Gluten is a blend of proteins found naturally in wheat, rye, barley, and hybrids of these grains such as triticale. A gluten-free diet is a diet that excludes all foods that contain or are contaminated with gluten to control the symptoms of celiac disease and other gluten-related conditions. Celiac disease is an autoimmune disease in which gluten triggers an immune system activity that damages the lining of the small intestine. Over time, this damage prevents nutrients from being absorbed from food.