The antigen-specific signal in T-lymphocyte activation by alloantigen.

: Current concepts of the mechanism of alloactivation of cytotoxic lymphocytes and the identity of the target cell determinants with which they interact are reviewed. The results of a quantitative analysis of the stimulatory activity of different forms of antigen and the role of a costimulator factor are presented. These lead to the conclusions that SD antigens are not the target cell antigens and that existing theories of cytotoxic T cell activation are inadequate. A new theory of activation of alloareactive cytotoxic lymphocytes is proposed.