Serious respiratory tract infections are rare in the healthy individual and most of the nuisance morbidity that occurs results from nasopharyngeal viral infections that many people get once or twice a year. The economic impact from these upper respiratory tract infections is appreciable, however, in terms of absenteeism from school or work, but unfortunately there is little that can be done to ward them off in a practical way. Pneumonia is an infrequent lifetime experience for most non-smoking adults and when it occurs, unusual circumstances may pertain-a particularly virulent microorganism is in circulation, or perhaps one has been exposed to a newly recognized germ, such as has occurred with Legionella species in the past 8 years or so. What protects us the great majority of the time is a very effective network of respiratory tract host defenses. These include many mechanical and anatomical barrier mechanisms concentrated in nose and throat; mucociliary clearance, coughing and mucosal immunoglobulins in the conducting airways and in the air-exchange region of the alveolar structures, phagocytes, opsonins, complement, surfactant and many other factors combine to clear infectious agents. The ability to mount an inflammatory response in the alveoli may represent the maximal and ultimate expression of local host defense. In some way these host defenses are combating constantly the influx of micro-organisms, usually inhaled or aspirated into the airways, that try to gain a foothold on the mucosal surface and colonize it. But many general changes in overall health such as debility, poor nutrition, metabolic derangements, bone marrow suppression and perhaps aging promote abnormal microbial colonization and undermine the body's defenses that try to cope with the situation. It is a dynamic struggle. The departure from normal respiratory health may not be obvious immediately to the patient or to the physician and repeated episodes of infection or persisting symptoms of cough, expectoration and sinus or ear infections may develop before serious assessment of the situation is taken and appropriate diagnosis gotten underway. Obvious explanations for respiratory infections may be apparent and, nowadays, side effects from antineoplastic chemotherapy or immunosuppressive therapy for a variety of diseases that create an immunocompromised host are common. In a few subjects, especially young adults who present with a cumulative history of frequent but mild infections in childhood and youth, a subtle deficiency in host defenses may exist and have been partially masked because of attentive pediatric medical care and prompt use of broad spectrum antibiotics. Some of these situations have been reviewed here, emphasizing diseases of cilia ultrastructure causing faulty clearance of airway secretions, cystic fibrosis and immunoglobulin deficiencies especially of IgG subclasses and functional destruction of secretory IgA by bacterial proteases. A number of "situational" pneumonias can occur when alveolar macrophages are confronted with intracellular parasites or bacteria that they cannot kill readily unless cellular activation through cell-mediated immunity develops. Such opportunistic infections with Pneumocystis carinii and Legionella pneumophilia highlight the problem, but other related problems with phagocytosis and intracellular killing can result from insufficient opsonic antibodies and lack of other components of humoral and complement cascade mediated immunity. Sometimes the fault lies in generating a vigorous inflammatory reaction. Usually granulocytopenia as part of bone marrow insufficiency is the cause, but other intrinsic defects in polymorphonuclear granulocyte function can be found, too. The message is to be alert and suspect the unexpected when dealing with troublesome respiratory infections that do not fit the norm or have a peculiar natural history.
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