Celiac disease (CD) is an illness in which the body's immune system causes damage to the lining of the intestine (gut). It involves a reaction to gluten found in food. Patients may suffer abdominal pain and problems in many other organs and are at risk for cancer of the immune system (called lymphoproliferative malignancy or lymphoma). Eliminating all gluten from food is essential in patients with CD. If the disease is controlled, then the damage to the gut lining usually heals. Diagnosing CD involves a biopsy of the gut, and some patients have follow-up biopsies after they have had treatment to see if the disease has been controlled.