Page 308 - Veterinary Immunology, 10th Edition

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FIG. 11.4 Arrangement of the major loci within the MHC of the
horse—a typical mammalian MHC.

Polymorphism

In mice and humans, the class Ia genes show extreme
polymorphism. This is a result of variations in the amino acid
sequences in their α and α domains. The most extreme
1 2
polymorphism is restricted to three to four small regions within the
α and α domains. In these variable regions, two or three
1 2
alternative amino acids can occur at each position. The other
domains of MHC class Ia molecules show little variation.
The α and α domains of MHC class I molecules fold together to
1 2
form an open-ended groove. A flat β sheet forms the floor of this
groove, and its walls are formed by two α helices (Fig. 11.5). This
groove can bind antigenic peptides that are 8 to 10 amino acids

long. The variable regions located along the walls of this groove
determine its shape. The shape of the groove in turn determines
which peptides can be bound and thus trigger immune responses.

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