Host defenses and pathogenesis

The lung is protected by an array of interacting defense mechanisms.These include mechanical defenses that filter and remove particulates from the inhaled air as well as mechanisms to capture and remove particulates that penetrate into the lung as well as chemical and cellular defenses that can kill invading microorganisms. Optimal lung defense requires coordinated action of these various defenses. However, whereas failure of any defense mechanism increases the risk of infection, the various mechanisms provide, at least to a certain extent, redundant protection. As a result, many individuals with compromise of a single defense mechanism may not become clinically ill. Syndromes characterized by chronic infection of the lower respiratory tract are therefore associated with several defects in respiratory tract defense.