ABSTRACT
The lungs serve two major functions: gas exchange and host defense. The very large surface area in contact with the external environment makes host defense a critical function. Many aspects of the anatomy and physiology of the lungs are well suited for host defense. The structure of the airways results in the deposition of inhaled microorganisms and particulate matter on the mucous lining of the airways, and airway epithelial cells are equipped with cilia that move the inhaled pathogens back up the airway into the pharynx, where they are swallowed. The epithelial cells are capable of responding to inhaled matter by generating regulatory cytokines, chemokines, and other mediators that can initiate an inflammatory response. Any pathogens or particulates that reach the alveolar spaces encounter the alveolar macrophages, which phagocytose and kill many pathogens without initiating recruitment of other leukocytes. The capillary blood contains a high concentration of neutrophils, often termed the marginated pool, which is readily recruitable if needed.
