Inflammatory bowel diseases (IBDs) are chronic inflammatory disorders of the intestine encompassing Crohn's disease (CD) and ulcerative colitis (UC). The inflammation in CD can extend anywhere along the alimentary canal, whereas in UC the inflammation is limited to the large bowel. In CD the inflammation is typically patchy, transmural, and contains noncaseating granulomata, whereas in UC the inflammation is relatively superficial with ulcer formations. Nonetheless, 10% of the cases are clinically and histologically identical. Both disorders are idiopathic, because no etiological agent can be found, and both show autoimmune characteristics. However, the underlying immunological mechanism is still very poorly understood and controversial. Therefore the treatment of IBD, has, thus far, been unsatisfying, because it involves the administration of rather unspecific immunosuppressive agents. To find more specific therapies, it is necessary to study the underlying immunopathological mechanism of the disease.