Celiac disease is caused by a reaction to gluten, a protein found in wheat, rye, and barley. People with celiac disease who eat food with gluten develop an immune response to the protein. The immune response causes damage to the lining of their small intestine, which makes it difficult to digest food and absorb nutrients. Classic symptoms of celiac disease are abdominal pain, diarrhea, and weight loss. Some people with the disease have no symptoms at all. Some have only mild symptoms that are nonspecific, such as mild fatigue and weakness. Treatment is careful avoidance of foods that contain gluten.