Page 304 - Veterinary Immunology, 10th Edition

VetBooks.ir Major Histocompatibility Complex

All vertebrates possess an MHC. Each mammalian MHC contains
about 200 expressed genes divided into three regions (I, II and III)

(Fig. 11.2). The class I region contain genes coding for MHC
molecules expressed on most nucleated cells. Class I genes can be
subdivided into those that are highly polymorphic (class Ia genes)
and those that show very little polymorphism (class Ib, Ic, or Id
genes). (Polymorphism refers to structural variations between

proteins.) Class Id genes are located outside the MHC on a different
chromosome. Genes in class II regions encode polymorphic MHC
molecules usually restricted to professional antigen-presenting cells

(dendritic cells, macrophages, and B cells) (Table 11.1). Genes
within the MHC class III region code for a mixture of proteins,
many of which are important in innate immunity such as
complement. Although each MHC contains all three gene regions,
their gene content and arrangement vary among species.

FIG. 11.2 The three major classes of genes located within the
major histocompatibility complex are grouped together in regions.
The products of each class have a unique distribution and function.

TABLE 11.1

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