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Anatomy and Pathophysiology of Normal Host Defense Mechanisms

The respiratory tract (Fig. 102-1) is in constant contact with the environment and exposed to direct inoculation by infectious and noninfectious agents. To defend itself against these agents, the respiratory tract is equipped with several types of defense mechanisms including both mechanical and immune-mediated mechanisms. Large size airborne particles (>5 microns) are filtered by the nose and are trapped by the nasal cilia. Intermediate size (1–5 microns) particles are deposited in the trachea and bronchi, and small size particles (0.01–1 microns) and infectious agents often are deposited in the bronchioles and the alveolar space (Fig. 102-2).

Figure 102-1

Anatomy of the respiratory tract.

Figure 102-2

Schematic diagram of the airway.

The respiratory tract from the trachea to the bronchioles is lined with ciliary cells and goblet cells that secrete a thin layer of mucus. The rhythmic beat of the cilia move the mucus-trapped material upward, which is then cleared externally by the cough mechanism or swallowed interiorly and eliminated by the gastrointestinal tract. This mechanism is called the mucociliary escalator (Fig. 102-3) and is a very important defense mechanism that plays a major role in clearing infectious and noninfectious particles from the respiratory tract.

The alveolar macrophages are another important line of defense. Macrophages clear bacteria and nonliving particles that reach the alveolar space through phagocytosis and digestion by cellular lysosomes. Opsonization, which involves coating of the invading agents by antibodies secreted by lymphocytes present in the mucosa, can further enhance phagocytosis. The alveolar macrophages also secret cytokines upon interaction with foreign particles that help recruit and activate neutrophils, lymphocytes, and other inflammatory cells to clear the foreign material.

The lungs and pleura are also richly supplied by an extensive lymphatic drainage system that helps transport phagocytosed particles and infectious agents out of the lungs to the regional lymph nodes where certain infectious agents (mycobacteria, fungi) can remain in a latent phase potentially for prolonged periods of time.

The interaction between host and pathogen factors determines if an infection is established or cleared. Certain host factors or conditions like altered mental status and alcoholism, for example, may predispose to the development of aspiration pneumonia as a result of the absence of protective gag reflexes. Other factors like endotracheal intubation or chest tube placement may predispose to ventilator-associated pneumonia and surgical wound infection, respectively, by bypassing normal host defenses. Infection could also be initiated when pathogen virulence factors overwhelm the host defense mechanisms.

In this chapter, we discuss lung infections associated with impaired host defense mechanisms ...

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