Pulmonary host defenses and oropharyngeal pathogens.

The lower respiratory tract is repetitively inoculated with oropharyngeal bacteria and yet pneumonia is an infrequent event. Efficient mechanisms of antibacterial defense are present in the respiratory tract that eliminate microbes before their presence or multiplication leads to disease in the majority of instances. Resident pulmonary defenses consist of aerodynamic defenses, the mucociliary apparatus, alveolar macrophages, complement, and surfactant. These resident defenses can be augmented by the development of an inflammatory response or the development of specific immunity. Significant species variability exists in the efficiency and mechanisms of clearance for oropharyngeal organisms. Streptococci are cleared promptly, Branhamella catarrhalis is cleared slowly, whereas non-typable Haemophilus influenzae multiply before being cleared. A dual phagocytic system of alveolar macrophages and recruited polymorphonuclear leukocytes is required for clearance of most oropharyngeal microbes. Systemic immunization can significantly enhance clearance of non-typable H. influenzae, suggesting immunoprophylaxis might be possible for this organism.