Interleukin-21 (IL-21) belongs to a family of cytokines that includes IL-2, IL-4, IL-7, IL-9, and IL-15, all of which bind to private (or shared) receptors as well as the common cytokine receptor γ-chain as a component. Most cytokines in this family are critically important for both the maintenance and function of T cells and B cells. The receptor for IL-21 is widely distributed on lymphohematopoietic cells, and IL-21 plays many biologic roles, including maintenance and function of CD8+ memory T cells and natural killer cells, as well as promoting the generation of Th17 cells in the mouse. One principal non-redundant role of IL-21 is the promotion of B-cell activation, differentiation or death during humoral immune responses. Furthermore, increased IL-21 production is characteristic of certain autoimmune diseases and is likely to contribute to autoantibody production as well as pathologic features of autoimmune disease. In contrast, IL-21 may function as a co-adjuvant to enhance antibody responses and thereby facilitate host defense to malignances and infectious diseases. The critical role of IL-21 in promoting humoral immune responses makes it an important focus of potential therapeutic interventions in conditions characterized by either overproduction of pathogenic autoantibodies or under production of protective antibodies.