IL-21 is produced by CD4+ T cells in response to antigenic stimulation. Its action enhances antigen-specific responses of immune cells. The biological effects of IL-21 include induction of differentiation of T-cells-stimulated B-cells into plasma cells and memory B-cells，stimulation （in conjuction） with IL-4 of IgG production，and induction of apoptotic effects in naive B-cells and stimulated B-cells in the absence of T-cell signaling. Additionally，IL-21 promotes the anti-tumor activity of CD8+ T-cells and NK cells. IL-21 exerts its effect through binding to a specific type I cytokine receptor，IL-21R，which also contains the gamma chain （°C） found in other cytokine receptors including IL-2，IL-4，IL-7，IL-9 and IL-15. The IL-21/IL-21R interaction triggers a cascade of events which includes activation of the tyrosine kinases JAK1 and JAK3，followed by activation of the transcription factors STAT1 and STAT3.
