CELLULAR DISTRIBUTION OF THE MHC ANTIGENS
Class I MHC molecules (HLA-A, B, and C alleles in humans and H2-K, D, and L alleles in mice) are expressed on all nucleated cells with only two exceptions: neurons and striated muscle cells. They are particularly abundant on the surface of lymphocytes (1,000–10,000 molecules/cell).
Class II MHC molecules (the I-A and I-E alleles of the mouse H2 complex and the DP, DQ, and DR alleles of the human HLA system) are exclusively expressed in two groups of leukocytes: B lymphocytes and cells of the monocyte-macrophage family, which includes all antigen-presenting cells (Langerhans cells in the skin, Kupffer cells in the liver, microglial cells in the central nervous system, and dendritic cells in the spleen and lymph nodes). While resting T lymphocytes do not express MHC-II molecules, these antigens can be detected after cell activation. Several other types of cells have been shown to express MHC after activation. It must be stressed that all cells expressing class II MHC simultane-ously express class I MHC.