A first or dominant immunization with one antigen markedly inhibited specific cytolytic T lymphocyte (CTL) responses to a second unrelated alloantigen without suppressing antibody responses to other antigens. Suppression was induced rapidly, became systemic, and could be transferred passively with only serum. Suppression did not result from elimination of cells capable of responding to the second antigen. The mechanisms responsible for this "priority of the first response" may be the same that help protect the fetus during pregnancy, promote renal allograft survival after multiple blood transfusions, and prevent effective CTL-mediated immunity to variants of tumor cells or infectious agents that arise during tumor progression or chronic infections.

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