Celiac disease is an autoimmune disease in which people can’t eat gluten because it will damage their small intestine. If you have celiac disease and eat foods with gluten, your immune system responds by damaging the small intestine. Gluten is a protein found in wheat, rye, and barley. It is found mainly in foods but may also be in other products like medicines, vitamins and supplements, lip balm, and even the glue on stamps and envelopes. Celiac disease affects each person differently. Symptoms may occur in the digestive system, or in other parts of the body. One person might have diarrhea and abdominal pain, while another person may be irritable or depressed. Irritability is one of the most common symptoms in children. Some people have no symptoms. (see also Dermatitis Herpetiformis).