Page 273 - Veterinary Immunology, 10th Edition

endogenous antigen presentation
VetBooks.ir • Describe the MHC class II pathway of exogenous antigen presentation.

The innate immune defenses have evolved to destroy microbes as
soon as they enter the body. Most invaders, especially if they are of

low virulence, are rapidly eliminated. However, in addition to
being uncomfortable and damaging, inflammation is not a
foolproof process. If the body is to be defended effectively, an
animal must have defenses that detect and eliminate all microbial

invaders without the damage and discomfort associated with
inflammation. This is the task of the adaptive immune system.
In order to trigger this adaptive immunity, a sample of foreign
material must first be captured, processed, and presented in the

correct fashion to cells that can recognize it. This is the
responsibility of antigen-processing cells.
Antigen-processing cells are attracted by microbial products and
tissue damage and are activated by the same stimuli that trigger

inflammation. Indeed, dendritic cells and macrophages serve both
as sentinel cells and antigen-processing cells. As a result, antigen
processing can be initiated at the same time as the invader is being
eliminated by the innate defenses. Once an invader has been

eliminated, an animal can develop adaptive immunity against a
second attack by the same organism.
Antigen processing involves breaking large protein molecules
into small peptides within a cell. These peptides are then attached

to specialized antigen-presenting receptors called major
histocompatibility complex (MHC) molecules. Once bound to MHC
molecules, the peptides are carried to the cell surface. Adaptive
immunity is triggered when these MHC-bound peptides are

recognized by lymphocytes bearing specific receptors for the
antigen. These lymphocytes (called T cells) bind and respond only
to foreign peptides that have been correctly processed and
presented. This ensures that adaptive immune responses do not

proceed indiscriminately.
The organisms that trigger adaptive immune responses are of
two general types. One type is typified by the bacteria that invade

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