Interleukins (ILs) are a large group of cytokines that are produced mainly by leukocytes, although some are produced by certain phagocytes and auxiliary cells. Each IL acts on a specific, limited group of cells through a receptor specific for that IL. Interleukin 1 (IL1), originally known as lymphocyte activating factor (LAF), activates T cells and lymphocytes, which then proliferate and secrete interleukin 2. IL1 is primarily released from stimulated macrophages and monocytes, but also is released from several other cell types and is thought to play a key role in inflammatory and immune responses. The two closely related agents, interleukin 1 alpha (IL1 alpha) and interleukin 1 beta (IL1 beta) bind to the same cell surface receptor, elicit nearly identical biological responses and share 25% homology in their amino acid sequence.