IL-21 is a pleiotropic cytokine produced by CD4+ T cells in response to antigenic stimulation. Its action generally enhances antigen-specific responses of immune cells. The biological effects of IL-21 include: inducing the differentiation of T-cell-stimulated B-cells into plasma cells and memory B-cells; the stimulation of IgG production in conjunction with IL-4;and the induction of apoptotic effects in naïve 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 γ chain (γc) 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.