Interferons (IFNs) are proteins made and released by host cells in response to the presence of pathogenssuch as viruses, bacteria, or parasitesor tumor cells. They allow 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. Recombinant interferon-beta (IFN-beta) is medically used to treat multiple sclerosis (MS), most likely achieving its beneficial effect by reducing inflammation.