Clinical meaning
Ulcerative colitis (UC) is a chronic inflammatory bowel disease characterised by diffuse, continuous mucosal and submucosal inflammation of the colon and rectum. Unlike Crohn disease, UC invariably involves the rectum and extends proximally in a continuous pattern without skip lesions. The disease is classified by extent: ulcerative proctitis (rectum only, 30-40%), left-sided colitis (up to the splenic flexure, 30-40%), and pancolitis (extending beyond the splenic flexure, 20-30%). The pathogenesis shares features with Crohn disease: a dysregulated immune response to commensal gut bacteria in genetically predisposed individuals. However, the immunologic profile differs. UC is characterised by a Th2-dominant response with increased production of IL-4, IL-5, IL-13, and natural killer T cells. IL-13 is particularly important as it disrupts epithelial tight junctions, increases epithelial permeability, and induces epithelial cell apoptosis. Inflammation in UC is superficial, affecting the mucosa and submucosa only (not transmural as in Crohn disease). The inflammatory infiltrate damages crypts (the glands of the colonic mucosa), producing cryptitis (neutrophil invasion of crypts) and crypt abscesses (accumulation of neutrophils within crypt lumens). Progressive crypt destruction leads to mucosal ulceration, with residual...