Page 35 - Veterinary Immunology, 10th Edition

and destroy specific invaders, and then learn from the process so
VetBooks.ir that if they invade a second time, they will be destroyed even more

effectively. In this system, the more often an individual encounters
an invading bacterium or virus, the more effective will be its

defenses against that organism. This type of “smart” response is the
function of the adaptive immune system, so called since it adapts
itself to ongoing threats to the animal. Although it develops slowly,
when an animal eventually develops adaptive immunity, the

chances of successful invasion by that organism decline
precipitously, and the animal is said to be immune. The adaptive
immune system provides the ultimate defense of the body. Its
essential nature is readily seen when its loss leads inevitably to

uncontrolled infections and death.
A key difference between the innate and adaptive immune
systems lies in their use of cell surface receptors to recognize
foreign invaders (Table 1.1). The cells of the innate system use a

limited number of preformed receptors that bind to molecules
expressed by many different microbes, and the response is
therefore generic in nature. In contrast, the cells of the adaptive
immune system generate enormous numbers of new, structurally

unique receptors that bind specifically to those foreign molecules
that induce them. Because the binding repertoire of these receptors
is generated randomly, they are assured of recognizing at least
some of the molecules found on almost any invading

microorganism.

TABLE 1.1

A Comparison of Innate and Adaptive Immunity

Innate Immunity Adaptive Immunity
Always “on” Turned on by Antigens
Cells engaged Macrophages, dendritic cells T and B cells
Neutrophils, NK cells*
Cell receptors Preformed and directed against common microbial Generated in response to foreign
molecules molecules
Evolutionary Ancient Recent
history
Onset Rapid (min-hr) Slow (days-weeks)
Specificity Common microbial structures Unique antigens
Potency May be overwhelmed Rarely overwhelmed
Memory None Significant memory
Effectiveness Does not improve Improves with exposure
* Note: Natural killer (NK) cells are a family of innate cells that can kill tumors and virus-

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