Inflammatory Bowel Disease III: Crohn's Disease

Crohn's disease develops in genetically susceptible individuals with mutations in NOD2, which impairs recognition of bacterial products; ATG16L1, which disrupts autophagy needed for clearing intracellular microbes; and IL23R, which influences T-cell differentiation and heightens inflammatory responses. These genetic variants collectively increase disease susceptibility.

Environmental exposures including cigarette smoking, prior gastrointestinal infections such as Salmonella or Campylobacter, and diets high in ultra-processed foods increase Crohn's disease risk. These factors impair epithelial barrier integrity and disturb the gut microbiome, allowing luminal antigens to interact directly with immune cells.

Genetic and environmental factors compromise epithelial integrity and alter the gut microbiome, increasing intestinal permeability. This allows luminal antigens, including bacterial proteins and metabolites, to access the lamina propria and disrupt immune tolerance to commensal microorganisms, leading to inappropriate immune activation.

Both innate and adaptive immune cells become overactive in Crohn's disease, including macrophages, dendritic cells, neutrophils, and T lymphocytes. These cells infiltrate the intestinal wall and release pro-inflammatory mediators, driving the exaggerated immune response against commensal gut bacteria.

Key cytokines such as tumor necrosis factor-alpha, interleukin-12, and interleukin-23 promote differentiation of Th1 and Th17 cells, amplifying inflammation. This cytokine-driven cascade produces transmural involvement characteristic of Crohn's disease, contributing to deep ulcerations, fistula formation, and strictures.

Impaired epithelial barrier function allows bacterial products and metabolites to interact directly with immune cells, causing immune system dysregulation. This leads to a loss of tolerance to commensal gut bacteria and exaggerated immune activation against these normally harmless organisms, perpetuating chronic inflammation.

Transmural inflammation in Crohn's disease affects the full thickness of the intestinal wall, producing hallmark complications including deep ulcerations, fistula formation, and strictures. This chronic inflammatory state is marked by recurrent episodes of gastrointestinal injury and impaired mucosal healing.