Interferon is a pharmaceutical preparation of proteins found in the human body that work as part of the immune system to destroy viruses, cancer cells, and other infections. In the body, it is produced by most cells in response to the double-stranded RNA that is typical of most viruses. It has the effect of interfering with the duplication of viruses and alerting the immune system to the existence of the virus. Interferon has a number of side effects, including flu-like symptoms and immunosuppression.