Interleukin 21 (IL21) belongs to a family of cytokines that bind to a composite receptor consisting of a private receptor (IL21R) and the common cytokine receptor gamma chain (gamma(C)). The IL21R is widely distributed on lympho-haematopoietic cells and IL21 impacts a number of cell types, including CD8+ memory T cells, NK cells and subsets of CD4 memory T cells. One essential role of IL21 is the promotion of B-cell activation and differentiation or death during humoral immune responses. Increased IL21 production is characteristic of certain autoimmune diseases and is likely to contribute to autoantibody production as well as pathological features of autoimmune disease. The critical role of IL21 in promoting humoral immune responses makes it an important focus of potential therapeutic interventions in conditions characterised by overproduction of pathogenic autoantibodies.