This chapter discusses immunology of the gut, where the entire epithelial surface of the gut is lined with mucus, which provides a physical barrier. Digestive enzymes present in gastrointestinal secretions reduce most of the potential antigens to an innocuous state. The highly acidic environment of the stomach is incompatible with the survival of most microorganisms. The high turnover of gastrointestinal epithelium ensures that any damage to the mucosa would be promptly repaired, and any toxic agent trapped in the epithelium would be soon removed from the body. All substances absorbed from the gastrointestinal tract into the bloodstream have to pass through the liver before entering the general circulation. Peyer’s patches are dense aggregates of lymphoid tissue, similar in architecture to the lymph nodes or tonsils. The blood supply of Peyer’s patches is characterised by postcapillary venules which bring lymphocytes to it. The epithelium prevents large molecules from entering the mucosa.