In regulation of specific immune responses the most important role play interleukin-2, IL-4, IL-5, IL-13, inter feron- (INF- ), transforming growth factor- (TGF- ) and lymphotoxin (LT). These signal molecules are produced mainly by T-lymphocytes after recognition of foreign antigens by specific receptors (TCR) placed on plasmalema. Some of mentioned cytokines stimulate proliferation and differentiation of various lymphocyte populations in the activation phase of T cell-de pendent immune responses, while the others activate and regulate the function of specialized effector cells, such as mono nuclear phagocytes, eosinophils, and neutrophils, to eliminate antigens in the effector phase of immune responses. In con trast to most of cytokines which have stimulating action on an initiation and course of humoral and cell-mediated immune responses, TGF- has an inhibitory effect on the activation and proliferation of T-lymphocytes and the other leukocyts