Description: The IL-21 receptor (IL-21R) combines with the common γ-chain to form a functional receptor for IL-21. IL-21R is expressed on several lineages including both resting and activated B, T, natural killer and dendritic cells. Binding of its ligand, IL-21, in these cells results in the activation of the Jak/Stat signal transduction pathway. The biological effects of ligand binding are diverse and depend on the cellular context and differentiation state of the cell. In the mouse, IL-21 binding to IL-21R induces the apoptosis of resting and activated B cells, enhances anti-CD3-mediated proliferation of T cells, and inhibits the maturation of dendritic cells and the IL-15-mediated expansion of NK cells.