Normally, the body's immune system is designed to protect it from foreign invaders. When people with celiac disease eat foods that contain gluten, their immune systems attack the lining of the intestine. This causes inflammation (swelling) in the intestines and damages the villi, the hair-like structures on the lining of the small intestine. Nutrients from food are absorbed by the villi. If the villi are damaged, the person cannot absorb nutrients and ends up malnourished, no matter how much he or she eats.