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  • There are two forms of idiopathic inflammatory bowel disease (IBD): ulcerative colitis (UC), a mucosal inflammatory condition confined to the rectum and colon, and Crohn disease (CD), a transmural inflammation of gastrointestinal (GI) mucosa that may occur in any part of the GI tract. The etiologies of both conditions are unknown, but they may have a common pathogenic mechanism.


  • Factors that cause IBD include infectious agents, genetics, the environment, and the immune system. This may involve abnormal regulation of the innate immune response or a reaction to various antigens. The microflora of the GI tract may provide an environmental trigger to activate inflammation in genetically susceptible individuals and is highly implicated in the development of IBD.

  • Suspect infectious agents include viruses, protozoans, mycobacteria such as Mycobacterium paratuberculosis or avium, and other bacteria such as Ruminococcus gnavus, Ruminococcus torques, Listeria monocytogenes, Chlamydia trachomatis, and Escherichia coli.

  • Th1 cytokine activity is excessive in CD, and increased expression of interferon-γ in the intestinal mucosa and production of IL-12 are features of the immune response in CD. In contrast, Th2 cytokine activity is excessive with UC (with excess production of IL-13). Tumor necrosis factor-α (TNF-α) is a pivotal proinflammatory cytokine that is increased in the mucosa and intestinal lumen of patients with CD and UC.

  • Antineutrophil cytoplasmic antibodies are found in a high percentage of patients with UC and less frequently with CD.

  • Smoking appears to be protective for UC but associated with increased frequency of CD. Use of nonsteroidal anti-inflammatory drugs (NSAIDs) may trigger disease occurrence or lead to disease flares.

  • UC and CD differ in two general respects: anatomical sites and depth of involvement within the bowel wall. There is, however, overlap between the two conditions, with a small fraction of patients showing features of both diseases (Table 26-1).

TABLE 26-1Comparison of the Clinical and Pathologic Features of Crohn Disease and Ulcerative Colitis

Ulcerative Colitis

  • UC is confined to the colon and rectum and affects primarily the mucosa and the submucosa. The primary lesion occurs in the crypts of the mucosa (crypts of Lieberkühn) in the form of a crypt abscess.

  • Local complications (involving the colon) occur in the majority of ...

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