Cytokines are the proteins secreted by the cells of immune system that control the immune responses by interaction between the neighboring cells. In other words Cytokines are the signaling molecules like hormones and are the end products of interaction among immune cells. Cytokines like other signaling molecules tend to bind the specific receptors on the target cells but the structure of cytokines and their receptors is very different. Once the cytokines bind to their receptors, transcription factors are produced as a result of changes in the cell behavior by the process called as signal transduction. Transcription factors stimulate the selected genes for transcription which then secrete new cytokines or signaling molecules.

23.1 Features of cytokines

1) Cytokines show redundancy i.e. different cytokines may have similar function, e.g. IL-1 and IL-6 both induce fever by acting on brain.

2) They are secreted in multiple numbers, e.g. macrophages secrete more than five interleukins and tumor necrosis factor.

3) Cytokines produce their effect on many cell types i.e. they are pleiotropic.

4) Cytokines need extreme regulation and are toxic in high doses.

23.2 Cytokine nomenclature

Most of the cytokines are named according to the Interleukin nomenclature subcommittee of the international union of immunological societies. Although the definition of cytokines is quite broad, but together they can be classified as lymphokines, interleukins, interferons , chemokines etc. depending on their function, cell of secretion, or target of action. The name interleukin was coined initially for those cytokines that mediate signaling between lymphocytes and leukocytes. In other words leukocytes were thought to be the principal target for interleukins but now this definition is no longer in function and the name interleukin is given to any new cytokine discovered routinely. Cytokines are produced mainly in response to viral infection or in response to immune attack. Interferons are the cytokines that interfere with viral RNA and protein synthesis to prevent viral replication.

Types of interferon – Interferons are mainly divided into two types.

Type I Interferons --- Interferon-α (IFN- α) and interferon-β (IFN-β).

Type II interferon --- Interferon-γ

Tumor necrosis factors (TNFs) can kill tumor cells and colony stimulating factors assist in regulation of stem cell activities. Chemokines play role in leukocyte recruitment and circulation.