Celiac disease is an autoimmune disorder that causes the immune system to interfere with the proper function of the small intestine, making it so the body cannot tolerate protein called gluten. With celiac disease, when gluten is eaten, the immune system responds by damaging cells and destroying the villi lining the small intestine. It is classified as both an autoimmune disorder, because the body is damaged by its own immune system, and a disease of malabsorption, since the body cannot absorb the nutrients it needs to function properly.