Figure 13.

Overview of cell types and innate immunity in the epithelium of the lung. The airway epithelium is a pseudostratified columnar epithelium involved in air conduction, lung moisture and protection. It lines the respiratory airways and is made up of different cell types. Basal cells serve as progenitor for other cells; goblet and serous cells produce and secrete antimicrobials and mucins as part of the airway surface liquid (ASL, composed of a periciliary layer and a more viscous upper mucosal layer); clara cells, with short microvilli, secrete defensive surfactants; and ciliated cells which act in mucociliary clearance. At the endpoint of the respiratory system, the alveolar epithelium that lines alveoli is a thin simple squamous epithelium made up of two cell types; AEC I are flattened squamous cells covering 95% of the surface, in close interaction with the capillaries to facilitate gas exchanges; and AEC II which are cuboidal cells producing and secreting the surfactants. Additionally, to ensure the host protection, many immune cells are resident; the DC, with their snorkels-shaped extension across epithelial tight junctions act as phagocytes and antigen presenting cells in contact with antigens; T cells, γδ T, and NKT cells are characterized by a rapid production and secretion of cytokines upon interactions with pathogens; and alveolar macrophages which are sentinel phagocytes in close proximity with AEC I and II in the alveoli. DC, dendritic cell; NKT, natural killer T cell; γδ T, gamma delta T cell; AEC I and AEC II, alveolar epithelial cells; SP-A, SP-D, surfactant protein A and D.