review article Medical Progress Airway Mucus Function and Dysfunction

The lungs are remarkably resistant to environmental injury, despite continuous exposure to pathogens, particles, and toxic chemicals in inhaled air. Their resistance depends on a highly effective defense provided by airway mucus,1-7 an extracellular gel in which water and mucins (heavily glyco-sylated proteins) are the most important components. Airway mucus traps inhaled toxins and transports them out of the lungs by means of ciliary beating and cough (Fig. 1). Paradoxically, although a deficient mucous barrier leaves the lungs vulner-able to injury, excessive mucus or impaired clearance contributes to the pathogen-esis of all the common airway diseases.1-4 This review examines the normal forma-tion and clearance of airway mucus, the formation of pathologic mucus, the failure of mucus clearance that results in symptoms and abnormal lung function, and the therapy of mucus dysfunction. S truc t ur e a nd Func tion of the Nor m a l A irwa y Epithelial surfaces in contact with the outside environment are protected by me-chanical barriers (e.g., keratinized skin) and chemical barriers (e.g., gastric acid). Mu-cosal surfaces are wet epithelia that have a mucous barrier as part of their protective mechanism.1-7 Mucus layers vary widely in composition and structure; for example, they are thick and adherent to the epithelium in the gut, but thin and mobile in the airway