small intestine
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The primary site of celiac disease — a 6-metre tube where almost all nutrient absorption happens, and where the immune attack unfolds.
The small intestine is the middle section of the digestive tract, running between the stomach and the large intestine. At ~6 metres long, it is responsible for the majority of nutrient digestion and absorption. Celiac disease primarily targets the proximal (upper) portion of the small intestine.
Three Sections
Duodenum (~25 cm) The first and shortest segment, immediately after the stomach. This is where:
- Pancreatic enzymes and bile are introduced to digest food
- Most iron and calcium absorption occurs
- The immune attack in celiac disease is most intense
- Biopsies for celiac are taken (beyond the duodenal bulb — the very first segment)
Jejunum (~2.5 m) The middle segment. Site of most carbohydrate, protein, and fat-soluble vitamin absorption. Also heavily affected in active celiac disease. Alternative biopsy site.
Ileum (~3.5 m) The terminal segment. Absorbs vitamin B12 (with intrinsic factor) and bile salts. Less severely affected in standard celiac disease, but damaged in severe or longstanding cases.
Why the Duodenum and Jejunum Are Hit Hardest
The proximal small intestine receives the highest concentration of gliadin from recently digested food — before much dilution or motility has occurred. It also has the highest density of HLA-DQ2/DQ8-bearing immune cells in the lamina propria. The combination of high antigen concentration and immunologically primed tissue makes the proximal gut the epicentre of celiac damage.
Structure Relevant to Celiac
- villi — finger-like projections maximising absorptive surface area; destroyed in villous-atrophy
- Crypts of Lieberkühn — produce replacement enterocytes; enlarge in crypt-hyperplasia
- tight-junctions — seal gaps between enterocytes; loosened by zonulin
- Lamina propria — connective tissue beneath the epithelium; contains immune cells (T cells, plasma cells, macrophages) that drive the immune attack
- IELs — immune sentinels embedded in the epithelium
Recovery
On a strict gluten-free-diet, the small intestinal lining gradually regenerates. Villi regrow, crypts normalise, and IEL counts fall. Complete mucosal healing takes months to years — and is slower in adults than children.
Related Concepts
villi | enterocytes | villous-atrophy | malabsorption | intraepithelial-lymphocytes | endoscopy-biopsy | tight-junctions