J Widdicombe
Department of Physiology, St. George's Hospital Medical School, London, United Kingdom.

Airway surface liquid (ASL) is complex and comes from many sources, in particular glands and epithelium. The mucoglycoproteins present bind to bacteria. Bacterial membranes contain adhesins that bind to receptors on the mucus. The bound bacteria multiply and release toxins that diffuse to the epithelium and damage or destroy it, inhibiting mucociliary transport. The damaged epithelium releases products such as phospholipids into the ASL. These change the physical properties of the mucus and also promote mucus secretion, which may block smaller airways. Airway surface liquid contains constituents such as immunoglobulins, lysozyme, and lactoferrin and neutrophil products such as proteases that act on bacteria. Few bacteria adhere to healthy epithelium. To adhere, most require damaged tissue with membrane receptors that encourage bacterial invasion. If the epithelium is destroyed, bacteria adhere to the basement membrane or extracellular matrix. A damaged epithelium can also cause hyperresponsiveness of airway secretory mechanisms, with increased gland secretion that in turn interacts with the bacteria.

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