Role of the alveolar macrophage in pulmonary bacterial defense

This review concerns the role of the alveolar macrophage as part of the coordinated mucociliary, macrophge and immune bacterial defense mechanisms of the lung. Alveolar macrophages are end-stage phagocytes that are derived from two precursor sources; an uncommitted pleuripotential hematopoietic stem cell, and a committed differentiated pulmonary precursor which can renew itself, as well as mature into functional alveolar macrophages. Sufficient numbers of alveolar macrophages are distributed throughout the lungs to ensure their proximity to any bacteria that penetrates alveolar regions. Studies with rodents have shown that these alveolar macrophages ingest, inactivate, and degrade inhaled microorganisms within eight hours of their entrance into alveolar regions. The biochemical mechanisms responsible for this antibacterial function involve the elaboration of chemotactic factors consequent to the interaction of bacteria, antibody, and complement, and the presence of bactericidal substances within the macrophage itself. Normally, these cellular mechanisms enable the alveolar macrophage system to maintain the lungs bacteria-free. However, if macrophage function is impaired due to pollutant or viral exposure, the host-parasite balance is upset and bacterial proliferation ensues. In such circumstances, polymorphonuclear leucocytes and additional macrophages enter the area of bacterial proliferation to produce the classical inflammatory reaction of pneumonia.