One of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. IL-3 is made mainly by a type of T lymphocyte. It increases the number of blood cells made by the bone marrow. IL-3 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. IL-3 is a type of cytokine. Also called interleukin-3.