Interferon refers collectively to a family of three proteins that nonspecifically inhibit viral replication inside host cells. In response to infection by a virus, various cell types secrete interferon into the extracellular fluid. Interferon then binds to plasma membrane receptors on nearby cells, whether they are infected or not. It also enters the circulation and reaches cells at far-removed sites. Thus cells can synthesize interferon provide it to cells that cannot.

How does interferon prevent viral replication? Its binding to the plasma membrane triggers the synthesis of serveral enzymes by the cell. If the cell is infected or eventually becomes infected, these enzymes block the synthesis of proteins the virus requires for replication. It must be reemphasized that interferon is not specific. Many, but not all, viruses induce interferon synthesis, and interferon in turn can inhibit the multiplication of many kinds of viruses.