells exert their toxic activity only after binding to susceptible cells or tissues in an animal. Certain animals may lack an appropriate target cell or specific type of cell receptor for the toxin to bind to and may therefore be nonsusceptible to the activity of the toxin. For example, injection of diphtheria toxin fails to kill the rat. The unchanged toxin is excreted in the urine. If a sample of the rat urine (or pure diphtheria toxin) is injected into the guinea pig, it dies of typical lesions caused by diphtheria toxin.

Individual resistance

There are many reasons why individuals of the same animal species may exhibit greater or lesser susceptibility to the same ineffective agent.

Age. Usually this relates to the development and status of the immunological system which varies with age. It may also be associated with changes in normal flora coincidental to developmental changes in the animal.

Sex. Usually this is linked to the presence and/or development of the sex organs. For example, mastitis and infectious diseases leading to abortion will obviously occur only in the female; orchitis would occur only in males. It could also be due to anatomical structure related to sex (bladder infections are 14-times more common in females than males), and possibly the effects of sex hormones on infections.

Stress. Stress is a complex of different factors that apparently has a real influence on health. Undue exertion, shock, change in environment, climatic change, nervous or muscular fatigue, etc. are factors known to contribute to increases in susceptibility to infection. The best explanation is that in time of stress the output of cortisone from the adrenal cortex is increased. This suppresses the inflammatory processes of the host and the overall effect may be harmful. There are also a number of relationships between stress-related hormones and the functioning of the immune defenses.

Diet, malnutrition. Infections may be linked with vitamin and protein deficiencies, and this might explain partly why many infectious diseases are more prevalent and infant mortality rates are highest in parts of the world where malnourishment is a problem. Also, overfed and obese animals are more susceptible to infection. Diets high in sucrose predispose individuals to dental caries.

Intercurrent disease or trauma. The normal defenses of an animal are impaired by organic diseases such as leukemia, Hodgkin's disease, diabetes, AIDS, etc. Frequently, inflammatory or immune responses are delayed or suppressed. Colds or influenza may predispose an individual to pneumonia. Smoking tobacco predisposes to infections of the respiratory tract. Burned tissue is readily infected by Pseudomonas aeruginosa.

Therapy against other diseases. Modern therapeutic procedures used in some diseases can render an individual more susceptible to infection. Under these conditions not only pathogens, but organisms of the normal flora and nonpathogens in the host's environment, may be able to initiate infection. Examples of therapeutic procedures that reduce the efficiency of the host's defenses are treatment with corticosteroids, cytotoxic drugs, antibiotics, or irradiation.

Anatomical Defenses

The structural integrity of the body surfaces, i.e., the skin and mucous membranes, forms an effective barrier to initial lodgment or penetration by microorganisms. The skin is a very effective barrier to bacterium, so that no bacterium by itself is known to be able to penetrate unbroken skin. Of course, a puncture, cut or scrape in the skin could introduce infectious bacteria. The mucous membranes are more vulnerable to penetration by infectious bacteria but still pose a formidable barrier of mucus and antimicrobial substances.

The anatomical defenses are associated with all other aspects of noncellular immunity, including individual resistance, mechanical resistance, chemical resistance and resistance established by the normal flora (Figure 3)

Figure 3. Anatomical defenses associated with tissue surfaces

chapter continued

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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