The Intestinal Immune Barrier

Abstract

As the largest interface with the external environment, the mature intestinal epithelium has remarkable physiologic responsibilities. The intestine must function first as an organ of absorption and digestion while simultaneously protecting itself from the vast bacterial flora, array of food antigens, and physiochemical stresses caused by digestive and microbial products. Thus, the intestinal epithelium must be permeable to nutrients and fluids, yet remain impermeable to potentially injurious chemicals and pathogens. The intestine also must differentiate between commensal organisms whose growth is encouraged and enteric pathogens that need to be suppressed or eliminated. The single layer of cells comprising the intestinal epithelium is critical for maintenance of this dynamic barrier. The developing neonatal gut faces an additional daunting challenge of successfully negotiating the transition from a sterile lumen devoid of digestive and microbial metabolic products to the fully realized “bioreactor” of the adult gut. This review describes the unique structural features and elements of innate host defense elaborated by this dynamic tissue.