Interferons are species specific proteins which are able to protect body cells from virus infection. Three main types of interferon are known: interferon α (produced by leukocytes), interferon β (produced by fibroblasts) and interferon γ (immune interferon produced by T-cells). Infected cells produce interferon and yield it to the interstitial liquid and to the bloodstream. The interferon binds to receptors of neighbouring and distant cells and induces the production of antiviral proteins, which block virus replication. Interferons are one of the body's first antiviral defence mechanisms, therefore most of viruses developed counter strategies against interferon.