Interleukin-21 (IL-21) belongs to the Type I four helix bundle cytokines, and shares the common cytokine receptor ? chain with IL-2, IL-4, IL-7, IL-9, and IL-15. IL-21 is expressed by CD 4+ T cells, natural killer (NK) T cells, and Th17 cells. The IL-21 receptor is highly expressed on CD 4+ and CD...