Class I antigens are coded for by the HLA-A, B, and C genes. Each gene is highly
pleomorphic, which means it occurs in various distinct forms. The A gene is
currently recognized to have at least 23 specificities, the B gene 50 specificities,
and the C gene 8 specificities. HLA-A, B, and C antigens are found on virtually
every human cell.
Class II antigens are different structurally and functionally from Class I. The exact
number of genes in the D region is uncertain but three are widely accepted. These
are DR, DP, and DQ. HLA-DR, DP, and DQ are found chiefly on the surface of
immunocompetent cells, including macrophage/monocytes, resting T lymphocytes,
activated T lymphocytes, and particularly B lymphocytes.
Class III genes are the structural genes for proteins C2, C4A, C4B, and factor B of
the complement system.
End of Lesson 4