Page 315 - Veterinary Immunology, 10th Edition

VetBooks.ir MHC of Domestic Animals

Every mammalian MHC contains class I, class II, and class III
regions. When the MHCs of different mammals are compared,

some regions such as class III are conserved, whereas others are
highly diverse. Likewise, the precise arrangement and number of
loci varies among species (Fig. 11.8). In general, genes within the
class II and class III regions possess obvious orthologs in all species.
That is, they are clearly derived from a single ancestor and have not

usually been subjected to major rearrangements during evolution
(ruminant class II genes are an exception). Class I genes, in contrast,
have been reorganized so many different times by deletion and

duplication that their amino acid sequences differ widely, and it is
very difficult to compare class I genes in different species. They are
said to be paralogous.

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