An allergic inflammatory disease of the airways, involving mast cells, eosinophils, macrophages, fibroblasts, and neutrophils. Such inflammatory changes are associated with widespread airflow obstruction, which is variable and improves (reverses) spontaneously or with appropriate therapy. Inflammation progresses to increased airway irritability (hyperresponsiveness) and episodes of airflow obstruction induced by the inhalation of allergens, cold air, and occupational factors. Although a bronchospasm can be induced immediately after exposure to a specific allergen in an appropriately sensitised recipient, it is the late allergic response that most resembles the inflammatory reaction occurring in asthma. Central to this reaction is the release from mast cells, eosinophils, and lymphocytes of chemical mediators such as histamine, leukotrienes (potent bronchoconstricting agents), prostaglandins, and various cytokines that perpetuate the response. Potent neurohumoral agents derived from neural pathways contribute further to the bronchospasm.