Human interleukin 5 (IL5) is the major hematopoietin that stimulates the proliferation, migration and activation of eosinophils and is implicated in the pathogenesis of inflammatory and other myeloproliferative diseases. IL5 functions through the signaling of a common receptor subunit β (βc), in a receptor activation process that requires initial recruitment of an IL5 specific receptor subunit α (IL5Rα), for cytokine presentation to βc. Important advances have been made to understand molecular mechanisms of cytokine recognition and receptor antagonism. Mutational studies indicate that a pair of charge complementary regions play an essential role in specific interaction between IL5Rα and IL5. Moreover, peptide studies with the IL5 system have identified a cyclic peptide inhibitor, AF17121, which binds specifically to IL5Rα by mimicking the cytokine. A key receptor-recognition pharmacophore has been identified in this peptide inhibitor, and sites of inhibitor recognition can be proposed in the homology-deduced structural model of IL5Rα. These results provide an experimental platform to derive enhanced-potency peptidomimetic inhibitors. Such inhibitors have potential use as tools to evaluate the role of eosinophilia in disease and as potential leads to antagonists to treat hyper-eosinophilic diseases such as eosinophilic esophagitis, asthma and chronic myeloproliferative leukemias.