Interleukin-5 Receptor Alpha (IL-5RA), also known as CD125, belongs to the Type 5 subfamily in the type I cytokine receptor family. It is composed of a ligand-specific alpha subunit and a signal-transducing beta subunit shared by the receptors for IL-3 and GM-CSF. IL-5RA is mainly expressed on eosinophils and basophils, and plays important roles in the immunobiology of these cell types. It is reported that when stimulated by IL-5, eosinophils down-regulate surface IL-5RA expression to attenuate their IL-5 responsiveness. Elevated IL-5 production may induce immune cell infiltration which leads to allergic inflammation. IL-5RA has also been reported to promote the differentiation of basophils and B cells.