Immunologic aspects infections in the lung of surface The hallmark of cystic fibrosis is progressive bronchopulmonary d a m a g e associated with chronic infection with Pseudomonas aeruginosa, leading to respiratory failure and, ultimately, death. P. aeruginosa is an essentially nonvirulent organismin an immunocompetent host, but it has been shown to colonize the respiratory tract progressively in more than 80% of patients with CF. Patients with CF do not exhibit any evidence of immunologic compromise, and the infection is limited to the mucosal surfaces of the respiratory tract. Thus, P. aeruginosa in bronchopulmonary disease is a unique chronic mucosal infection resulting in a progressive pathologic process. It is therefore conceiv- able that locally induced alteration in the pulmonarymucosal defense is a major mechanism underlying the development of lung disease in patients with CF. An understanding of immunologic homeostasis in the mucous membranes and in the bronchopulmonaryepithelium should provide a better perspective on the factors contributing to the acquisition and chronicity of P. aeruginosa infection in the lower respiratory tract and the development of progressive pulmonary damage unique to CF. (J PEDIATR 1986;108(2):817-823) Pedro Piedra, M.D., a n d P e a r a y L. O g r a , M.D. From the Departments of Pediatrics and Microbiology, School of Medicine, State University of New York at Buffalo, and the Division of Infectious Diseases and Microbiology Laboratories, Children's Hospital, Buffalo, New York M U C O S A L I M M U N E S Y S T E M Most microbial agents and environmental macromole- cules gain access to the human host via the mucosal portals of the intestinal or respiratory tracts and, less frequently, through the genital tract or conjunctiva. Several secretory products and cellular elements in the mucosal epithelium determine the outcome of initial exposure to the microor- ganism in the mucosal lumen (Table I). Mucus glands, goblet cells, and epithelial secretions provide a mucus blanket, the characteristics of which are unique to the lung. The mucus coat functions, in part, as a "flytrap," providing an effective barrier against microbial penetration. Mucus protects against microbial attachment through specific chemical interactions with glycoproteins, glycolipids, and other surface moieties of the epithelium. Mucus blocks theadherence of microorganisms to the Reprint requests: Pearay L. Ogra, M.D., Children's Hospital, 219 Bryant St., Buffalo, NY 14222. epithelial surface, and enhances the shedding of microbial receptors from the mucosal epithelium. Recent evidence suggests that components mimic microbial receptors, resulting in decreased availability of microorganisms to adhere to the epithelial receptors. Mucus can promote bacterial growth by reducing phagocytosis and by impair- BALT Bronchus-associated lymphoid tissue CF Cystic fibrosis GALT Gut-associated lymphoid tissue IEL lntraepithelial lymphocyte MMC Mucosal mast cell PMN Polymorphonuclear leukocyte slgA Secretory lgA ing the host's ability to recognizebacteria as antigens (Table II). The epithelium of the respiratory and intestinal mucosa contains several distinct cellular components that contrib- ute to the initial sequence of host-pathogen interactions. These include intraepithelial lymphocytes, mucosal mast 817