One of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. IL-6 is made mainly by some T lymphocytes. It causes B lymphocytes to make more antibodies and also causes fever by affecting areas of the brain that control body temperature. IL-6 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. IL-6 is a type of cytokine. Also called interleukin-6.