Monocytes are the main source of secreted IL1; expressing mostly IL1-beta, while human keratinocytes express large amounts of IL1-alpha. Overall, IL1-beta is the predominant form in humans while IL1-alpha is the one in mice. Both isoforms of IL1 bind to the same receptor and therefore show similar, if not identical, biological activities. The IL1-beta precursor though must be processed before it can bind to the receptor. IL1 proteins are involved in the inflammatory response, being identified as endogenous pyrogens, and are reported to stimulate the release of prostaglandin and collagenase from synovial cells. A monomer, IL1B stimulates thymocyte proliferation, B-cell maturation and proliferation, and fibroblast growth factor activity.