One of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. IL-5 is made mainly by some T lymphocytes. It causes B lymphocytes to make more antibodies and increases the number of eosinophils. IL-5 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. It is a type of cytokine. Also called interleukin-5.