Mepolizumab is a fully humanized monoclonal antibody (IgG1/κ) targeting human interleukin-5 (IL-5), a key haematopoietin needed for eosinophil development and function. Mepolizumab blocks human IL-5 from binding to the α-chain of the IL-5 receptor complex on the eosinophil cell surface, thereby inhibiting IL-5 signalling.