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