IL-3 is a strong stimulator of proliferation, differentiation, and survival of pluripotent hematopoietic stem cells. IL-3 has been known by many different names through the years, including MCGF, HCGF, BPA, multi-CSF, WEHI-3, etc. It is secreted by many hematopoietic cells but also various other cells including T-, NK, mast, epithelial, stromal, neuronal, and astrocyte cells.

Naturally, IL-3 is a glycosylated 133 amnio acid protein. It binds through a heterodimeric receptor. The alpha subunit is specific for IL-3 while the beta subunit is shared with IL-5 and GM-CSF receptors. Whether alone, or in conjunction with IL-1, IL-6, stem cell factor, G-CSF, MGDF; IL-3 is a potent proliferator of hematopoietic stem cells and progenitor cells. Outside of stem cells, IL-3 can stimulate growth in other cells such as endothelial, smooth muscle, and neuronal cells.

In summary, IL-3 appears to be an important growth and differentiation factor, though studies in knock out mice suggest the factor is not key to development of immunity. It is often measured with other growth factors and hematopoietic factors such as IL-1, IL-5, IL-6, GM-CSF, and EPO.