Immunoglobulin E (IgE) plays a central role in the development of allergic diseases. In sensitized individuals, IgE antibodies bind to receptors on mast cell and basophil surfaces, releasing preformed and newly generated mediators that initiate an immunologic cascade and inflammatory symptoms. Omalizumab (Xolair) is a humanized monoclonal antibody designed to bind specifically to IgE. It was approved by the United States Food and Drug Administration in 2003 for the treatment of patients with moderate-to-severe persistent asthma that is inadequately controlled with inhaled corticosteroids (ICS) and who have a positive skin test or in vitro reactivity to a perennial aeroallergen. In clinical trials in such patients, omalizumab reduced the incidence of asthma exacerbations, severe exacerbations, the use of rescue medication, and improved both symptoms and quality of life (QOL).