Crohn’s disease (CD) arises when environmental triggers and gut microbial dysbiosis provoke a poorly regulated mucosal immune response across an impaired epithelial barrier in a genetically susceptible host.^[1][2] No single gene, microbe, or environmental factor is sufficient on its own. The result is a self-perpetuating cycle of barrier breakdown, microbial translocation, and Th1/Th17-skewed transmural inflammation, which current therapies interrupt at specific points.^[3][2] More than 200 susceptibility loci have been identified. They converge on four processes: innate microbial sensing (NOD2), autophagy (ATG16L1), epithelial barrier integrity, and IL-23/Th17 signaling.^[4][5] The hallmark pathologic features are:

• Transmural inflammation
• Skip lesions
• Non-caseating granulomas
• A propensity for stricture and fistula formation

CD reflects the interplay of four interdependent elements rather than a linear sequence:^[1][3][2]

• Genetic susceptibility lowers the threshold for immune activation.
• Environmental triggers act largely through the microbiome and the epithelial barrier. These include smoking, diet, antibiotic exposure, and enteric infection.
• Microbial dysbiosis expands pathobionts and depletes protective commensals.
• Dysregulated innate and adaptive immunity produces transmural, granulomatous inflammation.

The genetics point to a “network dyshomeostasis” across interconnected pathways rather than a single defective pathway. These pathways cover microbial sensing, barrier integrity, autophagy, cytokine signaling, and cell-stress responses.^[5]

• Genome-wide association studies have identified more than 200 inflammatory bowel disease (IBD) risk loci, which confirms a polygenic architecture.^[1][4]
• Most loci confer only modest risk (odds ratios ~1.1–1.2) and lie in regulatory regions. Known variants explain only roughly 19–26% of CD heritability, which underscores the contribution of environment and gene–environment interaction.^[6]
• Monozygotic twin concordance is approximately 50% in CD, compared with approximately 15% in ulcerative colitis (UC).^[4]
• Risk alleles differ by ancestry:^[4]
  • Coding variants in NOD2, ATG16L1, and IL23R predominate in populations of European ancestry.
  • These variants are essentially monomorphic in East Asian cohorts, where TNFSF15 is the leading CD locus.

• CD remains a clinical diagnosis. Routine genotyping is not part of standard care.

• An impaired epithelial barrier is now viewed as a central pathogenic factor rather than a bystander.^[1]
• Several defects allow luminal bacteria and metabolites to reach the lamina propria and activate immune cells:^[15][16]
  • Reduced mucus
  • Dysregulated tight junctions
  • Fewer or dysfunctional Paneth cells with impaired antimicrobial peptide secretion
  • Reduced intestinal stem cell function
• Barrier dysfunction and subclinical gut inflammation have been detected before clinically overt CD in at-risk individuals. This supports an upstream, potentially causal role.^[17]
• Environmental exposures, including smoking, are best understood as acting through the barrier and the microbiome. Psychological stress may modulate disease course but is not a core etiologic pillar.^[17][18]

• The microbiota is essential to CD pathogenesis, but no single causative pathogen has been identified.^[3][6]
• CD is characterized by reduced microbial diversity, with two main shifts:^[1][15]
  • Loss of Firmicutes, notably the butyrate producers Faecalibacterium prausnitzii and Roseburia spp.
  • Expansion of Proteobacteria, including adherent-invasive Escherichia coli (AIEC).
• In the pathobiont concept, otherwise commensal organisms become pathogenic under disease-promoting host conditions. Dysbiosis is both a cause and a consequence of inflammation.^[3][2]
• Inflammation itself drives “late dysbiosis.” It increases luminal oxygen, nitrate, and iron, which favors facultative anaerobes.^[7]
• Loss of protective microbial metabolites further weakens barrier and immune homeostasis. These metabolites include short-chain fatty acids, secondary bile acids, and tryptophan/indole derivatives.^[19]

• CD shows deficient innate responses, including reduced macrophage bactericidal activity and impaired neutrophil recruitment. These deficits permit microbial persistence and translocation.^[20]
• NLRP3 inflammasome activation processes IL-1β and IL-18.^[20]
• Lamina propria macrophages that engulf penetrating bacteria secrete TNF, IL-6, IL-12, and IL-23.^[16]
• Dendritic cells present luminal antigens and polarize naive T cells toward inflammatory effector phenotypes.^[4]

• CD is driven by excessive Th1 and Th17 responses to IL-12, IL-18, and IL-23 from antigen-presenting cells, together with impaired regulatory T cell function.^[13][4]
• Th1/Th17 cells secrete IFN-γ, IL-17, and TNF, which perpetuate transmural inflammation.^[4][20]
• The characteristic granuloma reflects this cell-mediated, rather than antibody-mediated, response.^[4]
• The older strict “Th1 = CD, Th2 = UC” paradigm has been revised. Th17 cells and the IL-12/IL-23 axis are now recognized as central drivers. The two cytokines share the p40 subunit.^[13]
• Gut-selective lymphocyte trafficking depends on α4β7 integrin–MAdCAM-1 interactions, a validated therapeutic target.^[4][18]

Sources for table: ^[21][16]

• Chronic transmural inflammation activates fibroblasts and other mesenchymal cells, including through oncostatin M signaling. This produces the strictures and penetrating/fistulizing complications that distinguish CD from UC.^[14][5]
• Fibrosis may progress despite control of inflammation. No therapy is approved to reverse established fibrostenosis.^[14]

• Transmural, patchy inflammation with skip lesions. The terminal ileum and proximal colon are involved most often.^[2]
• Creeping fat (mesenteric fat wrapping) and a thickened, rubbery bowel wall.^[4]
• Strictures, which appear as the string sign on contrast imaging.
• Aphthous ulcers that progress to linear ulcers, cobblestoning, fissures, and fistulas.^[4]

• Transmural inflammation with lymphoid aggregates throughout the bowel wall.^[4]
• Focal crypt injury:
  • Neutrophilic infiltration of crypts, causing cryptitis or crypt abscess
  • Crypt architectural distortion and branching
• Non-caseating granulomas are highly specific for CD but are present in only a subset of cases. Their absence does not exclude the diagnosis.^[4]
• Paneth cell metaplasia and villous blunting.^[22]