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.