Pulmonary defense mechanisms . Abrupt changes in direction of airflow in the nasal passages can trap potential pathogens. The epiglottis and cough reflex prevent introduction of particulate matter in the lower airway. The ciliated respiratory epithelium propels the overlying mucous layer (right) upward toward the mouth. In the alveoli, cell-mediated immunity, humoral factors, and the inflammatory response defend against lower respiratory tract infections. (C, complement.) Adopted from Schaechter.