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'''Cytokines''' are a group of proteins and peptides that are used in organisms as signaling compounds. These chemical signals are similar to hormones and neurotransmitters and are used to allow one cell to communicate with another. The cytokine family consists mainly of smaller water-soluble proteins and glycoproteins (proteins with an added sugar chain) with a mass of between 8 and 30 kDa. While hormones are released from specific organs into the blood and neurotransmitters are released by nerves, cytokines are released by many types of cells. They are particularly important in both innate and adaptive immune responses. Due to their central role in the immune system, cytokines are involved in a variety of immunological, inflammatory and infectious diseases. However, not all their functions are limited to the immune system, as they are also involved in several developmental processes during embryogenesis.

Cytokines are produced by a wide variety of cell types (both [[Hematopoietic stem cell|haemopoietic]] and non-haemopoietic) and can have effects on both nearby cells or throughout the organism. Sometimes these effects are strongly dependent on the presence of other chemicals and cytokines. 

==Effects==
Each cytokine binds to a specific [[Receptor (biochemistry)|cell-surface receptor]]. Subsequent [[biochemical cascade|cascades]] of intracellular signalling then alter cell functions. This may include the upregulation and/or downregulation of several [[genes]] and their [[transcription factors]], in turn resulting in the production of other cytokines, an increase in the number of surface receptors for other molecules, or the suppression of their own effect by [[enzyme inhibitor|feedback inhibition]].

The effect of a particular cytokine on a given cell depends on the cytokine, its extracellular abundance, the presence and abundance of the complementary receptor on the cell surface, and downstream signals activated by receptor binding; these last two factors can vary by cell type.  Interestingly, cytokines are characterized by considerable "redundancy", in that many cytokines appear to share similar functions.

Generalization of functions is not possible with cytokines; nonetheless, their actions may be comfortably grouped as:
*[[autocrine signalling|autocrine]], if the cytokine acts on the cell that secretes it
*[[paracrine signalling|paracrine]], if the action is restricted to the immediate vicinity of a cytokine's secretion
*[[endocrine system|endocrine]], if the cytokine diffuses to distant regions of the body (carried by blood or plasma) to affect different tissues.

Cytokines binding to [[antibody|antibodies]] paradoxically have a stronger immune effect than the cytokine alone. This may lead to lower therapeutic doses and perhaps fewer side effects.

Overstimulation of cytokines can trigger a dangerous syndrome known as a [[cytokine storm]]; this may have been the cause of severe adverse events during a clinical trial of [[TGN1412]].

==Nomenclature==
Cytokines have been variously named as [[lymphokines]], [[interleukins]] and [[chemokines]], based on their presumed function, cell of secretion or target of action. Because cytokines are characterized by considerable redundancy and pleiotropism, such distinctions, allowing for exceptions, are obsolete.

* The term ''interleukin'' was initially used by researchers for those cytokines whose presumed targets are principally [[White blood cell|leukocytes]].  It is now used largely for designation of newer cytokine molecules discovered every day and bears little relation to their presumed function. 
* The term ''chemokine'' refers to a specific class of cytokines that mediates chemoattraction ([[chemotaxis]]) between cells. 

Of note, IL-8 ([[interleukin-8]]) is the only chemokine originally named an interleukin.

==Classification==
===Structural===
Structural homology has been able to partially distinguish between cytokines that do not demonstrate a considerable degree of redundancy so that they can be classified into four types:

<ul>
<li>The four α-helix bundle family - Member cytokines have three-dimensional structures with four bundles of [[alpha helix|α-helices]]. This family in turn is divided into three sub-families:
  <ol>
  <li> the [[interleukin 2|IL-2]] subfamily
  <li> the [[interferon|interferon (IFN)]] subfamily
  <li> the [[interleukin 10|IL-10]] subfamily
  </ol>The first of these three subfamilies is the largest.  It contains several non-immunological cytokines including [[erythropoietin]] (EPO) and [[thrombopoietin]] (THPO). Alternatively, four α-helix bundle cytokines can be grouped into ''long chain'' and ''short chain'' cytokines.
<li> the [[interleukin 1|IL-1]] family - It primarily includes IL-1 and [[interleukin|IL-18]].
<li> the [[interleukin|IL-17]] family - It has yet to be completely characterized, though member cytokines have a specific effect in promoting proliferation of T-cells that cause cytotoxic effects.
<li> [[Chemokines]]
</ul>

===Functional===
A more clinically and experimentally useful classification divides immunological cytokines into those that promote the proliferation and functioning of helper T-cells, type 1 ( IFN-γ etc.) and type 2 ([[interleukin 4|IL-4]], [[interleukin 10|IL-10]], [[interleukin 13|IL-13]], TGF-β etc.), respectively. 

