IL5RA and hypereosinophilic syndrome: The second-line treatments of idiopathic HES are hydroxyurea, interferon-α, imatinib, and some new agents such as monoclonal antibodies targeting IL-5/IL-5Rα (mepolizumab) and anti-CD52 antibody (alemtuzumab), but thus far mepolizumab is not available outside of clinical trials.[22]