Crohn's disease (CD) and ulcerative colitis (UC) are chronic inflammatory disorders of the gastrointestinal tract that share clinical and pathologic characteristics. The most credible hypothesis is that CD and UC result from an inappropriate and exaggerated mucosal immune response to normal constituents of the mucosal microflora that is in part genetically determined. However, there is reason to believe that the main pathologic processes in these two diseases are distinct. For example, the CARD15/NOD2 gene has been identified as a susceptibility gene for CD but not for UC. Moreover, the study of patients and mouse models of inflammatory bowel disease has clearly shown that, in CD, the tissue-damaging inflammatory reaction is driven by interleukin-12-activated Th1 cells, whereas a humoral response predominates in UC.