Figure 81

The major histocompatibility complex (MHC) is a group of closely linked genes that was first appreciated because it was found to contain the structural genes for transplantation antigens. A, The MHC, located on the short arm of chromosome 6, is now recognized to include many other genes important in the regulation of immune responses. B, Regions of the MHC classes I, II, and III. The MHC can be divided into three regions, of which the class I and II regions contain the loci for the human histocompatibility antigen or human leukocyte antigen (HLA). Genes in the class I

region encode the a or heavy chain of the class I antigens, HLA-A, B, and C. The class I region is composed of other genes, most of which are pseudogenes and are not expressed. The MHC class II region is more complex, with structural genes for both the a and P chains of the class II molecules. The class II region includes four DP genes, one DN gene, one DO gene, five DQ genes, and a varying number of DR genes (two to 10), depending on the halotype. Many other immune response genes are coded within the class III region. TNF—tumor necrosis factor.

Specific locus

The major histocompatibility complex in humans

Provisional specificity

Specific antigen

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