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Countless processes operating in the body progressively fail with age, each one an inconvenience at the outset, and many turning from that to an ultimately fatal reduction in vital capabilities over the decades of later life. The defense against pathogens offered by innate immune functions, including generation of mucus to trap and expel pathogens, holds up relatively well with advancing age, in comparison to many organs, but it is nonetheless is reduced in capacity with age. Researchers note here that the mucosal systems of the lung suffer detrimental changes with aging, with the consequence of increased vulnerability to inhaled pathogens and particles.
The lung is exposed to a myriad of substances with every breath we take. To protect itself from pollutants, dust, particulate matter, allergens, viruses, bacteria, and fungi that exist in the air around us, the lung has evolved a highly tuned innate immune system. One of the first lines of defense against inhaled matter is mucociliary clearance, which is performed by the airway epithelium of the trachea and the central conducting airways.
The conducting airways of the lung are lined by ciliated airway epithelial cells. The ciliated cells are covered by a thin periciliary sol layer that is approximately the same height as the cilia. The periciliary layer is low viscosity and facilitates ciliary beating. Interspersed with the ciliated cells are mucus-producing cells. In the conducting airways, goblet cells are the most prevalent mucus-producing cell. The apical cytoplasm of goblet cells is filled with membrane-bound secretory granules filled with mucins. These granules are secreted to form the mucus layer. Mucus is a thick, gel-like material that consists of water, salts, mucins, proteoglycans, lipids, and proteins. This blanket of mucus is free-floating over the respiratory epithelium. When an insoluble foreign substance is inhaled and deposits in the airway, it is trapped in a blanket of mucus. The cilia then beat in a coordinated manner to expel mucus from the lungs.
Cough is also an important mechanism of clearing the airways. In humans, cough increases as mucociliary clearance slows. Impairment in cough sensitivity can lead to recurrent pneumonia. Mucociliary clearance can be affected by changes in the quantity, viscosity, or composition of mucus or changes in ciliary number, structure, beating, or coordination. Aging can cause changes in many of the mucociliary clearance apparatus components, include structural changes in the airway epithelium, changes in cilia function, as well as changes in mucus quality, leading to a higher propensity for chronic lung disease and infection with aging.