The apical surface of the intestinal mucosa faces a luminal external milieu with remarkably diverse and changeable ecological conditions. This includes variable pH, electrolytes, nutrients, digestive enzymes, bile salts, and their micelles; transient alterations in luminal osmotic pressure; beneficial and pathogenic microbes and their metabolites; as well as toxigenic or potentially antigenic molecules [68,82]. Consequently, one of the central functions of the intestinal epithelium, intraepithelial lymphocytes, and lamina propria lymphoid and effector cells is to provide a barrier, protecting the internal environment from noxious or damaging elements of the external mileu of the intestine. A key component of this gut barrier concerns the mechanisms that exclude foreign antigens and toxigenic macro-molecules from the mucosa and internal environment [34,51,61,68,82]. In addition to its protective role in excluding antigens, the gut barrier permits some macromolecules that may be of benefit to the host to cross the epithelium. This 52includes orally derived antigen induction of systemic immune tolerance and absorption of potential trophic factors.