IL-21 and IL-21 receptor: a new cytokine pathway modulates innate and adaptive immunity.

Interleukin (IL)-21 is a new member of the type I cytokine superfamily. Although it is most homologous to IL-15, it has a unique receptor chain, IL-21R, that pairs with the gamma-common cytokine receptor chain. The first experiments examining the biology of the IL-21 pathway reveal that it is a cytokine with effects on natural killer (NK) cells, T cells, and B cells. Mice deficient in the IL-21R have also been made, and are being examined for the effects of the IL-21/IL-21R pathway in vivo. Here we summarize our current knowledge of this new cytokine pathway, and its role in innate and adaptive immunity.