IL-1 was originally identified and cloned as a lymphocyte mitogen and much later, was found to be comprised of two closely related but distinct proteins, interleukin 1 alpha (IL-1 alpha) and interleukin 1 beta (IL-1 beta). Both these proteins bind to the same cell surface receptor. IL-1 is primarily released from stimulated macrophages, but is also released from several other cell types. Along with other IL-1 gene family members, IL-1 beta falls within a cytokine gene cluster on chromosome 2.
