Interleukin 3 (IL-3) is a hematopoietic growth factor, which is produced mainly by activated T cells, but is also secreted by other cell types, including mast cells, eosinophils, and keratinocytes. The broad spectrum of biological activities of IL-3 includes the stimulation of the proliferation and differentiation of immature pluripotent hematopoietic stem cells and various lineage-committed progenitor cells, leading to the production of most of the major blood cell types. In addition, IL-3 also affects the functional activity of mature mast cells, basophils, eosinophils, and macrophages.