"Interferons (IFNs) are proteins made and released by host cells in response to the presence of 
pathogens—such as viruses, bacteria, or parasites—or tumor cells. They allow for communication between cells to trigger the protective defenses of the immune system that eradicate pathogens or tumors."
"IFNs belong to the large class of glycoproteins known as cytokines. Interferons are named after their ability to "interfere" with viral replication within host cells. IFNs have other functions: 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. Certain host symptoms, such as aching muscles and fever, are related to the production of IFNs during infection."