A key focus of interest has been that cytokines in one of these two sub-sets tend to inhibit the effects of those in the other. This tendency is under intensive study for its possible role in the pathogenesis of [[autoimmune disorder]]s.

== Cytokine receptors ==
In recent years, the cytokine receptors have come to demand the attention of more investigators than cytokines themselves, partly because of their remarkable characteristics, and partly because a deficiency of cytokine receptors have now been directly linked to certain debilitating immunodeficiency states. In this regard, and also because the redundancy and pleiomorphism of cytokines are in fact a consequence of their homologous receptors, many authorities are now of the opinion that a classification of cytokine receptors would be more clinically and experimentally useful.

A classification of cytokine receptors based on their three-dimensional structure has therefore been attempted. (It must be noted that such a classification, though seemingly cumbersome, provides with several unique perspectives for attractive pharmacotherapeutic targets.)
* [[Immunoglobulin superfamily|Immunoglobulin (Ig) superfamily]], which are ubiquitously present throughout several cells and tissues of the vertebrate body, and share structural [[homology (biology)|homology]] with immunoglobulins ([[antibody|antibodies]]), [[cell adhesion molecule]]s, and even some cytokines. Examples: IL-1 receptor types.
* Haemopoietic [[Growth Factor]] (type 1) family, whose members have certain conserved motifs in their extracellular [[amino-acid]] domain. The IL-2 receptor belongs to this chain, whose γ-chain (common to several other cytokines) deficiency is directly responsible for the x-linked form of [[Severe Combined Immunodeficiency]] ([[X-SCID]]).
* [[Interferon]] (type 2) family, whose members are receptors for IFN β and γ. 
* [[Tumour Necrosis Factor]] (TNF) (type 3) family, whose members share a [[cysteine]]-rich common extracellular binding domain, and includes several other non-cytokine [[ligands]] like [[CD40]], [[CD27]] and [[CD30]], besides the ligands on which the family is named (TNF).
* [[7TM Receptor|Seven transmembrane helix]] family, the ubiquitous receptor type of the animal kingdom. All [[G-protein coupled receptors]] (for hormones and neurotransmitters) belong to this family.  It is important to note that chemokine receptors, two of which act<!--gerund needed here--> as binding proteins for [[HIV]] ([[CXCR4]] and [[CCR5]]), also belong to this family.

== Cysteine-knot cytokines ==
Members of the [[transforming growth factor beta superfamily]] belong to this group, including [[TGF beta 1|TGF-β1]], [[TGF beta 2|TGF-β2]] and [[TGF beta 3|TGF-β3]].

==References==

* Gallin J, Snyderman R (eds). Inflammation: Basic Principles and Clinical Correlates. 3rd edition, Philadelphia, Lippincott William and Wilkins, 1999.
* Janeway CA et al. (eds). Immunobiology. The immune system in Health and Disease, 4th edition, New York, Garland, 1999.
* Roitt I et al. (eds.) Immunology. 5th edition, London, Mosby, 2002.
* ''[[Science (journal)|Science]]'' Vol. 311 No. 5769, pp. 1875 - 1876, 31 March 2006 [[Digital object identifier|DOI]]: [http://dx.doi.org/10.1126/science.1126030 10.1126/science.1126030]

== See also ==
* [[Adipokine]]s
* [[Apoptosis]]
* [[Chemokine]]s
* [[Secretion assay|Cytokine secretion assay]]
* [[Cytokine storm]]
* [[ELISA]] assays
* [[ELISPOT]] assays
* [[Interleukin]]s
* [[Interferon]]
* [[Granulocyte-colony stimulating factor]]
* [[Signal transduction]]
* [[Tumor necrosis factors]]

==External links==
*[http://microvet.arizona.edu/Courses/MIC419/Tutorials/cytokines.html Cytokine Tutorial]
*[http://www-immuno.path.cam.ac.uk/~immuno/part1/lec09/lec10_99.html Cell Interactions: Cytokines]
*[http://www.emedicine.com/neuro/topic602.htm Reperfusion Injury in Stroke]. 
*[http://www.copewithcytokines.de Cytokines Online Pathfinder Encyclopaedia]
*[http://www.cytokines.ru Cytokines and inflammation, science and practical journal]

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