Editing Cytokine (section)

== Receptors ==
{{Main|Cytokine receptor}}
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 has now been directly linked to certain debilitating immunodeficiency states. In this regard, and also because the redundancy and pleomorphism of cytokines are, in fact, a consequence of their homologous receptors, many authorities think 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. Such a classification, though seemingly cumbersome, provides 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]] with immunoglobulins ([[antibody|antibodies]]), [[cell adhesion molecule]]s, and even some cytokines. Examples: IL-1 receptor types.
* [[Hemopoietic growth factor|Hemopoietic 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.
* [[Tumor necrosis factor]]s (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.
* [[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. Chemokine receptors, two of which act as binding proteins for [[HIV]] ([[CD4]] and [[CCR5]]), also belong to this family.{{Citation needed|date=January 2011}}
* [[Interleukin-17 receptor]] (IL-17R) family, which shows little homology with any other cytokine receptor family. Structural motifs conserved between members of this family include: an extracellular fibronectin III-like domain, a transmembrane domain and a cytoplasmic SERIF domain. The known members of this family are as follows: IL-17RA, IL-17RB, IL-17RC, IL17RD and IL-17RE.<ref name="pmid19575028">{{cite journal | vauthors = Gaffen SL | title = Structure and signalling in the IL-17 receptor family | journal = Nat. Rev. Immunol. | volume = 9 | issue = 8 | pages = 556–567 | date = August 2009 | pmid = 19575028 | pmc = 2821718 | doi = 10.1038/nri2586 }}</ref>