MHC Genes
MHC gene families are found in all vertebrates, though they vary widely, chickens having among the smallest known MHC regions (19 genes). In humans the MHC region occurs on chromosome 6, between the flanking genetic markers MOG and COL11A2 (from 6p22.1 to 6p21.3 ~29Mb to 33Mb on the hg19 assembly), and contains 140 genes spanning 3.6 mega base pairs (3.6 Mb or 3 600 000 bases). About half have known immune functions.
The same markers in the gray short-tailed opossum (Monodelphis domestica), a marsupial, span 3.95 Mb yielding 114 genes, 87 shared with humans. Marsupial MHC genotypic variation lies between eutherian mammals and birds—taken as minimal MHC encoding—but is closer in organization to that of nonmammals, and MHC class I genes of marsupials have amplified within the class II region, yielding a unique class I/II region.
Class III function very differently from class I and class II, but its locus occurs between the other two classes—on chromosome 6 in humans—and are frequently discussed together.
| Class | Encoding | Expression |
| I | (1) peptide-binding proteins, which select short sequences of amino acids for antigen presentation, as well as (2) molecules aiding antigen-processing (such as TAP and Tapasin). | One chain, called α, whose ligands are the CD8 receptor—borne notably by cytotoxic T cells—and inhibitory receptors borne by NK cells. |
| II | (1) peptide-binding proteins and (2) proteins assisting antigen loading onto MHC class II's peptide-binding proteins (such as MHC II DM, MHC II DQ, MHC II DR, and MHC II DP). | Two chains, called α & β, whose ligands are the CD4 receptors borne by helper T cells. |
| III | Other immune proteins, outside antigen processing and presentation, such as components of the complement cascade (e.g., C2, C4, factor B), the cytokines of immune signaling (e.g., TNF-α), and heat shock proteins buffering cells from stresses. | Various. |
Read more about this topic: Major Histocompatibility Complex
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