This is a fully humanized IgG1 monoclonal antibody using the same sequences as the therapeutic antibody mepolizumab. It specifically targets interleukin-5 (IL5) and is adminstered in combination with other therapies for the treatment of severe asthma, eosinophilic granulomatosis with polyangiitis, and hypereosinophilic syndrome. Mepolizumab functions by antagonizing IL5 to reduce blood eosinophil levels, typically achieving a decrease of 60-90% from baseline depending on dosage. It has a relatively long half-life ranging between 16 and 22 days, which provides sustained therapeutic benefits and allows for a dosing schedule of once every four weeks. By binding to IL5 with a dissociation constant of 100pM, mepolizumab effectively prevents IL5 from interacting with and subsequently activating IL5R+ cells. This reduction significantly lowers circulating blood eosinophil levels, and thereby confers beneficial effects in patients suffering eosinophilic diseases.