Pathophysiology Local Host Defenses

Local host defenses of both the upper and lower respiratory tract along with the anatomy of the airways are important in preventing infection. Upper respiratory defenses include the mucociliary apparatus of the nasopharynx, nasal hair, normal bacterial flora, IgA, and complement. Local host defenses of the lower respiratory tract include cough, mucociliary apparatus of the trachea and bronchi, antibodies (IgA, IgM, and IgG), complement, and alveolar macrophages. Mucous lines the cells of the respiratory tract forming a protective barrier for the cells that minimizes the ability of organisms to attach to the cells and initiating the infectious process. The squamous epithelial cells of the upper respiratory tract are not ciliated but those of the columnar epithelial cells of the lower tract are. The cilia beat in a uniform fashion upward, moving particles up and out of the lower respiratory tract.

Particles greater than 10 microns are efficiently trapped by mechanisms of the upper airway and are removed from the nasopharynx either by swallowing or by expulsion. The mucociliary apparatus of the trachea and bronchi along with the sharp angles of the bronchi, often are effective at trapping and eliminating particles that are

2 to 10 microns in size. Particles in the range of 0.5 to 1 micron may consistently reach the alveolar sacs of the lung. Microorganisms fall within this size range and if they reach the alveolar sacs, then infection may result if alveolar macrophages and other defenses cannot contain the organisms.

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