Celiac disease is an autoimmune disorder that causes a serious allergic reaction to gluten. Gluten is a protein found in wheat, barley, and rye. It's also found in certain products, including some toothpastes, lipsticks, and medicines. A celiac disease test looks for antibodies to gluten in the blood. Antibodies are disease-fighting substances made by the immune system. Normally, your immune system attacks things like viruses and bacteria. If you have celiac disease, eating gluten makes your immune system attack the lining of the small intestine, as if it were a harmful substance. This can damage your digestive system and may prevent you from getting the nutrients you need.