Mepolizumab is a monoclonal antibody that affects the actions of the body’s immune system. Mepolizumab is used together with other medicines to help control severe asthma in adults and children who are at least 12 years old. Mepolizumab is an interleukin-5 antagonist. It blocks the binding of interleukin-5 (a molecule produced by immune cells) to the eosinophils and thereby reduces the production, maturation, and circulation of the eosinophils.