Interferon gamma(IFNγ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. Wheelock as a product of human leukocytes stimulatedwith phytohemagglutinin, and by others as a productofantigen-stimulatedlymphocytes ortuberculin-sensitized mouseperitoneal lymphocytes challenged with PPD.The resulting supernatants were shown toinhibit growth of vesicular stomatitisvirus. IFNγis a cytokine that is critical for innate and adaptive immunityagainst viral, some bacterial and protozoal infections. IFNγ is an important activator of macrophages and inducer of Class II major histocompatibility complex (MHC) molecule expression. The importance of IFNγ in the immune system stems in part from its ability to inhibit viral replication directly, and most importantly from its immunostimulatoryand immunomodulatoryeffects. IFNγ is produced predominantly by natural killer (NK) and natural killer T (NKT) cells as part of the innate immune response, and by CD4 Th1 and CD8 cytotoxic T lymphocyte (CTL) effector T cells once antigen-specific immunity develops.