Page 31 - Veterinary Immunology, 10th Edition

FIG. 1.1 The basic arrangement of the innate and adaptive
VetBooks.ir adaptive immunity backs it up.
immune systems. Innate immunity is the first line of defense;

Physical Barriers

Because the successful exclusion of microbial invaders is essential
for survival, it is not surprising that the animal body employs
multiple, overlapping layers of defense (Fig. 1.2). For example, a

microbe that has succeeded in breaking through the first layer of
defenses is then confronted with the need to overcome a second,
higher barrier, and so forth. The first and most obvious of these
defenses are the physical barriers to invasion. Intact skin provides

an effective barrier to microbial invasion. If skin is damaged,
microbes may invade; but wound healing ensures that this is
repaired promptly. On other body surfaces, such as in the
respiratory and gastrointestinal tracts, simple physical defenses

include the “self-cleaning” processes: coughing, sneezing, and
mucus flow in the respiratory tract; vomiting and diarrhea in the
gastrointestinal tract; and urine flow in the urinary system. The
presence of huge populations of commensal bacteria on the skin,

respiratory tract, and in the intestine also excludes many potential
invaders. Well-adapted commensal organisms adapted to living on
body surfaces can easily outcompete poorly adapted pathogenic
organisms. The microbiota thus plays an essential role in resistance

to invasion.

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