Clinical meaning
Inflammatory bowel disease (IBD) encompasses Crohn disease (CD) and ulcerative colitis (UC), chronic immune-mediated inflammatory conditions of the GI tract with distinct pathological features. Crohn disease can affect ANY part of the GI tract from mouth to anus (most commonly terminal ileum and colon), involves TRANSMURAL inflammation (all layers of the bowel wall), has a patchy/skip lesion distribution, and characteristically forms non-caseating granulomas, strictures, fistulas (enterocutaneous, enterovesical, perianal), and abscesses. Ulcerative colitis is LIMITED to the colon and rectum, involves only MUCOSAL and SUBMUCOSAL inflammation, extends CONTINUOUSLY from the rectum proximally (no skip lesions), and characteristically produces crypt abscesses, pseudopolyps, and bloody diarrhea with mucus. The pathogenesis involves dysregulated mucosal immune responses to commensal gut bacteria in genetically susceptible individuals. Key cytokines: TNF-alpha (central inflammatory mediator — target of infliximab, adalimumab), IL-12/23 (target of ustekinumab), integrins (alpha-4-beta-7 — target of vedolizumab, which blocks gut-specific lymphocyte trafficking). Treatment follows a step-up or top-down approach: 5-ASA (mesalamine — UC only, mild-moderate), corticosteroids (induction only — NEVER maintenance), immunomodulators (azathioprine/6-MP, methotrexate — steroid-sparing maintenance), and biologics (anti-TNF, anti-integrin, anti-IL-12/23, JAK inhibitors). Surgical indications:...