People who have celiac disease cannot tolerate gluten, a protein in
wheat, rye, and barley. Gluten is found mainly in
foods but may also be found in
everyday products such as medicines, vitamins, and lip balms.
The small intestine is shaded above.
When people with celiac disease eat foods or use products containing gluten, their immune system responds by damaging or destroying villi—the tiny, fingerlike protrusions lining the small intestine.