in·ter·leu·kin-·1. A cytokine, derived primarily from mononuclear phagocytes, that enhances the proliferation of T-helper cells and the growth and differentiation of B cells. Il-1 induces the release of Il-2. When secreted in larger quantities, IL-1 is a mediator of inflammation, entering the bloodstream and causing fever, inducing synthesis of acute phase proteins, and initiating metabolic wasting.