one of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. interleukin-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 lymphocyte. interleukin-4 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. interleukin-4 is a type of cytokine. also called il-4.