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).
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).
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. .[3]
^Brooks, Andrew J.; Dehkhoda, Farhad; Kragelund, Birthe B. (2017). "Cytokine Receptors". Principles of Endocrinology and Hormone Action. Springer International Publishing. pp. 1–29. doi:10.1007/978-3-319-27318-1_8-2. ISBN 9783319273181.
^Arimont A, Sun S, Smit MJ, Leurs R, de Esch IJ, de Graaf C (2017). "Structural Analysis of Chemokine Receptor-Ligand Interactions". J Med Chem. 60 (12): 4735–4779. doi:10.1021/acs.jmedchem.6b01309. PMC 5483895. PMID 28165741.
