IL-5 is a homodimeric glycoprotein that was initially identified by its ability to support the in vitro growth and differentiation of mouse B cells and eosinophils. IL-5 induces eosinophil progenitor cell proliferation, terminal differentiation, and activation. In animal models of allergic diseases or helminth infection, IL-5 induces a massive proliferation of eosinophil progenitors in the bone marrow, promotes eosinophil recruitment with eotaxins, and prolongs eosinophil survival in local tissues. IL-5 regulates genes involved in B cell terminal differentiation. Thus, IL-5 induces CD38-activated splenic B cells to differentiate into immunoglobulin M-secreting cells and go through m to g1 class switch recombination at the DNA level, resulting in immunoglobulin G1 (IgG1) production. IL-5 binds the IL-5R complex, which consists of an IL-5Rα chain specific for IL-5 and a common β-chain that is shared by the receptors for IL-3 and GM-CSF. The alpha subunit is required for ligand-specific binding whereas association with the beta subunit results in increased binding affinity. IL-5 plays important roles in the pathogenesis of asthma, hypereosinophilic syndromes, and eosinophil-dependent inflammatory disease.