Interferon gamma (IFN-gama ) is a multifunctional protein first observed as an antiviral activity in cultures of Sindbis virus-infected leukocytes stimulated by PHA . Produced by T lymphocytes and natural killer (NK) cells, IFN-γ is now known to be both an inhibitor of viral replication and a regulator of numerous immunological functions.

IFN-γ influences the class of antibody produced by B cells, upregulates class I and II MHC complex antigens and increases. the efficiency of macrophage-mediated killing of intracellular parasites. Most of the activities attributed to IFN-γ are believed to be mediated by IFN-γ-induced proteins. The appearance of such proteins is a consequence of IFN-γ binding to a specific receptor that is distinct from the receptor for IFN-αandβ.