[Physiology and pathophysiology of bronchial secretion]

The mechanisms of bronchial secretion are an important part of the innate defense system that protects the airways against pathogens and environmental toxins. Bronchial secretions are mainly produced by goblet cells and submucosal glands but also small amounts of surfactant from clara cells and some other fluids are part of the airway epithelium fluid. Together with the ciliary system the bronchial secretions are essential for the bronchial clearance ("mucociliary clearance"). Cilia beat within a periciliary layer with low viscosity ("sol-phase"). They move the overlying mucous sheet ("gel-phase") by their tips towards the nose to remove those mucous particles together with foreign material and pathogens from the airways. The gel-layer of the airway epithelium fluid is formed mainly by water, mucins (MUC) and free proteins. Mucins are highly glycosylated macromolecules, to date more than 18 different MUC-genes have been described. In addition the airway epithelium fluids contains many antibacterial proteins and peptides including lysozyme, lactoferrin, secretory IgA, complement, beta-defensines as well as many others. Acute inflammatory or toxic stimuli can promote hypersecretion of mucins mediated by a large variety of cytokines and chemokines or even directly like some toxins. Chronic inflammatory conditions like asthma or COPD are associated with hyperplasia of goblet cells and submucosal glands thus increasing the secretory capacity of the airways. The system of mucociliary clearance forms a functional unit together with the coughing mechanisms discussed elsewhere in this journal.