Interleukin 3 (IL-3) is a pleiotropic cytokine that can stimulate proliferation and differentiation of pluripotent hematopoietic stem cells as well as various lineage committed progenitors. IL-3 exerts its activity through binding to a specific cell surface receptor known as IL-3 R. IL-3 R is a heterodimeric structure composed of a 70 kDa IL-3 R alpha subunit (CD123 or beta common) and a 120-140 kDa IL-3 R beta subunit (CD131). CD131 also associates with the receptor for Erythropoietin, forming a tissue-protective hetero-receptor complex.

IL-3 R alpha binds IL-3 with relatively low affinity. In the presence of IL-3 R beta, however, IL-3 R alpha has a much higher affinity for IL-3. It is not clear how signal transduction occurs following IL-3 binding. The IL-3 R alpha chain has a very short intracellular domain while the IL-3 R beta chain has a very large cytoplasmic domain. Recent studies suggest that formation of IL-3 R beta-beta homodimers initiates signaling events. The IL-3 R beta chain is also shared by the receptors for IL-5 and GM-CSF. Cells known to express IL-3 receptors include hematopoietic progenitors, epithelial cells, double negative T cells, mast cells, basophils and blood monocytes.