Defects in Immune Defenses

Hosts with defects in their specific and/or nonspecific immune defenses are prone to infection.

■ Primary defects. Congenital defects in the complement-dependent phagocytosis system are rare, as are B and T lymphocyte defects.

■ Secondary defects. Such effects are acquired, and they are much more frequent. Examples include malnutrition, very old and very young hosts, metabolic disturbances (diabetes, alcoholism), autoimmune diseases, malignancies (above all lymphomas and leukemias), immune system infections (HIV), severe primary diseases of parenchymatous organs, injury of skin or mucosa, immunosuppressive therapy with corticosteroids, cytostatics and immuno-suppressants, and radiotherapy.

One result of progress in modern medicine is that increasing numbers of patients with secondary immune defects are now receiving hospital treatment. Such "problem patients" are frequently infected by opportunistic bacteria that would not present a serious threat to normal immune defenses. Often, the pathogens involved ("problem bacteria") have developed a resistance to numerous antibiotics, resulting in difficult courses of antibiotic treatment in this patient category.

Normal Flora

Commensals (see Table 1.3, p. 9) are regularly found in certain human micro-biotopes. The normal human microflora is thus the totality of these commensals. Table 1.7 lists the most important microorganisms of the normal flora with their localizations.

Bacteria are the predominant component of the normal flora. They proliferate in varied profusion on the mucosa and most particularly in the gastrointestinal tract, where over 400 different species have been counted to date. The count of bacteria per gram of intestinal content is 101-105 in the duodenum, 103-107 in the small intestine, and 1010-1012 in the colon. Over 99% of the normal mucosal flora are obligate anaerobes, dominated by the Gram-neg. anaerobes. Although life is possible without normal flora (e.g., pathogen-free experimental animals), commensals certainly benefit their hosts. One way they do so is when organisms of the normal flora manage to penetrate into the host through microtraumas, resulting in a continuous stimulation of the immune system. Commensals also compete for living space with overtly pathogenic species, a function known as colo-

Table 1.7 Normal Microbial Flora in Humans

Microorganisms Microbiotopes

Microorganisms Microbiotopes

Table 1.7 Normal Microbial Flora in Humans

Skin

Oral

Intes-

Upper re-

Genital

cavity

tine

spiratory tract

tract

Staphylococci

+++

+

+

++

++

Enterococci

++

+

a-hemolytic streptococci

+

+++

+

+

+

Anaerobic cocci

+

+

+

Pneumococci

+

+

Apathogenic neisseriae

+

+

+

Apathogenic corynebacteria

++

+

+

+

+

Aerobic spore-forming bacteria

(+)

Clostridia

+++

(+)

Actinomycetes

+++

+

Enterobacteriaceae

(+)

(+)

+++

(+)

+

Pseudomonas

+

Haemophilus

+

++

(+)

Gram-neg. anaerobes

+++

+++

+++

+++

Spirochetes

++

+

(+)

Mycoplasmas

++

+

+

++

Fungi (yeast)

++

+

+

+

+

Entamoeba, Giardia, Trichomonas

+

+

+++ = numerous, ++ = frequent, + = moderately frequent, (+) = occasional occurrence

+++ = numerous, ++ = frequent, + = moderately frequent, (+) = occasional occurrence

nization resistance. On the other hand, a potentially harmful effect of the normal flora is that they can also cause infections in immunocompromised individuals.

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