A class of small (15-28 kD) protein and glycoprotein cytokines (15-28 kD) produced by T cells, fibroblasts, and other cells in response to viral infection and other biologic and synthetic stimuli. IFNs bind to specific receptors on cell membranes. Their effects include inducing enzymes, suppressing cell proliferation, inhibiting viral proliferation, enhancing the phagocytic activity of macrophages, and augmenting the cytotoxic activity of T lymphocytes. Interferons are divided into five major classes (alpha, beta, gamma, tau, and omega) and several subclasses (indicated by Arabic numerals and letters) on the basis of physicochemical properties, cells of origin, mode of induction, and antibody reactions. [interfere + -on]Commercially available IFNs are produced by genetically altered colonies of Escherichia coli or Chinese hamster ovary cells, or are induced by controlled viral infection in pooled human leukocytes. Alpha IFNs have found the widest application in medicine. (The spelling alpha is used with respect to naturally occurring interferons. In compliance with international conventions for generic drug names, the spelling alfa appears in names of pharmaceutical formulations.) Alpha IFNs are used in the treatment of chronic hepatitis B and hepatitis C, hairy cell leukemia, chronic myelogenous leukemia, AIDS-related Kaposi sarcoma, melanoma, condylomata acuminata and recurrent respiratory papillomatosis due to human papillomavirus, and infantile hemangiomatosis. About 50% of patients treated for chronic hepatitis B with IFN-alfa show disappearance of hepatitis Be antigen (HBeAg) and reversion of alanine aminotransferase to normal. The response rate in chronic hepatitis C is lower (15-25%), but better results are achieved by using more aggressive therapy (daily rather than thrice weekly administration) and continuing it longer (a minimum of 12 months). Beta IFNs reduce clinical recurrences and progression of myelin damage in multiple sclerosis. Gamma IFN is effective in retarding tissue changes in osteopetrosis and systemic scleroderma and in reducing the frequency and severity of infections in chronic granulomatous disease. Administration of IFNs is parenteral (intravenous, intramuscular, subcutaneous, intranasal, intrathecal, or intralesional) and several weeks of treatment may be required before clinical response is noted. More than 50% of people treated with IFNs experience a flulike syndrome of fatigue, myalgia, and arthralgia. Gastrointestinal and CNS side-effects are also common, and marrow suppression may occur with prolonged treatment.