Host defense and bacterial pneumonia

Despite advances in antibiotic therapy, bacterial pneumonia remains a significant cause of morbidity and mortality. Pulmonary host defense has both an innate component, consisting of nonspecific antimicrobial factors, as well as an acquired component, which is pathogen-specific. Defects in either arm of the immune system can have a profound impact on the other because these are highly interactive systems. From the upper airway to the respiratory alveolus, defense mechanisms are in place to maintain sterility of the lower respiratory tract. These features include anatomical barriers, nonspecific antimicrobial peptides, the mucociliary escalator, and the airway lining fluid. In the airspaces, the alveolar macrophage is the cell responsible for early pathogen clearance and subsequent initiation of the acute inflammatory response. Neutrophil recruitment and acquired immune responses are dependent on cytokine secretion by these resident tissue phagocytes. This article reviews the salient features of innate and acquired immunity against bacterial pathogens and how host factors (such as alcoholism) undermine these antibacterial defenses.