Interleukin 3 (IL-3) is a typical member of the family of four helix-bundle cytokines. It is synthesized and released by T lymphocytes when they are activated by specific antigens, and mast cells and eosinophils when multivalent antigens crosslink cytophilic antibodies bound to Fc receptors on their surface. IL-3 stimulates the growth, differentiation, survival, and function of a very broad range of cells derived from the pluripotential hematopoietic stem cell, including stem cells themselves, and progenitor cells and mature cells of many lineages, but is particularly important in stimulating the production of mast cells and basophils.