T-cell-T-cell collaboration in allograft responses.

T-cell-T-cell collaboration in allogeneic responses traditionally has been viewed as the requirement for CD4+ T helper cells in the activation of CD8+ cytotoxic T cells. In this regard, the role of the CD4+ T cell is primarily to provide growth factors, such as interleukin-2, on which the CD8+ T cell is dependent. However, expanding information concerning the function of T-cell subsets, and the roles of antigen-presenting cells and cytokines in regulating immune responses, requires that the basic tenets of T-cell interactions be re-evaluated.