Interferon-gamma (IFN-gamma, also known as Type II interferon or immune interferon) is a cytokine produced primarily by T-lymphocytes and natural killer cells. The protein shares no significant homology with IFN-beta or the various IFN-alpha family proteins. Mature IFN-gamma exists as non-covalently-linked homodimers. Human IFN-gamma is highly species specific and is biologically active only in human and primate cells. IFN-gamma was originally characterized based on its antiviral activities. The protein also exerts anti-proliferative, immuno-regulatory and pro-inflammatory activities and is thus important in host defense mechanisms. IFN-gamma induces the production of cytokines, upregulates the expression of class I and II MHC antigens, Fc receptor and leukocyte adhesion molecules. It modulates macrophage effector functions, influences isotype switching and potentiates the secretion of immunoglobulins by B cells. IFN-gamma also augments TH1 cell expansion and may be required for TH1 cell differentiation.