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A basic, fundamental feature of an antigen-stimulated antibody response is the specificity of the binding capacity of the antibodies produced for determinants on that stimulating antigen. The exquisite specificity that is evidenced by antibodies for their antigenic determinants is also observed in T cell-mediated killing responses directed against cell surface antigenic molecules. The major target molecules for such cytotoxic T effector cells are associated with the major histocompatibility complex (MHC) 1, which is the H-2 complex in the mouse. Although "cross-reactive" killing of cellular targets obtained from mice with different H-2 haplotypes is frequently observed, recent evidence suggests that such cross-reactivity arises through the activation of distinct prekiller cell clones. Although the majority of the cell clones reactive to MHC antigens recognize the immunodominant specificity that is distinct for a given haplotype ("private" specificity), other distinct prekiller cell clones are specific for MHC determinants that are shared by a number of different H-2 haplotypes ("public" specificities) (1-4). Thus, the development of cytolytic effector T cells directed against cell surface H-2 antigenic determinants appears to follow the laws of clonal selection and immunological specificity. Similar rules concerning the generation of antigen-specific help for T cytolytic effector cell development have not been readily evident. T cell help for alloreactivity can be generated with virtually any H-2I-region

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