final portion of this section discusses the various mediators of inflammation and hyperreactivity in airways and the ways in which these mediators may influence the airway response to inhaled allergens.

Epithelium and Other Airway Structural Tissues and Cells

EPITHELIAL CELLS

The resident cells and tissues of the upper and lower airways include the epithelial covering of the upper airways as well as the lining of the bifurcations of the bronchi and extending to the respiratory bronchioles. The epithelial cells that cover these structures are ciliated and known as epithelial brush-like cells (or type III epithelial cells). These bronchial brush cells have surface cilia that beat material and proteins caught in mucus toward the upper airway and out of the lower and upper airway system. A similar cell is the alveolar brush cell, which is found in the alveolar space (Hunninghake et al., 1979).

In addition to the type III alveolar epithelial or brush cells, two other types of epithelial cells line the bronchial mucosa or overlay the alveolar structure. Known as type I and type II alveolar epithelial cells, these have various and diverse functions. Both the bronchial brush cell with columnar epithelium (the type III epithelial cell noted above) and type I epithelial cells are end-stage differentiated cells that cover the vast majority of the respiratory epithelium. Type II alveolar epithelial cells are capable of dividing and may give rise to both type I and type III cells through their proliferative activity (Hunninghake et al., 1979). They are the source of phospholipid-rich surfactant, which maintains the integrity of the alveolar spaces; in addition, they may produce factors that are involved in nonspecific host defense (Hunninghake et al., 1979). Goblet cells or mucus-producing cells found in the bronchi are also prominent; such cells can produce materials, in particular, mucus, that blanket the bronchial brush cells and enhance their functioning.

SUBEPITHELIAL STRUCTURES

Below the overlay of epithelial cells in the respiratory mucosa is a rich network of subepithelial structures. Obviously, blood vessels are present, which contain both endothelium and smooth muscle; also to be found are other resident cells that are involved in maintaining ground structure. These include fibroblasts; mast cells in a resting or basal, nonactivated state; nerve cells; and a rich biochemical network of ground substances including collagen, fibronectin, and other structural proteins and proteoglycans that contribute to the integrity of the airways. Within the bronchi, structural integrity



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