Skip to content
Explore atlas
PH-GI04

Crohn's disease

How an immune response to gut bacteria in susceptible people inflames the whole thickness of the bowel wall in patches anywhere from mouth to anus, and why that leads to strictures, fistulae and malabsorption.

By the end, you can

  • Describe the layers of the gut wall and the barrier that keeps bacteria away from the immune system.
  • Explain how genetic susceptibility, the microbiota and Th1/Th17 responses drive chronic inflammation and granulomas.
  • Explain what transmural inflammation and skip lesions mean, and why the terminal ileum is so often affected.
  • Relate deep fissuring ulcers and fibrosis to fistulae, abscesses and strictures, and ileal disease to B12 and bile-salt loss.
  • Recognise the classic gross and microscopic features, the Montreal classification and the key tests of Crohn’s disease, and explain how each main treatment acts on its mechanism.

What it is

Crohn’s disease is a chronic, relapsing inflammatory bowel disease in which inflammation extends through the whole thickness of the bowel wall (transmural) and affects patches of the gut anywhere from mouth to anus, most often the terminal ileum and colon, with normal bowel between (skip lesions).

4 types to tell apart
Inflammatory
Inflammation without narrowing or penetration; the usual pattern at diagnosis.
Stricturing
Repeated inflammation and healing lay down fibrous tissue and thicken the muscle, fixing a narrowing of the lumen.
Penetrating
Deep fissures pass through the wall to form fistulae or abscesses.
Perianal disease
Fistulae, abscesses, fissures and skin tags round the anus; may occur with or without disease elsewhere.

Normal function

The gut wall and its barrier

The gut wall has four layers: the mucosa, the submucosa, the muscularis propria (inner circular and outer longitudinal muscle) and the serosa. The mucosa is separated from the submucosa by the thin .

A single layer of epithelium separates the immune system from trillions of bacteria. Mucus from , antimicrobial peptides from at the bases of the , tight junctions and secreted IgA keep bacteria at a distance. Immune cells in the and lymphoid aggregates (Peyer’s patches, most numerous in the ileum) sample what gets through and normally respond with tolerance rather than attack.

The terminal ileum also has two special jobs: it absorbs vitamin B12 bound to intrinsic factor, and it reclaims bile salts so the liver can reuse them.

Point to it on the model