Interferons may be regarded as polypeptide hormones because of their role in communicating from cell to cell a specific set of instructions that lead to a wide variety of effects. Viruses induce type I interferon, subdivided into alpha-interferon, produced by leukocytes or lymphoblastoid cells, and beta-interferon, produced by fibroblasts. Mitogens and antigenic stimuli induce in lymphocytes type II, immune, or gamma-interferon. The biologic effects of human interferons, including increment of histocompatibility antigens, are mediated through species-specific receptors. Human interferons are not active, for example, in mouse cells. The human interferon-gamma receptor is a heterodimer of IFNGR1 and IFNGR2. IFNGR1 is the ligand-binding subunit.