Interferon gamma (IFN-γ) 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 stimulated with phytohemagglutinin, and by others as a product of antigen-stimulated lymphocytes. It was also shown to be produced in human lymphocytes. or tuberculin-sensitized mouse peritoneal lymphocytes challenged with Mantoux test (PPD); the resulting supernatants were shown to inhibit growth of vesicular stomatitis virus. Those reports also contained the basic observation underlying the now widely employed IFN-γ release assay used to test for tuberculosis. In humans, the IFN-γ protein is encoded by the IFNG gene.

Through cell signaling, IFN-γ plays a role in regulating the immune response of its target cell. A key signaling pathway that is activated by type II IFN is the JAK-STAT signaling pathway. IFN-γ plays an important role in both innate and adaptive immunity. Type II IFN is primarily secreted by adaptive immune cells, more specifically CD4⁺ T helper 1 (Th1) cells, natural killer (NK) cells, and CD8⁺ cytotoxic T cells. The expression of type II IFN is upregulated and downregulated by cytokines. By activating signaling pathways in cells such as macrophages, B cells, and CD8⁺ cytotoxic T cells, it is able to promote inflammation, antiviral or antibacterial activity, and cell proliferation and differentiation. Type II IFN is serologically different from interferon type 1, binds to different receptors, and is encoded by a separate chromosomal locus. Type II IFN has played a role in the development of cancer immunotherapy treatments due to its ability to prevent tumor growth.