Soluble IgE receptors--elements of the IgE network.

Soluble isoforms of three human IgE Fc receptors, namely FcεRI, FcεRII, and galectin-3, can be found in serum. These soluble IgE receptors are a diverse family of proteins unified by the characteristic of interacting with IgE in the extracellular matrix. A truncated form of the alpha-chain of FcεRI, the high affinity IgE receptor, has recently been described as a soluble isoform (sFcεRI). Multiple soluble isoforms of CD23 (sCD23), the low affinity IgE receptor also known as FcεRII, are generated via different mechanisms of extracellular and intracellular proteolysis. The second low affinity IgE receptor, galectin-3, only exists as a secretory protein. We here discuss the physiological roles of these three soluble IgE receptors as elements of the human IgE network. Additionally, we review the potential and current use of sFcεRI, sCD23, and galectin-3 as biomarkers in human disease.