The Phylogeny of T Cell Antigens

Human T cells express surface markers which can be identified by means of monoclonal antibodies (mAbs). It is possible to subdivide the T cells into functionally distinct subsets (1–5). Moreover, for some of these markers it is now known what role they play in the immune response. Thus, CD4 and CD8 are presumably involved in binding to the MHC class II or I antigens, respectively (6–8), of the target cells in the CML assay. The CD3 marker is involved in the lytic event of the CML after specific binding has taken place (9). The CD2 marker is also involved in the CML reaction, although its precise function is as yet unknown (10). Not all mAbs identifying these markers are able to block the above-mentioned functions, most likely because the epitopes recognized by the nonblocking antibodies are not involved in the immunological function of that molecule. It can be postulated that functional epitopes with a rather general function in the immune response not related to the specific immunological adaptation of one species will be preserved during evolution. On the other hand, functional epitopes which are responsible for immune functions related to the immunological adaptation of the species might be much less conserved throughout evolution.