Interleukin-21 receptor is also known as IL-21 receptor, IL-21R, CD360. In humans, it is encoded by the IL21R gene. It belongs to the type I cytokine receptor family. Type 4 subfamily contains 2 fibronectin type-III domains. Interleukin-21 receptor is selectively expressed in lymphoid tissues and highly expressed in thymus and spleen. IL-21 is produced by CD4+ T cells in response to antigenic stimulation. Its action enhances antigen-specific responses of immune cells. The biological effects of IL-21 include induction of differentiation of T-cells-stimulated B-cells into plasma cells and memory B-cells. It also promotes the anti-tumor activity of CD8+ T-cells and NK cells.