This course is intended for further in-depth study of the gastrointestinal physiology and immunology. Functions of the gastrointestinal tract include motility, digestion, absorption, secretion and barrier. The GI tract plays dual roles, one serving as the first line for nutrient uptake and the other as a defense system against microbial pathogens. The gastrointestinal tract is the largest reservoir of commensal bacteria in the human body, providing nutrient and space for the survival of microbes while concurrently operating mucosal barriers to confine and restrict the microbial population. The turnover rate of intestinal epithelial cells along the crypt/villus axis that involves processes of proliferation, differentiation, apoptosis and shedding is crucial for maintenance of gut homeostasis. Recent scientific studies suggest dynamic interaction among enteric nervous system, immune system and the gastrointestinal mucosa. The lymphoid tissues in the gastrointestinal tract contain the largest number of immune cells in the body, termed gut-associated lymphoid tissue (GALT). The gastrointestinal immunity protects against invasion of potential pathogens, and provides oral tolerance to food proteins without developing an adverse immune response to unharmful dietary particles. The balance between immunity and tolerance in the gut plays a crucial role in the pathogenesis of various gastrointestinal diseases.