Nonspecific Defenses-Interferon

Interferons (IFNs) are natural cell-signaling proteins produced by the cells of the immune system of most vertebrates in response to challenges such as viruses, parasites and tumor cells. Interferons belong to the large class of glycoproteins known as cytokines. Interferons are produced by a wide variety of cells in response to the presence of double-stranded RNA, a key indicator of viral infection. Interferons assist the immune response by inhibiting viral replication within host cells, activating natural killer cells and macrophages, increasing antigen presentation to lymphocytes, and inducing the resistance of host cells to viral infection.

Types of interferon
There are three major classes of interferons that have been described for humans according to the type of receptor through which they signal:

•  Interferon type I: All type I IFNs bind to a specific cell surface receptor complex known as the IFN-α receptor (IFNAR) that consists of IFNAR1 and IFNAR2 chains. The type I interferons present in humans are IFN-α, IFN-β and IFN-ω.
  
•  Interferon type II: Binds to IFNGR. In humans this is IFN-γ. 
•  Interferon type III: Signal through a receptor complex consisting of IL10R2 (also called CRF2-4) and IFNLR1 (also called CRF2-12). Interferon type III is not accepted as a separate classification in mainstream Medicine. 

Natural function and synthesis

Interferons in general have several effects in common. They are antiviral and possess antioncogenic properties, macrophage and natural killer cell activation, and enhancement of major histocompatibility complex glycoprotein classes I and II, and thus presentation of foreign (microbial) peptides to T cells. In a majority of cases, the production of interferons is induced in response to microbes such as viruses and bacteria and their products (viral glycoproteins, viral RNA, bacterial endotoxin, bacterial flagella, CpG sites), as well as mitogens and other cytokines, for example interleukin 1, interleukin 2, interleukin-12, tumor necrosis factor and colony-stimulating factor, that are synthesised in the response to the appearance of various antigens in the body. Their metabolism and excretion take place mainly in the liver and kidneys. They rarely pass the placenta but they can cross the blood-brain barrier.

The therapeutically used forms are denoted by Greek letters indicating their origin: leukocytes, fibroblasts, and lymphocytes for interferon-alpha, -beta and -gamma, respectively.