Respiratory tract 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 mucodliary apparatus of the nasopharynx, nasal hair, normal bacterial flora, IgA antibodies, and complement. Local host defenses of the lower respiratory tract include cough, mucodliary apparatus of the trachea and bronchi, antibodies (IgA, IgM, and IgG), complement, and alveolar macrophages. Mucus lines the cells of the respiratory tract, forming a protective barrier for the cells. This minimizes the ability of organisms to attach to the cells and initiate the infectious process. The squamous epithelial cells of the upper respiratory tract are not ciliated, but those of the columnar epithelium of the lower tract are. The cilia beat in a uniform fashion upward, moving particles up and out of the lower respiratory tract. 

Similar to acute otitis media, acute sinusitis usually is preceded by a viral respiratory tract infection that causes mucosal inflammation." " This can lead to obstruction of the sinus ostia—the pathways that drain the sinuses. Mucosal secretions become trapped, local defenses are impaired, and bacteria from adjacent surfaces begin to proliferate." " " The pathogenesis of chronic sinnsitis has not been well studied. Whether it is caused by more persistent pathogens or there is a snbtle defect in the host s immnne fnnction, some patients develop chronic symptoms after their acnte infection." " " ... 

To develop pneumonia, virulent microorganisms must invade the lung parenchyma, either as the result of a defect in defense mechanisms of the host or by an overwhelming inoculum. The normal human respiratory tract has a variety of defense mechanisms such as anatomic barriers, cough reflex, cell and humoral-mediated immunity, and a dual phagocytic system... 

Figure 1 Summary of risk factors contributing to colonization and infection of the lower respiratory tract Important risk factors include the inoculum and virulence of the infecting agents, response of the pulmonary host defenses in the lung. (From Ref 181, with permission.)...

Devices that circumvent host defenses and facilitate the transmission of bacteria into the lung have been identified as risk factors for NP (17,22,24,29-31,35,41,55,59,74,128) (Tables 3 and 4). This includes devices in contact with the respiratory tract for diagnostic purposes (e.g., bronchoscopes) or therapeutic purposes (e.g., endotracheal tubes, nebulizers). Devices are readily colonized by nosocomial bacteria with transmission frequently occurring by way of contaminated hands of health care workers from other patients, devices, or other body sites of the same patient. Certain devices also predispose the patient... 

Several factors could explain the possible association between bronchoscopy and VAP, including the use of large volumes of BAL that impede the clearance of bacteria from the lower respiratory tract and the introduction of nosocomial pathogens into the lower airway by dislodging bacterial aggregates from the endotracheal tube surface. As discussed previously, when bacteria encased in biofilm embolize to different areas of the lung, they may be particularly difficult for host defenses to clear effectively (145,146). These pathogenic mechanisms are theoretical, and prospective cUnical studies are required to evaluate the actual risk of bronchoscopy in ventilated patients. 

If these first lines of defense are sufficiently stimulated or breached by an inhaled agent, the respiratory tract usually responds with the recruitment of additional phagocytic monocytes and neutrophils an increased output of protective fluids, cytokines, and mediators and for antigenic substances, the development of speciflc immunity. Most of these defense mechanisms are appropriate and protect the host from permanent injury, although the immune and inflammatory... 

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