a. Class I Antigens. Class I molecules are coded for by the HLA-A, B, and C
genes. Each gene is highly pleomorphic, that is, occurring 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. The Class I antigen consists of two
polypeptides: one MHC-encoded peptide and the other non-MHC encoded peptide,
beta microglobulin, which is a small globular peptide. HLA-A, B, and C antigens are
found on virtually every human cell.
b. Class II Antigens. They 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. DP and DQ replace the earlier designation SB
and DC, respectively. The number of alleles for each gene is not known. At least six
different specificities for each of the DP and DQ genes are known. Each Class II
protein consists of two peptides, an alpha chain and a beta chain that is slightly smaller.
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.
c. Class III Antigens. Four gene loci, located between the HLA-B and the
HLA-D loci on chromosome 6, are associated with the complement system. These
Class III genes are the structural genes for proteins C2, C4A, C4B, and factor B of the