One of a group of related proteins made by leukocytes (white blood cells) and other cells in the body. Interleukin-1 is made mainly by one type of white blood cell, the macrophage, and helps another type of white blood cell, the lymphocyte, fight infections. It also helps leukocytes pass through blood vessel walls to sites of infection and causes fever by affecting areas of the brain that control body temperature. There are two forms of interleukin-1, alpha and beta, which act the same. Interleukin-1 made in the laboratory is used as a biological response modifier to boost the immune system in cancer therapy. Interleukin-1 is a type of cytokine. Also called IL-1.