The gene is localized to the short arm of chromosome 19. It contains 11 exons, and spans 13390 bps on minus strand.

Normally expressed in B-cells. Its expression has been shown on T-cells, Langerhans cells, monocytes, macrophages, platelets, and eosinophils as well.
Through alternative splicing 2 major isoforms are produced, FCER2a (CD23a) and FCER2b (CD23b), both of which are expressed on B-cells. FCER2a is constitutively expressed, whereas FCER2b is induced by several cytokines, especially by IL4. There are 3 other splice forms, whose biological significance remains to be determined.

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Essential role in the regulation of IgE production as well as differentiation of B cells.

FCER2 is a Ca²⁺-dependent C-type lectin. The protein in humans contains 321 amino acids and has a molecular weight of 45 kDa. It has a single membrane-spanning domain. The extracellular domain consists of an alpha-helical coiled coil domain, a lectin head, followed by a short tail region containing an inverse RGD sequence. The coiled coil domain and the lectin head section are critical for the formation of trimers, and IgE binding, respectively. The RGD sequence is a common recognition site of integrins. Through an autocatalytic process involving matrix metalloproteases, the membrane-bound FCER2 is turned into a series of soluble fragments (sFCER2) of molecular weight ranging between 17 and 37 kDa. The soluble forms are also biologically active.

FCER2 expression was first described in the thymic medullary B-lymphocytes, and subsequently, in a population of large thymic B-cells with dendritic features which were referred to as asteroid cells. FCER2 is mainly expressed on B cells. It is also expressed on T cells, Langerhans cells, monocytes, macrophages, platelets, and eosinophils. On B cells 2 isoforms are present: a constitutively active form (FCER2a) and an inducible form (FCER2b).
FCER2 expression is upregulated by its own ligand, i.e. IgE. Furthermore, upon treatment with IL4, increased FCER2 expression can be observed in several cell types including eosinophils, neutrophils, macrophages, monocytes, and T cells. In addition, EBV induces CD23 transcription through a regulatory element in intron 1 of FCER2a.
FCER2 expression levels are differentially higher (70%) in mediastinal diffuse large B-cell lymphoma (med-DLBCL) and lower in nonmediastinal nodal DLBCL (15%) and extranodal DLBCL (9%).

Although FCER2 expression is mainly localized to the surface of B-lymphocytes, it is widely distributed on the surface of various other cells including follicular dendritic cells (FDC), and even airway smooth muscle cells.

FCER2 is the low-affinity immunoglobin E receptor. It mediates various biological functions. Mainly, it is involved in the down regulation of IgE production by B cells. However, because of its ability to associate with various ligands, its effects on IgE regulation might be stimulatory as well. In fact, FCER2 has been implicated in a variety of cellular functions ranging from cellular adhesion, antigen presentation, growth and differentiation of B and T cells, rescue from apoptosis, release of cytotoxic mediators, and regulation of IgE synthesis.
FCER2 displays susceptibility to proteolytic cleavage which produces a soluble form of FCER2. As the soluble FCER2 does not have the ability to bind to the cell surface, this results in disruption of the feedback inhibition mechanism and increase of IgE production. As a result, soluble FCER2 has mitogenic properties. Furthermore, the proteolytic cleavage of FCER2 is probably the basis of various allergic reactions.
These observations have been supported by cellular and animal models. Antibodies directed against the soluble FCER2 have been shown to inhibit the synthesis of IgE, while cross linking of membrane-bound FCER2 inhibits B-cell growth and differentiation. Furthermore, FCER2-deficient mice produce higher levels of IgE, whereas mice overexpressing membrane-bound FCER2 exhibit weaker IgE responses.
Lumiliximab is an anti-CD23 antibody that is proposed in the therapy of chronic lymphocytic leukemia (CLL). Lumiliximab binds to cell surface membrane FCER, and induce apoptosis. Although previous reports indicated activation of the caspase cascade, the exact mechanism for apoptosis remains to be elucidated.

At this time a paralogous gene is not known in humans. It does not show similarity to FCER1, the other member of the immunoglobin E receptor family.
Orthologues have been shown in mouse, rat and cow, with up to 58% homology at the protein level. However, some of the domains required for the proper function of human FCER2 is absent in orthologues. For example, the murine FCER2 does not have the RGD motif nor does it retain the IgE-binding affinity.

An arginine to tryptophane substitution of amino acid 62 (p.R62W) have been suggested to affect the receptor function and the mitogenic role of the protein. This suggestion was based on in vitro studies which showed the mutant protein was resistant to proteolytic cleavage following treatment with a broad range of proteases.

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