Schematic representation showing cytokines released by Th1 and Th2 cells

Advanced T cells secrete an extraordinary array of cytokines with a bewildering number of functions. Such cytokines have been divided into two general groups according to their general mode of action, Th1 and Th2. In general, allograft rejection is mediated by T cells that produce Th1 cytokines. In contrast, transplantation tolerance has been associated with T cells that produce Th2 cytokines; however, the casual link between such cytokine production and the induction and maintenance of the tolerant state remains controversial.