Role of leukocytes in lung defenses.

Alveolar macrophages are the primary cells that protect the lung against inhaled or aspirated microbes. They kill microorganisms intracellularly by both oxidative and non-oxidative mechanisms. Alveolar macrophages secrete antimicrobial factors, including lysozyme, peptides, and transferrin, which are found in the bronchoalveolar lining fluid and may kill microbes extracellularly. Macrophage secretory products help initiate a controlled inflammatory response and a limited influx of granulocytes, which appears to be important for complete elimination of many bacterial pathogens from the lung.