Definition:
Inflammatory Bowel Disease (IBD) is a group of chronic, relapsing inflammatory conditions of the gastrointestinal (GI) tract. The two main types are:
- Crohn's Disease (CD): A transmural, granulomatous inflammation that can affect any part of the GI tract from mouth to anus, most often terminal ileum and colon
- Ulcerative Colitis (UC): A superficial inflammation restricted to the mucosa and submucosa of the colon and rectum.
Etiology:
- Genetic predisposition (e.g., NOD2 mutations in Crohn's)
- Environmental triggers:
- Diet
- Smoking (protective in UC but worsens Crohn’s)
- Hygiene hypothesis
- Immune dysregulation:
- Abnormal mucosal immune response to intestinal flora
- Autoimmune mechanisms
- Breastfeeding, early childhood exposures (protective)
Pathogenesis:
- Defective epithelial barrier ("leaky gut") allows bacterial antigens to enter lamina propria.
- Inappropriate activation of mucosal immune system:
- Crohn’s: TH1 and TH17-mediated response → granulomatous inflammation
- Ulcerative Colitis: TH2-like cytokines → superficial mucosal inflammation
- Chronic inflammation leads to:
- Mucosal ulceration
- Crypt architectural distortion
- In Crohn’s: transmural fibrosis and fistula formation