Cross-reactive T-cell cytotoxicity is seen when cytotoxic responses are generated in mixed lymphocyte cultures either between mouse strans which differ at the major histocompatibility complex, H-2, or between H-2b mutant strains and the strain from which they were derived. This cross-reactivity can be measured with [51Cr] labeled target cells from a number of different H-2 haplotypes, and the pattern of cross-reaction indicates that the target antigens are unlikely to be any of the serologically defined public specificities. In contrast, the specificity of H-2 restricted cytotoxic responses, such as that to the male-specific antigen, H-Y, is exquisite, and male cells from strains of mice carrying H-2 haplotypes other than the responder have never been found to act as appropriate targets. The contrast between the specificity of anti-H-2 and H-2 restricted responses may argue for a greater idiotypic homogeneity of the cells makiing H-2 restricted responses, and the greater specificity of these responses may be necessary for their biological function.

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