Interleukin 21 (IL-21) is a 133 amino acid (molecular weight 15.4 kDa) cytokine that plays a key role in enhancing antigen specific immune responses. It is primarily secreted by CD4+ T cells after antigenic stimulation. IL-21 binds to a heterodimeric receptor composed of IL-21R and a common gamma chain subunit used by other cytokines such as IL-2 and IL-4. IL-21 has been shown to promote the activation and proliferation of CD8+ T cells, B cell maturation and differentiation, as well as driving TH-17 cell based immune responses.