Useful Notes on Major Histocompatibility Complexes

B and T cells contain mem­brane receptors that react with specific antigenic de­terminants.

B cells are activated by interaction with the antigen either in its dissolved form or while it is still a part of the surface of a pathogen. In contrast, the T cells of an animal are only activated when the antigen is displayed on the surface of a cell that also possesses markers of the animal’s own identity.

These markers are proteins encoded by a cluster of genes called the major histocompatibility complex (MHC) and are called the MHC glycoproteins.

The MHC glycoproteins were discovered as a result of tissue-transplantation or tissue-grafting experi­ments. Grafts involving donors and recipients from the same strain of experimental animals (i.e., the equivalent of identical twins) are usually accepted by the donor’s body. However, when the donor and recipi­ent are not genetically identical, the graft is rejected because the recipient mobilizes an immune response to the transplanted cells and destroys them.

It appears that different (i.e., unrelated) individuals express different sets of MHC genes and, like the an­tibodies and T-cell receptors, these proteins are in­credibly diverse. However, unlike antibodies and T-cell receptors, which differ among the millions of different clones of cells of an individual, the MHC proteins dif­fer only among individuals, that is, all cells of a single individual bear the same MHC proteins.

Human MHC proteins are called human leukocyte-associated anti­gens or HLA antigens (they were first identified in leukocytes) and can be divided into two major classes: class I MHC antigens and class II MHC antigens (Fig. 25-16).

Class I antigens are found in the surfaces of nearly all cells. They consist of two polypeptide chains: a large A chain that is similar in size and organization to an immunoglobulin heavy chain arid a smaller chain called a β₂-microglobulin (Fig. 25-16). Amino acid se­quence homologies between these polypeptides and immunoglobulin’s suggest that an evolutionary link ex­ists among them.

Class II antigens are not found in the surfaces of all types of cells; rather, they are limited to a few types of cells that play a role in the immune response. For ex­ample, they are found in most B cells, some T cells, and some antigen-presenting macrophages. Class II antigens are composed of two poly­peptide chains: an alpha chain and a beta chain, each about the same size as an immunoglobulin light chain (Fig. 25-16). Like the class I antigens, the class II an­tigens exhibit some degree of homology with the immunoglobulin’s. The MHC antigens play a role in de­termining the actions of T cells.