Page 50 - Veterinary Immunology, 10th Edition

VetBooks.ir How Invaders Are Recognized

The innate immune system is activated when the body senses that it
is under attack. It uses alarm signals generated either by the

presence of invading microorganisms or by dead, damaged, and
dying cells. Microbial invaders express a diverse mixture of
molecules that can be recognized by the body as foreign.
Collectively, these are called pathogen-associated molecular
patterns (PAMPs). Likewise, molecules released from damaged

cells, collectively called damage-associated molecular patterns
(DAMPs) also generate alarm signals. Together, the PAMPs and
DAMPs bind to pattern-recognition receptors (PRRs) found on

sentinel cells located throughout the body. Triggering of
inflammation requires the binding of either PAMPs or DAMPs to
PRRs.

Pattern-Recognition Receptors

Microbes not only grow very fast but also are highly diverse and
can alter their surface molecules very rapidly. For this reason, the
PRRs of the innate immune system cannot recognize all possible
microbial molecules. Rather, the PRRs recognize abundant,

essential molecules. Because they are essential, these molecules tend
be structurally conserved and may be shared by entire classes of
pathogens. They form, in effect, conserved molecular patterns. For
example, the walls of Gram-positive bacteria are largely composed

of peptidoglycans (chains of alternating N-acetylglucosamine and
N-acetylmuramic acid cross-linked by short peptide side chains)
and lipoteichoic acids (Fig. 2.2). Likewise, the walls of Gram-
negative bacteria consist of peptidoglycans covered by a layer of

lipopolysaccharide (LPS). Acid-fast bacteria are covered in
glycolipids. Yeasts have a mannan- or β-glucan-rich cell wall.
Viruses have unique nucleic acids. PRRs can recognize all of these
molecules.

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