Inflammatory bowel diseases (ulcerative colitis, Crohn disease)

💡 IBD = group of diseases characterized by an exaggerated and destructive mucosal immune response of the colon and small intestine; includes Crohn’s disease and ulcerative colitis

• Both are chronic relapsing inflammatory disorders of unknown origin
• Tissue injury = abnormal local immune response against normal gut flora and self-antigens in genetically susceptible individuals

Pathogenesis:

• Loss of balance between immune activation (microbes, dietary antigens) and host defenses
• Involves: genetic susceptibility + failure of immune regulation + microbial factors
• Both diseases: MHC class 2 alleles + mutated IL-23 receptor gene
• Crohn’s: NOD2 mutation (host response to bacteria)
• Primary damaging agent: CD4+ T-cells; TNF plays an important role in Crohn’s
• Sites (ileum and colon) are rich in bacteria that provide the antigenic trigger
• Outcome: impaired mucosal barrier + loss of surface epithelium → intermittent bloody diarrhea

---

A) Crohn’s Disease (CD)

• May affect any portion of the GI-tract (esophagus to anus); usually the terminal ileum
• Systemic inflammatory disease; peak incidence 20–30 years of age; females slightly more affected

Morphology

• Skip lesions: sharp demarcation of diseased segments from adjacent normal bowel
• Serpentine fissures: deep ulcers in the long axis separated by nodular mucosal thickenings
• Fibrosis → stricture of involved segments (esp. terminal ileum)
• Extension of fissures → fistula formation (to bowel loops, bladder, vagina, perianal skin) + peritoneal abscesses

Clinical

• Recurrent episodes of diarrhea, crampy abdominal pain, fever; slow onset (sometimes abrupt)
• Melena (black tarlike stool = digested blood) in ~50% of cases
• Symptom-free periods followed by relapses

Complications

• Fistulas, abdominal abscesses, intestinal stricture/obstruction
• Extraintestinal (immune origin): uveitis, sacroiliitis, migratory polyarthritis, erythema nodosum, bile duct disorders, obstructive uropathy

---

B) Ulcerative Colitis (UC)

• Ulceroinflammatory disease limited to the colon (mucosa + submucosa); starts in the rectum and extends proximally in a continuous fashion; may involve the whole colon
• Peak 20–25 years of age; same extraintestinal associations as CD

Key differences from Crohn’s Disease

Feature	Crohn’s Disease	Ulcerative Colitis
Location	Any GI tract; terminal ileum	Colon only
Skip lesions	Yes	No
Granulomas	Yes	No
Depth of ulcers	Transmural	Mucosa + submucosa
Fibrosis/stricture	Yes	Little
Mural thickening	Yes	No
Carcinoma risk	Lower	Higher

Morphology

• Macro: broad-based ulcers; pseudopolyps (narrow strands of edematous hyperemic mucosa bulging upward between ulcers)
• Micro: PMNL infiltration → crypt abscess → mucosal destruction; crypt atrophy → dysplasia

Clinical

• Bloody mucoid diarrhea + cramps, tenesmus, lower abdominal pain; chronic relapsing
• Complications: mucosal atrophy, bleeding/anemia, epithelial dysplasia → carcinoma

💡 High-yield: CD = skip lesions + serpentine fissures + transmural + fistulas + terminal ileum; NOD2. UC = continuous from rectum + pseudopolyps + crypt abscess + higher carcinoma risk; no granulomas. Both: CD4+ T-cells; TNF (CD); extraintestinal = uveitis, sacroiliitis, erythema nodosum.