IL-5 receptor alpha (IL-5 Rα), also known as CD125, is a subunit of the functional heterodimeric IL-5 receptor complex (1). IL-5 is produced primarily by activated T cells and mast cells, and has diverse biological effects on a variety of cell types. It is a potent eosinophil differentiation and activation factor. The IL-5Rα chain is exclusively expressed by eosinophils, some basophils and murine B1 cells or B cell precursors (2). The high affinity receptor complex for IL-5 consist of the ligand binding α chain, IL-5 Rα, and the common beta chain, CD131 (3). The α-chain exclusively binds IL-5 and the intra-cellular portion of IL-5 Rα is associated with JAK2, a protein tyrosine-kinase essential in IL-5 signal transduction (4-5). The beta subunit is shared with the high affinity receptor complexes IL-3 and GM-CSF. Like many other cytokine receptors, alternative splicing of the IL-5Rα gene results in expression of either a membrane bound or soluble form. The soluble form does not lead to signal transduction and therefore has an antagonistic effect on IL-5 signaling. Soluble IL-5 Rα up-regulation has been shown to be involved with the eosinophil-associated disease nasal polyposis (6).

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