CD23, the low affinity receptor for IgE, belongs to the C-type lectin structural family and plays a role in the regulation of IgE synthesis and IgE mediated activities. It is found both as a transmembrane receptor protein and in a soluble form, which is generated by proteolytic cleavage of membrane bound CD23. The predominant soluble form of CD23 (sCD23) consists of 172 amino acids corresponding to the extracellular domain of the full length precursor. sCD23, in addition to binding IgE, also exerts a number of IgE independent activities, such as promoting the activation and differentiation of B-cells and stimulating the release of pro-inflammatory cytokines from monocytes. Recombinant human sCD23 is a 19.2 kDa non-glycosylated protein conta