Any of numerous cytokines that are produced by leucocytes and other cell types and which perform a range of regulatory functions for cells of the immune system, including cell differentiation, proliferation, maturation, migration, adhesion, and death. Interleukin-1 (IL-1) is secreted by antigen-activated macrophages and activates liver cells to produce acute-phase proteins. It has a pro-inflammatory effect. Interleukin-2 (IL-2) stimulates the proliferation of T cells, which also secrete it, and also enhances the activity of B cells, natural killer (NK) cells, and macrophages, promoting cytotoxicity, interferon secretion, and antibody production. Interleukin-3 is a growth factor for lymphoid and myeloid cells, and interleukin-4 induces T cells and B cells to proliferate and the latter to produce antibodies. Interleukin-6 is transiently produced at sites of inflammation and in response to infection, and stimulates the acute-phase response, the production of blood cells (haemopoiesis), and other immune responses. Over 30 interleukins are now known to exist, and some are manufactured using recombinant DNA technology, for use as therapeutic agents.