Page 27 - Veterinary Immunology, 10th Edition

VetBooks.ir The Microbial World

Historically, our concerns regarding infectious diseases have
caused us to regard all microbes as potential enemies. Dangerous

microbial invaders include not just bacteria and viruses, but also
fungi, protozoa, arthropods, and helminths (worms). Nevertheless,
the real situation is much more complex. Bacteria find animal hosts
to be a rich source of nutrients and a great place to shelter. As a
result, enormous numbers colonize our body surfaces, especially

within the intestine, in the airways, and on our skin. Most of these
bacteria—our normal microbiota—do not even try to invade the
body and do not normally cause damage. They share resources

with us and so are regarded as commensal organisms.
The presence of this microbiota and the diversity of molecules it
generates must either be tolerated or ignored if an animal is to
remain healthy. An animal cannot afford to act aggressively toward
its own microbiota. Any response must be carefully regulated and

must not happen unless necessary for the defense of the body. The
immune system is aware of the intestinal microbiota. Numerous
bacterial molecules cross the intestinal epithelium and influence the

immune responses. They do not, however, automatically trigger
strong defensive responses unless tissue damage occurs. The
response is measured, proportional, and carefully controlled. The
immune system has to watch them warily, but they rarely cause
trouble. In fact, they are needed for the proper digestion of food as

well as a stimulus that keeps our defenses in peak operating
condition.
A small number of other, more aggressive bacteria, try to invade

animal tissues and do cause damage. This is normally prevented, or
at least controlled, by our immune defenses. If these organisms
succeed in invading the body and overcoming the immune
defenses, they may cause sufficient damage that results in disease
or death. On the other hand, organisms such as the viruses are

intracellular parasites that can survive for only a limited time
outside the animal body. These invaders will only survive if they
can avoid the host's defenses for sufficient time to replicate and

transmit their progeny to a new animal host. While it is essential for

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