Legionella pneumophila infection is acquired by breathing aerosolized water contaminated with the organism. Healthy people are quite resistant to infection, but smokers and those with impaired host defenses from chronic diseases such as cancer and heart or kidney disease are susceptible. The organisms lodge in and near the alveoli of the lung, and their porin proteins bind complement component C3b, which aids phagocytosis of L. pneumophila by macrophages. A surface protein of L. pneumophila, macrophage invasion potentiator (Mip), aids entry into the macrophages. The bacteria are not killed by the phagocytes, but multiply within them and are released upon death of the macrophages to infect other tissues. Necrosis (tissue death) of alveolar cells and an inflammatory response result, causing multiple small abscesses, pneumonia, and pleurisy. Bacteremia is often present. Fatal respiratory failure, meaning that the lungs can no longer adequately oxygenate the blood or expel carbon dioxide, occurs in about 15% of hospitalized cases. Curiously, L. pneumophila infections remain confined to the lung in most cases. ■ porin proteins, p. 59

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