Page 274 - Veterinary Immunology, 10th Edition

the body from outside and then grow in the tissues and
VetBooks.ir extracellular fluid. These are called exogenous antigens, and they

are processed by specialized antigen-processing cells. A second
type of invading organism is typified by viruses that invade a cell

and force it to make viral proteins. These new proteins are called
endogenous antigens. Endogenous antigens are processed by the
cells in which they are produced. There are therefore two classes of
MHC molecules, class I and class II. MHC class I molecules are

made by all nucleated cells and present endogenous antigens. MHC
class II molecules, in contrast, are restricted to specialized antigen-
processing cells, and they present exogenous antigens. The body
mainly employs three specialized cell families, dendritic cells,

macrophages, and B cells, to process exogenous antigens. The most
important of these are dendritic cells (DCs) (Fig. 10.1).

FIG. 10.1 The three major populations of antigen-presenting cells:
B cells, DCs, and macrophages. Of these, only DCs can activate
naïve T cells and trigger a primary immune response.

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