- One of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. IL-12 is made mainly by B lymphocytes and macrophages. It causes other immune cells to make cytokines and increases the growth of T lymphocytes. It may also block the growth of new blood vessels. IL-12 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. IL-12 is a type of cytokine. Also called interleukin-12.