Celiac disease (CD) auto-antibodies against tissue transglutaminase (anti-tTG) are produced in the intestinal mucosa even when not measurable in serum. By using the phage display libraries technique it is possible to investigate in vivo (intestinal biopsy) early antibody responses in autoimmune disease. In particularly, this technique demonstrated that the humoral response against tissue transglutaminase occurs at the intestinal mucosal level, and that the human VH5 gene is the commonly used variable region by the celiac patients to build the anti-tTG. The intestinal mucosa production of IgA anti-tTG could be important in the diagnostic work-up of early-stage CD, when mucosal histology is not yet diagnostic.