Innate immunity and the gut

The intestinal epithelium encounters a unique environment consisting of mic robes, both commensals and pathogens, as well as dietary nutrients and anti gens. This complex composition necessitates the presence of a dynamic syste m of defense to contain both pathogenic and commensal bacteria within the l umen yet allow for nutrient absorption. Tight junctions provide protection of the intercellular spaces while other surface molecules, such as intestin al trefoil factor, help to maintain the structural integrity of the epithel ium. Other more active processes, including upregulated expression and acti vation of antimicrobial peptides and enhanced fluid secretion, provide a se cond level of innate defense. Despite providing the interface between an ex uberant immune system and a highly antigenic lumenal environment, the intes tinal epithelium must remain quiescent; As such, several novel antiinflamma tory mechanisms were recently identified. Studies that elaborate the variou s aspects of these pathways are discussed in this review. (C) 2000 Lippinco tt Williams & Wilkins, Inc.