One of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. IL-7 is made by cells that cover and support organs, glands, and other structures in the body. It causes the growth of T lymphocytes and B lymphocytes. IL-7 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. IL-7 is a type of cytokine. Also called interleukin-7 and lymphopoietin-1.