As humans breathe air, the respiratory system is constantly screening the external environment. To maintain homeostasis and prevent inflammation, the airways must maintain their ability to remove inhaled microorganisms, allergens, and debris.

The respiratory system is divided into two segments: the conducting section and the respiratory section. From the nasal cavity to the bronchi, the majority of the respiratory tree is lined by pseudostratified columnar ciliated epithelium. The bronchioles are lined by simple columnar to cuboidal epithelium, while the alveoli have a thin squamous epithelial lining that facilitates gas exchange.

Within the respiratory system, there are four major histological layers: respiratory mucosa, which contains the epithelium and supporting lamina propria, submucosa, cartilage and/or muscle layer, and adventitia. Respiratory epithelium is a columnar ciliated pseudostratified epithelium that lines the majority of the respiratory tract; it is absent from the larynx and pharynx. The epithelium is pseudostratified; while it is composed of a single layer of cells along the basement membrane, the nuclei are not aligned in the same plane, giving the appearance of numerous layers. This unusual form of epithelium serves as a barrier to pathogens and foreign particles, but it also prevents infection and tissue harm through the utilization of the mucociliary elevator.