Interferon, any of several related proteins that are produced by the body’s cells as a defensive response to viruses. They are important modulators of the immune response. Interferon was named for its ability to interfere with viral proliferation. The various forms of interferon are the body’s most rapidly produced and important defense against viruses. Interferons can also combat bacterial and parasitic infections, inhibit cell division, and promote or impede the differentiation of cells. They are produced by all vertebrate animals and possibly by some invertebrates as well. Interferons are categorized as cytokines, small proteins that are involved in intercellular signaling. Interferon is secreted by cells in response to stimulation by a virus or other foreign substance, but it does not directly inhibit the virus’s multiplication. Rather, it stimulates the infected cells and those nearby to produce proteins that prevent the virus from replicating within them. Further production of the virus is thereby inhibited and the infection is stemmed. Interferons also have immunoregulatory functions—they inhibit B-lymphocyte (B-cell) activation, enhance T-lymphocyte (T-cell) activity, and increase the cellular-destruction capability of natural killer cells.