IL-21 is a type I cytokine and a member of the family of cytokines, along with IL-2, IL-4, IL-7, IL-9, and IL-15, that utilizes the common cytokine receptor gamma chain (γc) as part of its receptor complex. IL-21 is expressed predominantly by CD4+ T cells and binds to the IL-21 receptor (IL-21R), expressed on lymphohematopoietic cells, including NK cells, T cells, B cells, monocytes, macrophages, and dendritic cells. Binding of IL-21 exerts a diverse array of biological effects, including increased CD4+ and CD8+ T cell proliferation, augmented function of CD8+ T cells and NK cells, promotion of IL-17-secreting Th17 cells, and enhancement of B cell activation and plasma cell differentiation. The effects of IL-21 on B cells are due partially to its autocrine activity on follicular helper T cells (TFH), CD4+ T cells that produce large amounts of IL-21 and are critical to the development and function of germinal centers. Furthermore, it is thought that IL-21 plays a role in the development of pathogenic autoantibodies in systemic lupus erythematosus (SLE) and rheumatoid arthritis.