Type I interferons are IFN-α and IFN-β. An infected cell makes them and sends them out. They do not save that cell. They warn the cells around it.
A warned cell enters an antiviral state. It builds enzymes that chew up viral genetic material, and it is ready to shut protein building down. A virus that gets in finds no factory to hijack.
IFN-γ is a different job. It mainly activates macrophages and bridges over to adaptive immunity.
Interferons are why a viral illness feels awful. Much of the fever, the aches, and the dragging tiredness of influenza is your own interferon at work, not the virus directly.
They are also medicines. Purified interferons treat certain viral hepatitis infections, some cancers, and multiple sclerosis.