CD4 and airway hyperresponsiveness: Asthma is characterized by reversible airway obstruction, airway hyperresponsiveness (AHR), increased production of immunoglobulin isotype IgE, airway inflammatory cell infiltrates of eosinophils and CD4+ T helper type 2 (Th2) lymphocytes, mucus hypersecretion, and airway remodeling (e.g., epithelial mucous cell metaplasia, intramural interstitial fibrosis).