Interferon gamma (IFN-γ) is a cytokine critical to both innate and adaptive immunity, and functions as the primary activator of macrophages, in addition to stimulating natural killer cells and neutrophils. A non-IgE-mediated anaphylactic reaction and severe bronchospasm have been reported once after the first injection of interferon gamma. IFN-γ activates cells via a different receptor than IFN-α and IFN-β, which accounts for the different physiological properties of the proteins. Production of IFN-γ is largely restricted to activated CD4+ TH1 T cells, CD8+ T cells, and natural killer cells. One of the most important consequences of IFN-γ secretion is the activation of macrophages. In addition, IFN-γ plays a central role in inflammatory responses by activating endothelial cells, promoting TH1 cell development and cellular immune responses, and up-regulation of major histocompatability complex protein expression on antigen-presenting cells.