Interferons (IFNs) are low molecular weight proteins that belong to the class of glycoproteins known as cytokines. IFNs are part of the non-specific immune system and are an important first line of defense against viral infections. They are released by host cells in response to the presence of pathogens such as viruses, bacteria, parasites or tumor cells.

IFNs have other functions as well: they activate immune cells, such as natural killer cells and macrophages; they increase recognition of infection or tumor cells by up-regulating antigen presentation to T lymphocytes; and they increase the ability of uninfected host cells to resist new infection by virus. Host symptoms, such as aching muscles and fever, are related to the production of IFNs during infection.