The role of airway submucosal glands in the airway mucosal defense system

Despite constant inhalation of air-borne particles including a variety of microbes and antigens, the normal lungs hardly ever develop infection or airway injury. This is because the normal lung is equipped with sophisticated defense mechanisms against foreign substances. It has been reported that the airway mucosa, especially the submucosal glands, play important roles not only in nonspecific defense using airway secretions but also in specific defense in cooperation with immune cells. In contrast to the nasopharyngeal or intestinal mucosa, which is always exposed to many foreign antigens, the mucosal surface of the lower respiratory tract in known to be kept in a germ-free condition. This fact indicates that immunological information derived from the antigen-rich mucosa, such as the intestine, might be transmitted to the airway mucosa, thus resulting in efficient removal of unwanted substances. This immunological elimination requires specific antibodies (Abs) against harmful antigens, and the major population of Abs in the airway is dimeric IgA. Airway submucosal glands synthesize a secretory component (SC), a transporter of secretory IgA, and immunoglobulin-containing plasma cells have been identified preferentially around the glandular tissue. Overall, the submucosal glands play a key role in the integrity of airway mucosal immunity.