Small airways are defined as bronchial tubes with an internal diameter of less than 2mm, which corresponds to the seventh or eighth bronchial generation.

Small airways are located at the interface between the alveoli and the conducting airways. Airway epithelial cells form a continuous lining in the airways and play a unique role as a protective physical and functional barrier against external deleterious agents. Small airway epithelial cells (SAEpiC) regulate immune responses by contributing to host defense through the production of chemokines and the expression of adhesion molecules. They also produce fluids that contribute to lung fluid balance. Many airway diseases, such as asthma, bronchiolitis, chronic obstructive pulmonary disease and cystic fibrosis, involve damage to the surface epithelium of the airways. The study of human SAEpiC may help identify new therapeutic options to prevent airway disorders.