A tissue transglutaminase IgA (tTg-IgA) test is used to help doctors diagnose celiac disease. In this autoimmune disorder, the immune system mistakenly thinks that gluten — a protein in wheat, barley, rye, and oats — is a foreign invader. It makes antibodies that attack an enzyme in the intestines called tissue transglutaminase (tTG). Antibodies (also called immunoglobulins) are proteins that recognize and get rid of germs.