The ileum is the final and longest segment of the small intestine, acting as the transition point before digested material moves into the large intestine. It follows the jejunum and completes the process of nutrient absorption that began in the upper small bowel. The ileum’s primary function is to reclaim specific substances the body needs to recycle or cannot absorb earlier in the digestive tract. This segment also plays a significant role in immune surveillance, guarding the body against the vast amounts of bacteria present in the gut.
Defining the Distal Ileum: Location and Anatomy
The ileum is the third segment of the small intestine, extending from the jejunum to the large intestine. While there is no clear boundary separating the jejunum from the ileum, it typically makes up about three-fifths of the small intestine’s total length. Its location is generally in the central and lower right regions of the abdomen.
The distal end of the ileum, often called the terminal ileum, connects to the cecum, the beginning of the large intestine. This connection is regulated by the ileocecal valve, a muscular sphincter that controls the flow of contents into the large bowel. The valve prevents the backflow of bacteria-rich material from the colon into the small intestine.
Histological differences exist between the ileum and the more proximal sections of the small bowel. The ileal walls are generally thinner, and its lumen is narrower compared to the jejunum. Structurally, the ileum has less prominent mucosal folds (plicae circulares) and fewer villi than the jejunum, reducing its surface area for general nutrient absorption. However, the ileum’s lining contains specialized structures, including aggregates of lymphoid tissue known as Peyer’s patches.
Specialized Absorption Functions
The ileum is uniquely equipped to absorb two specific substances not efficiently absorbed elsewhere in the digestive tract: Vitamin B12 and bile salts. This specialized absorption is the defining physiological function of the distal ileum.
Vitamin B12, or cobalamin, is an important nutrient required for red blood cell formation and neurological function. The absorption process is complex, requiring the vitamin to first bind to intrinsic factor, a protein secreted by the stomach. This intrinsic factor-B12 complex is then transported to the distal ileum, where specialized receptors on the epithelial cells, known as cubilin, recognize and take in the complex.
If the distal ileum is damaged or removed, the body cannot absorb Vitamin B12, leading to a deficiency and the development of pernicious anemia. This condition necessitates lifelong supplementation, usually through injections, as the oral absorption pathway is compromised. The ileum also recovers bile salts, which are compounds synthesized by the liver and released into the small intestine to aid in fat digestion.
Approximately 95% of the bile salts released are reabsorbed exclusively in the ileum via specific transport mechanisms. This process is part of the enterohepatic circulation, allowing the body to reuse the bile salts repeatedly, reducing the burden on the liver to synthesize new ones. If the ileum fails to reabsorb these salts, they pass into the large intestine, where they interfere with water absorption, potentially causing chronic diarrhea.
Immune System Role and Clinical Significance
Beyond its absorptive functions, the distal ileum is a highly active immune surveillance site within the digestive tract. This function is primarily concentrated in the Peyer’s patches, clusters of lymphoid tissue embedded in the ileal wall. These patches are a major component of the gut-associated lymphoid tissue (GALT) and contain various immune cells, including B cells, T cells, and macrophages.
Peyer’s patches function as sentinels, monitoring the vast population of bacteria and other antigens present in the intestinal lumen. Specialized cells within the patches, called M cells, sample antigens from the gut contents and present them to the underlying immune cells. This surveillance helps the body mount an appropriate immune response against harmful pathogens while maintaining tolerance to beneficial gut flora and food antigens.
The architecture of the Peyer’s patches makes the distal ileum a common target for specific infectious and inflammatory diseases. Inflammatory Bowel Disease, specifically Crohn’s disease, frequently affects this area, often referred to as terminal ileitis. This chronic inflammation is characterized by deep ulcerations and can lead to complications like strictures or fistulas.
Infectious agents also exploit the immune nature of this segment; for instance, the bacteria that cause typhoid fever, Salmonella enterica serotype Typhi, specifically target the lymphoid tissue of the ileum. The concentration of immune tissue in the distal ileum makes it a high-traffic area for both immune monitoring and the entry of specific pathogens. Consequently, the integrity of the terminal ileum is a major factor in intestinal health.