# Cytokine receptor

Cytokine receptors are receptors that bind to cytokines.
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 pleiotropy of cytokines are 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.

## Classification

A classification of cytokine receptors based on their three-dimensional structure has been attempted. (Such a classification, though seemingly cumbersome, provides several unique perspectives for attractive pharmacotherapeutic targets.)
* Type I cytokine receptors, 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).
* Type II cytokine receptors, whose members are receptors mainly for interferons.
* Immunoglobulin (Ig) superfamily, which are ubiquitously present throughout several cells and tissues of the vertebrate body
* Tumor necrosis factor receptor family, whose members share a cysteine-rich common extracellular binding domain, and includes several other non-cytokine ligands like receptors, CD40, CD27 and CD30, besides the ligands on which the family is named (TNF).
* Chemokine receptors, two of which acting as binding proteins for HIV (CXCR4 and CCR5). They are G protein coupled receptors.
* TGF-beta receptor family, which are Serine/threonine kinase receptors. Includes the TGF beta receptors

## Comparison

| Type | Examples | Structure | Mechanism |
| type I cytokine receptor | Type 1 interleukin receptors Erythropoietin receptor GM-CSF receptor G-CSF receptor growth hormone receptor prolactin receptor Oncostatin M receptor Leukemia inhibitory factor receptor | Certain conserved motifs in their extracellular amino-acid domain. Connected to Janus kinase (JAK) family of tyrosine kinases. Many have a FN-III superfamily domain and an immunoglobulin-like fold. | JAK phosphorylate and activate downstream proteins involved in their signal transduction pathways |
| type II cytokine receptor | Type II interleukin receptors interferon-alpha/beta receptor interferon-gamma receptor | Certain conserved motifs in their extracellular amino-acid domain. Connected to Janus kinase (JAK) family of tyrosine kinases. Many have a FN-III superfamily domain and an immunoglobulin-like fold. | JAK phosphorylate and activate downstream proteins involved in their signal transduction pathways |
| Many members of the immunoglobulin superfamily | Interleukin-1 receptor CSF1 C-kit receptor Interleukin-18 receptor | Share structural homology with immunoglobulins (antibodies), cell adhesion molecules, and even some cytokine. Includes with the two classes above. | |
| Tumor necrosis factor receptor family | CD27 CD30 CD40 CD120 Lymphotoxin beta receptor | cysteine-rich common extracellular binding domain | |
| chemokine receptors | Interleukin-8 receptor CCR1 CXCR4 MCAF receptor NAP-2 receptor | Seven transmembrane helix, rhodopsin-like receptor | G protein-coupled |
| TGF-beta receptor family | TGF beta receptor 1 TGF beta receptor 2 | Serine/threonine kinase receptors | Dimeric TGFBR2 binds to TGFB and phosphorylates TGFBR1, which phosphorylates the SMADs. See TGF beta signaling pathway. |

## Solubility

Cytokine receptors may be both membrane-bound and soluble. Soluble cytokine receptors are extremely common regulators of cytokine function. Soluble cytokine receptors typically consist of the extracellular portions of membrane-bound receptors. .