Page 36 - Veterinary Immunology, 10th Edition

infected cells. They are described in Chapter 19.
VetBooks.ir microbes, but it also destroys them and retains the memory of the
The adaptive immune system not only recognizes invading

encounter. If the animal encounters the same organism a second

time, the adaptive immune system responds more rapidly and
much more effectively. Such a sophisticated system must, out of
necessity, be complex.

Another reason for this complexity is the great diversity of
potential invaders, including: bacteria, viruses, fungi, protozoa, and
helminths (worms). These invaders may be classified into two
broad categories. One category consists of the organisms that
normally reside outside cells—extracellular invaders. This includes

most bacteria and fungi, as well as many protozoa and invading
helminths. The second category consists of organisms that originate
or live within the body's own cells—the intracellular invaders.

These include viruses and intracellular bacteria or protozoa. Each
category requires a different defensive strategy.
The adaptive immune system thus consists of two major branches
(Fig. 1.5). One branch is directed against the extracellular invaders.
The other is directed against intracellular invaders. Both branches

depend upon the use of specialized white blood cells called
lymphocytes. There are two major lymphocyte populations, B cells
and T cells. Immunity to extracellular invaders is mainly the

function of B cells. They produce proteins called antibodies that
promote the microbial destruction. This B-cell-mediated immune
response is sometimes called the “humoral immune response” since
antibodies are found in body fluids (or “humors”).

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