Page 312 - Veterinary Immunology, 10th Edition

VetBooks.ir

FIG. 11.6 Diagram showing the structure of a MHC class II antigen
located on a cell surface. Note that the antigen-binding site is
formed by the variable domains from both peptide chains.

Gene Arrangement

A “complete” MHC class II region contains three paired loci. In

primates, these are DPA and DPB, DQA and DQB, and DRA and
DRB. (The genes for the α chains are designated A, and the genes
for the β chains are called B.) Some of these genes are polymorphic.

There may also be additional non-polymorphic loci present such as
DM and DO in humans. The DM and DO gene products regulate
the loading of antigen fragments into the MHC groove. Not all
mammals possess a complete set of class II genes since nonprimates
lack DPA and DPB. Not all loci contain genes for both chains, and

some contain many pseudogenes. These pseudogenes serve as
DNA donors that can be used to generate class II polymorphism by
gene conversion.

Polymorphism

MHC class II proteins have an antigen-binding groove formed by

312