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The immune system has evolved with a highly diverse antigen-specific repertoire capable of recognizing an extraordinarily large universe of foreign antigens. As a corollary of this wide diversity, the precursor fre quency of T cells specific for any given foreign antigen is generally very low, so low, in fact, that proliferative responses of naive T-cell populations to these antigens are undetectable. However, prominent exceptions to this generalization have been identified. In multiple species, cell surface alloan tigens encoded by genes of the major histocompatibility complex (MHC) are recognized by T cells at a high precursor frequency and are capable of eliciting a strong proliferative mixed lymphocyte response by unprimed T cell populations ( 1 , 2). In the mouse, a second set of determinants, the minor lymphocyte stimulatory (Mis) determinants, also induce strong proliferative responses by naive T cells (3-5). The finding that T cells are reactive to MHC and to Mis determinants at extraordinarily high pre cursor frequencies (1 , 2, 6, 7) has generated interest in understanding the biologic significance of these two systems. A great deal of information has been accumulated concerning the structure and function of MHC genes and their products, and the biologic importance of MHC-encoded deter minants has been extensively documented (reviewed in 8). In contrast, although MIs gene products were described as lymphocyte
Abe et al. (Sat,) studied this question.