The ileum (/ˈɪliəm/) is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds.

[6] By the fifth week of embryological life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the primary intestinal loop.

In roughly 2−4% of humans, this duct fails to close during the first seven weeks after birth, leaving a remnant called Meckel's diverticulum.

Therefore, the ileum has an extremely large surface area both for the adsorption (attachment) of enzyme molecules and for the absorption of products of digestion.

The DNES (diffuse neuroendocrine system) cells of the ileum secrete various hormones (gastrin, secretin, cholecystokinin) into the blood.

Cells in the lining of the ileum secrete the protease and carbohydrase enzymes responsible for the final stages of protein and carbohydrate digestion into the lumen of the intestine.

The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver.

It is of importance in medicine as it can be affected in a number of diseases,[8] including: In veterinary anatomy, the ileum is distinguished from the jejunum by being that portion of the jejunoileum that is connected to the caecum by the ileocecal fold.