Structure and functions of CD23.

This review summarizes recent data on CD23, a low affinity receptor for IgE (FceRII). CD23 is the only FcR which does not belong to the immunoglobulin gene superfamily. The CD23 molecule was discovered independently as an IgE receptor on human lymphoblastoid B cells [1], as a cell surface marker expressed on Epstein-Barr-Virus-transformed B cells (EBVCS) [2] and as a B-cell activation antigen (Blast 2) [3]. CD23 was shown to be a low affinity receptor for IgE [4,5].Similar to most FcR, soluble forms of CD23 (sCD23) are released into extracellular fluids. The soluble fragments formed by proteolytic cleavage of surface CD23 are not only capable of binding IgE (IgE binding factors) but also exhibit multiple functions that are not IgE related. These observations together with the finding that CD23 displays significant homology with Ca2+-dependent (C-type) animal lectins, suggested the existence of natural ligands other than IgE. The recent finding that CD23 interacts with CD21, GD11b and CD1 1c indicates that...