CD23 is a calcium dependent transmembrane glycoprotein which is expressed in many cell types. The cytoplasmic region of the protein has two isoforms – the 'a' isoform that participates in IgE-regulated endocytosis, and the 'b' isoform, which is involved in IgE-regulated phagocytosis. It has been found that there are at least four soluble forms of the CD23 protein (sCD23) all of which are produced by the activity of metallo/cysteine proteases. In addition to binding IgE, sCD23 have been shown to promote the activation and differentiation of B-cells and stimulate the release of pro-inflammatory cytokines from monocytes.