Interleukin-3 (IL-3) is a secreted cytokine that stimulates the proliferation and differentiation of pluripotent hematopoietic stem cells and various lineage-committed progenitors. It also regulates the activation of mature mast cells, basophils, eosinophils, and macrophages. It is produced by activated T cells, thymic epithelial cells, activated mast cells, keratinocytes, neurons, and astrocytes. IL-3 signals through a receptor complex that contains IL-3 R alpha/CD123 and the common beta chain which is also a component of the receptors for IL-5 and GM-CSF. Receptors for IL-3 are present on bone marrow progenitors, macrophages, mast cells, eosinophils, megakaryocytes, basophils, and various myeloid leukemic cells. Human IL-3 does not show activity on mouse cells.