One of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. IL-4 is made by a type of T lymphocyte. It causes B lymphocytes to increase and to make antibodies and also increases the production of T lymphocytes. IL-4 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. IL-4 is a type of cytokine. Also called interleukin-4.