Interleukin 3, also known as IL-3, was initially described in the supernatant of cultures of viral-infected murine spleen cells. Mature rat IL-3 surprisingly shows low homology (54%) with its murine counterpart. IL-3 is the most potent growth factor for basophils followed by the granulocyte-macrophage colony-stimulating factor and IL-5. These cytokines also act on mature basophils through specific receptors, thereby mediating adhesion, migration, and release of the cells. IL-3 is highly expressed by mast cells. Rapid autocrine production of large amounts of the cytokine is responsible for mast cell IgE-induced survival in the absence of antigen. In addition, IL-3 and its receptors are expressed in neuronal progenitors and neurons. IL-3 has been associated to human brain volume variation by regulating proliferation and survival of neural progenitors. It has also been implicated in the pathogenesis of several chronic inflammatory diseases, which include asthma, atherosclerosis, and neurodegenerative disorders such as multiple sclerosis.