What is the role of interferon in defense against disease?

Interferon are small proteins (cytokines) that enhance the immune system and defense against viruses and bacterial infections.
Interferon stimulates the infected cells and those nearby to produce proteins. These proteins  prevent the virus from replicating within them. Further production of the virus is inhibited and the infection is stemmed. 
There are three types of interferon: gamma, alpha and beta. Alpha and beta can be produced from any cell, and gamma is produced only by natural killer cells and T lymphocytes. The  main purpose of gamma interferon is to signal the immune system to respond to infectious agents or cancerous growth.

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