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