Interleukin-3 is a pleiotrophic factor produced primarily by activated T cells that can stimulate the proliferation and differentiation of pluripotent hematopoietic stem cells as well as various lineage committed progenitors. IL-3 exerts its biological activities via a heterodimeric receptor composed of an IL-3 specific alpha chain and common beta chain that is shared with the IL-5 and GM-CSF high-affinity receptors. Receptors for IL-3 are present on bone marrow progenitors, and several mature myeloid cell types. Mature canine IL-3 shares <40% amino acid sequence identity with IL-3 of other mammals.