Page 32 - Veterinary Immunology, 10th Edition

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FIG. 1.2 The three major barriers that protect an animal's body
against microbial invasion. Each barrier provides a more effective
defense than the previous one. The adaptive immune system also
improves with experience so that it presents a progressively greater
barrier to invasion.

Innate Immunity

Physical barriers, though essential in excluding invaders, cannot be

totally effective by themselves. Given time and persistence an
invading microorganism will eventually overcome mere physical
obstacles. Nevertheless, most microbial attempts at invasion are
rapidly blocked before they can result in disease. All animals and

plants, even the least evolved, need to detect and eliminate
microbial invaders as fast and as effectively as possible. This
immediate response is the task of the innate immune system. Many
different innate defense mechanisms have evolved over time and

the mammalian innate immune system is therefore a diverse
collection of subsystems that work through many different
mechanisms. Collectively, they all respond rapidly to block
microbial invasion and minimize tissue damage (Fig. 1.3). Innate

immune responses are activated immediately when a pathogen
penetrates the epithelial barriers. These responses are generic; that
is, they detect microbes such as bacteria and viruses because they
differ structurally and chemically from normal animal tissues. Once

the invaders are recognized, multiple innate responses are available
to destroy them. For example, animals make many different
antimicrobial proteins that either kill invaders directly or promote

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