A cluster of genes on chromosome 6 in humans, encoding cell surface molecules that are polymorphic and that code for antigens which lead to rapid graft rejection between members of a single species which differ at these loci. Several classes of protein such as MHC class I and II proteins are encoded in this region. These in humans, are known as Human leukocyte antigens (HLA). Class I molecules are designated HLA-A, B, or C. Class II molecules are designated DP, DQ or DR.

• MHC class I molecule . A molecule encoded to genes of the MHC which participates in antigen presentation to cytotoxic T (CD8+) cells.
• MHC class II molecule . A molecule encoded by genes of the MHC which participates in antigen presentation to helper T (CD4+) cells.

The ability of T lymphocytes to respond only when they 'see' the appropriate antigen in association with "self" MHC class I or class II proteins on the antigen presenting cells is called MHC restriction .