Interleukin-21 also known as IL-21 is a protein that in humans is encoded by the IL21 gene.
Interleukin-21 is a cytokine that has potent regulatory effects on cells of the immune system, including natural killer (NK) cells and cytotoxic T cells that can destroy virally infected or cancerous cells.[1][4] This cytokine induces cell division/proliferation in its target cells.