Celiac disease is a condition in which the immune system reacts abnormally to a protein called gluten, leading to damage to the lining of the small intestine. Gluten is found in wheat, rye, barley, spelt and many prepared foods and is a protein that makes food stick together (pizza dough is a good example). Since the small intestine is the organ responsible for food absorption, when the intestinal lining is damaged, nutrients and proteins do not get absorbed. In some people this can result in diarrhea, weight loss, iron deficiency, bone loss, swelling of the legs, but others may have constipation and bloating, or no symptoms at all, just abnormal blood tests.