Lung mucosal immunity: immunoglobulin-A revisited

Mucosal defence mechanisms are critical in preventing colonization of the r espiratory tract by pathogens and penetration of antigens through the epith elial barrier. Recent research has now illustrated the active contribution of the respiratory epithelium to the exclusion of microbes and particles, b ut also to the control of the inflammatory and immune responses in the airw ays and in the alveoli. Epithelial cells also mediate the active transport of polymeric immunoglobulin-A from the lamina propria to the airway lumen t hrough the polymeric immunoglobulin receptor. The role of IgA in the defenc e of mucosal surfaces has now expanded from a limited role of scavenger of exogenous material to a broader protective function with potential applicat ions in immunotherapy. In addition, the recent identification of receptors for IgA on the surface of blood leukocytes and alveolar macrophages provide s an additional mechanism of interaction between the cellular and Immoral i mmune systems at the level of the respiratory tract.