Interleukin 5 (IL-5) is predominantly produced by Th2 cells to regulate differentiation and proliferation of eosinophils. As the major cytokine in eosinophil development, IL-5 induces eosinophil migration, activation, and survival. The primary source of IL-5 is TH2 lymphocytes, but mast cells are also a source in the airways. IL-5 is increased in bronchial biopsies and BAL fluid and serum from symptomatic asthmatic patients, knockout of IL-5 in mice eliminates AHR and eosinophilia, and overexpression or treatment with rIL-5 results in asthma-like lung histopathology. Blockade of IL-5 in humans with mepolizumab, an anti-IL-5 antibody, decreases eosinophils in the sputum and general circulation, and in asthma patients exhibiting hypereosinophilia and resistance to steroid therapy mepolizumab reduces exacerbations.