Interferons are a group of signaling proteins made and released by host
cells in response to the presence of several viruses. In a typical scenario, a virus-infected cell will release
interferons causing nearby
cells to heighten their anti-viral defenses. IFNs belong to the large class of proteins known as cytokines, molecules used for communication between
cells to trigger the protective defenses of the
immune system that help eradicate pathogens.
Interferons are named for their ability to "interfere" with viral
replication by protecting
cells from
virus infections. IFNs also have various other functions: they activate immune cells, such as natural killer
cells and macrophages; they increase host defenses by up-regulating antigen presentation by virtue of increasing the expression of major histocompatibility complex (MHC) antigens. Certain symptoms of infections, such as fever, muscle
pain and "flu-like symptoms", are also caused by the production of IFNs and other cytokines. More than twenty distinct IFN genes and proteins have been identified in animals, including humans. They are typically divided among three classes: Type I IFN, Type II IFN, and Type III IFN. IFNs belonging to all three classes are important for fighting viral infections and for the regulation of the immune system.
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