T cells, the MHC, and function.

The model presented relates the class of MHC molecule recognized with the activity of T lymphocytes. This occurs by postulating that the function of MHC molecules is to present determinants to differentiation directing (D) receptors on T lymphocytes. These D receptors are distributed nonclonally and interact with nonpolymorphic determinants on MHC molecules which are normally inaccessible. The result of this interaction is to allow for the expression of a functional phenotype for the T cell providing that helper signals are present. In the case of alloreactive CTL that recognize class II or Th that recognize class I molecules we postulate that these T cells require the presence of class I or class II molecules, respectively, on the target cell. Recognition of antigen in the absence of helper signals leads to tolerance of both Th.P and CTL.P. Since helper signals are required for the activation of CTL.P, a lack of specific Th, determined by class II alleles, will prevent development of a proportion of the CTL repertoire.