The contents of the gut are outside the body and are potentially dangerous to the body if they cross the gut wall. The contents include hydrochloric acid, protein-digesting enzymes and bacteria. We are protected from the gut contents by its mucosal lining and the cells (macrophages, dendritic cells, B-lymphocytes and T-lymphocytes) in the lymphoid tissue known as Peyer’s patches. The gut also contains ingested food molecules of protein, carbohydrate and triglyceride. Digestion (hydrolysis) is the disassembling of large food molecules and is necessary to reduce the very large polymerised molecules in food to particles small enough to pass into the cells lining the gut. The products of hydrolysis of proteins are amino acids and di- and tripeptides. Carbohydrates are hydrolysed into monosaccharides (glucose, fructose, galactose), while triglycerides are hydrolysed into free fatty acids and monoglycerides. Nucleic acids (DNA and RNA) are hydrolysed by pancreatic nucleases into nucleotides and then hydrolysed by brush border enzymes (nucleosidases and phosphatases) into their free bases, pentose sugars and phosphate ions.