Interferon (IFN) antagonism by viral protein. Molecular model of an orthopoxvirus IFN-gamma-binding protein tetramer bound to an IFN-gamma dimer. IFN-gamma is a cytokine and the only member of the type II class of interferons. These are proteins made and released by the immune system in response to the presence of pathogens. The viral IFN-gamma-binding proteins attach themselves to IFN-gamma, blocking its action, disrupting its signalling and its ability to induce an antiviral state within cells.