<P> 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 . </P> <P> 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 . </P> <Ul> <Li> Immunoglobulin (Ig) superfamily, which are ubiquitously present throughout several cells and tissues of the vertebrate body, and share structural homology with immunoglobulins (antibodies), cell adhesion molecules, and even some cytokines . Examples: IL - 1 receptor types . </Li> <Li> 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). </Li> <Li> Interferon (type 2) family, whose members are receptors for IFN β and γ . </Li> <Li> Tumor necrosis factors (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). </Li> <Li> 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 . </Li> <Li> 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 . </Li> </Ul> <Li> Immunoglobulin (Ig) superfamily, which are ubiquitously present throughout several cells and tissues of the vertebrate body, and share structural homology with immunoglobulins (antibodies), cell adhesion molecules, and even some cytokines . Examples: IL - 1 receptor types . </Li>

What is a cytokine and what is its function