Basic structure of the MHC gene complex in humans and in mice

The gene loci of the MHC region are grouped
into three classes (I–III). Class I in humans includes
HLA-A, HLA-B, and HLA-C; in mice, D, L,
and K of the H2 system. Many other loci also
belong to this class, such as HLA-E to -J. Class II
includes HLA-DP, -DQ, and -DR in humans and
I-A, and I-E (the letter I, not the Roman
numeral) in mice. Alleles are designated according
to a numerical system, e.g., HLA-A2,
-B5, -DR4, etc. The gene products of the alleles
of the HLA system (human leukocyte antigens;
also said by some to refer to Los Angeles, where
some of the first basic discoveries were made)
can be demonstrated by the toxicity of serum of
defined specificities to other leukocytes (serological
cytotoxicity). Cytolysis occurs unless the
specificities of the serum and of the cells being
tested are identical. The gene products of the alleles
of the HLA-D system are distinguished by
the mixed lymphocyte test, based on lymphocyte
proliferation as a reaction to foreign T cells.
Strictly speaking, the class III genes do not
belong to the MHC loci. They contain genes for
different complement proteins and a few other
genes.