Interleukin 21 (IL-21) has a strong regulatory effect on immune cells, including natural killer (NK) cells and cytotoxic T cells that can destroy viral infections or tumor cells. This cytokine induces target cell division/proliferation.IL-21 is expressed in human activated CD4+ T cells but not in most other tissues. In addition, IL-21 expression is up-regulated in the helper T cell subsets Th2 and Th17 cells as well as in follicular T cells. Moreover, IL-21 is expressed in NKT cells that regulate the function of these cells.IL-21 has a pathological role in immunoinflammatory diseases. Studies have shown the existence of a positive autocrine loop that amplifies and stabilizes IL-21-driven, T-cell-mediated responses. Moreover, it has antitumor activity and a role in autoimmune processes. In the clinical setting, IL-21 also serves as a potential antitumor agent.