The fate of alloreactive T cells. Upon antigen engagement and in the presence of adequate costimulatory signals, the alloreactive T cell becomes activated, proliferates, and is subject to a number of different fates. The first is differentiation to an effector cell, orchestrating the immune response directed toward the target antigen. Some of the T cells will differentiate into memory cells, able to provide rapid recall responses upon antigenic restimulation. Other cells will have their effector functions terminated either by anergy or by apoptotic death (which may be passive or may involve activation-induced cell death [AICD]), or as a consequence of regulation by other cells or soluble factors. CTL, cytotoxic T lymphocyte; DTH, delayed-type hypersensitivity.