IL-21 is a typical family I cytokine with broad pleiotropic actions and is primarily produced by T follicular helper cells, Th17, and natural killer T cells in response to antigenic stimulation. Il-21 generally enhances antigen-specific responses of immune cells. It controls the activation, proliferation, differentiation, cytotoxicity, and survival of various target immune cells. Additionally, IL-21 promotes the anti-tumor activity of CD8+ T-cells and NK cells. IL-21 elicits its effect through binding to IL-21R, which also contains the gamma chain found in other cytokine receptors such as IL-2, IL-4, IL-7, IL-9 and IL-15. 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. Recombinant Human IL-21 is a 15.4 kDa protein consisting of 132 amino acid residues.