One of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. IL-13 is made by a type of T lymphocyte. It reduces inflammation by blocking production of cytokines by macrophages. It also increases the number and activity of B lymphocytes. IL-13 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. It is a type of cytokine. Also called interleukin-13.