Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a group of chronic, relapsing immune-mediated inflammatory disorders of the gastrointestinal tract, comprising the two principal entities Crohn's disease and ulcerative colitis. It arises from a dysregulated mucosal immune response to the gut microbiota in genetically susceptible hosts, and its two forms are distinguished by characteristic patterns of inflammation and tissue change.

Definition

A category of chronic idiopathic inflammatory disorders of the bowel, principally Crohn's disease (transmural, segmental inflammation that may affect any part of the gut) and ulcerative colitis (continuous mucosal inflammation confined to the colon), driven by aberrant immune responses to luminal contents in genetically predisposed individuals.

Scope

This entry covers the pathology of Crohn's disease and ulcerative colitis: their distinguishing morphologic features, the immunopathogenic model of disease, the distribution and depth of inflammation, and the long-term tissue consequences including neoplastic risk. It frames IBD as a reference pathology topic rather than offering clinical management.

Core questions

• How do Crohn's disease and ulcerative colitis differ in the distribution, depth, and histology of inflammation?
• What immune, genetic, microbial, and environmental factors converge to produce chronic intestinal inflammation?
• How does chronic inflammation in IBD raise the risk of dysplasia and colorectal cancer?

Key concepts

• Crohn's disease versus ulcerative colitis
• Transmural versus mucosal inflammation
• Skip lesions and continuous colitis
• Non-caseating granulomas
• Dysregulated mucosal immunity to the microbiota
• Inflammation-associated dysplasia

Mechanisms

IBD results from a chronic, dysregulated immune response to the intestinal microbiota in genetically susceptible individuals, shaped by environmental factors and epithelial barrier dysfunction (Torres 2017; Ungaro 2017). The two forms differ morphologically. Crohn's disease produces transmural, segmental ('skip') inflammation that can involve any part of the gastrointestinal tract, often the terminal ileum, with features such as fissuring ulcers, a cobblestone mucosa, fibrosis with strictures, fistula formation, and non-caseating granulomas (Torres 2017). Ulcerative colitis produces continuous, predominantly mucosal inflammation beginning in the rectum and extending proximally within the colon, with crypt distortion, crypt abscesses, and, in chronic disease, pseudopolyps; granulomas are absent (Ungaro 2017). Long-standing colonic inflammation in both forms predisposes to dysplasia and colitis-associated colorectal cancer.

Clinical relevance

IBD pathology underpins the histologic distinction between Crohn's disease and ulcerative colitis on biopsy, the recognition of dysplasia in surveillance specimens, and the understanding of complications such as strictures and fistulae. This material is descriptive and educational and supports interpretation of how the disease is classified; it is not individualized clinical guidance (Torres 2017; Ungaro 2017).

Epidemiology

Once concentrated in Western, industrialised nations, IBD has become a global disease: incidence has stabilised at high levels in the West while rising rapidly in newly industrialised regions, making it one of the more prevalent chronic gastrointestinal conditions worldwide (Ng 2017). Disease onset is most common in adolescence and early adulthood.

Evidence & guidelines

The pathophysiologic account here rests on comprehensive disease-specific reviews of Crohn's disease and ulcerative colitis (Torres 2017; Ungaro 2017) and on a systematic review of global incidence and prevalence (Ng 2017). These are cited to support the descriptive content rather than as treatment recommendations.

History

Ulcerative colitis was described in the nineteenth century, and Crohn's disease was delineated as regional ileitis by Crohn, Ginzburg, and Oppenheimer in 1932. The two were subsequently recognised as the principal forms of idiopathic inflammatory bowel disease, and over the past century their separation has been refined through morphology, and more recently through genetics and immunology (Torres 2017).

Key figures

• Burrill Crohn
• Jean-Frédéric Colombel
• Siew Ng

Seminal works

• torres-2017-crohn
• ungaro-2017-uc
• ng-2017-epi

Frequently asked questions

What are the two main types of inflammatory bowel disease?

Crohn's disease and ulcerative colitis. Crohn's disease causes transmural, patchy inflammation that can affect any part of the gut, whereas ulcerative colitis causes continuous mucosal inflammation limited to the colon.

Why does inflammatory bowel disease raise colorectal cancer risk?

Long-standing chronic inflammation of the colon promotes an inflammation-dysplasia-carcinoma sequence, so patients with extensive, long-duration colitis carry an increased risk of dysplasia and colorectal cancer.