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Respiratory System Defense Mechanisms Against Infection

Pneumonias, sometimes recurrent, occur with increased frequency in children who have defective immune or pulmonary defense mechanisms. Conditions that are associated with an increased risk of pneumonia include abnormalities of antibody production (e.g., agammaglobulinemia), deficient mucociliary function (e.g., cystic fibrosis and dysmotile cilia syndrome), and abnormalities of polymorphonuclear leukocytes (e.g., chronic granulomatous disease). Infants with congenital heart disease also have a propensity for developing recurrent pneumonias.

Mechanical Defense Mechanism

The mechanical defense mechanism of the lung provides filtration and clearance functions. In the nasopharynx, filtration occurs by trapping of material in the nasal hairs and mucous and by turbulence created as air passes through the irregular, tortuous channels of the nasal cavity. The nasopharyngeal filtering system predominantly traps particles that are greater than 5 μm in diameter. With mouth breathing, minimal filtration effect is provided by the angular course that incoming air follows in the posterior aspect of the oropharynx. Particles that gain access to the tracheobronchial tree are predominantly deposited on the walls of the airway, usually at sites of bifurcation where airflow is turbulent. In the peripheral aspects of the lungs, particle deposition occurs predominantly by sedimentation and diffusion.

Various clearance mechanisms are present in the respiratory system to remove particles from the airways. Sneezing clears nasopharyngeal material, and coughing removes particles in the trachea and large bronchi. Mucociliary activity is an additional important clearance mechanism that functions in the nasopharynx and tracheobronchial tree. The 3 components of the mucociliary clearance mechanism are mucous, cilia, and a coupling mechanism. The mucus that lines the tracheobronchial tree serves to trap particles, humidify inspired air, lubricate the airways, dilute toxic substances, and support neutrophils and macrophages. Secretion of mucus is by goblet cells that are interspersed among the ciliated epithelial cells of the airways. The cilia are the second major component of the mucociliary clearance mechanism. A corrugated beating motion of the cilia propels the overlying mucus in a cephalad direction within the airways. Effective propulsion relies on a viscous-chemical coupling mechanism that links the overlying mucus to the beating action of the cilia.

A variety of factors can result in compromise of the mechanical pulmonary defense mechanisms. The nasal filtration mechanism is bypassed when patients with an upper respiratory viral infection resort to mouth breathing because of nasopharyngeal secretions and mucosal edema. The pharyngeal and upper tracheal mechanical barriers are bypassed in patients with endotracheal or tracheostomy tubes. Patients with a depressed cough reflex have diminished clearance of secretions and the infectious agents trapped in those secretions. Viral and Mycoplasma infections can be toxic to the respiratory cilia, thereby compromising the mucociliary transport mechanism and increasing susceptibility to bacterial infections. Intrinsic abnormalities of the cilia result in compromised mechanical pulmonary defense in patients with cystic fibrosis and dysmotile cilia syndrome.

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