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Reliable, easy-to-read patient information on common medical conditions — written in plain language, free of jargon, and grounded in current evidence. Browse the most frequent diagnoses, understand what they mean for everyday life, and learn which steps and questions help you have a focused conversation with your doctor.

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Crohn's disease causes

Introduction

Overview

Overview

Crohn’s disease (CD) has no single identifiable cause. The best-supported model is a dysregulated mucosal immune response to the gut microbiota in a genetically susceptible host, triggered and modified by environmental exposures, resulting in impaired epithelial barrier function and chronic transmural inflammation.[1][2] Genetic susceptibility, environmental exposure, and the microbiome each appear necessary but not individually sufficient.[3] More than 200 susceptibility loci have been identified, with NOD2 the strongest single-gene risk factor in populations of European ancestry. Defective innate handling of intestinal bacteria (NOD2 and autophagy pathways) and a Th1/Th17–IL-23–driven effector response are central mechanisms.[4] Established environmental contributors include cigarette smoking and early-life antibiotic exposure. No single infectious agent has been proven causal; the role of Mycobacterium avium subspecies paratuberculosis (MAP) remains unproven and controversial.

Causes

Causes

Genetic Susceptibility

  • Family history is the strongest single risk factor; an affected first-degree relative confers an incidence rate ratio of approximately 7.8 (95% CI 7.05–8.56).[1]
  • Twin studies: concordance in monozygotic twins is approximately 50% for CD (vs approximately 15% for ulcerative colitis), indicating strong heritability alongside an essential environmental contribution.[4]
  • Polygenic architecture: Genome-wide association studies have identified more than 200 IBD risk loci. Most confer modest individual risk (odds ratios approximately 1.1–1.2) and lie in regulatory regions. Known variants explain only a minority of estimated heritability.[1][4]
  • Population differences: Coding risk variants in NOD2, ATG16L1, and IL23R predominate in populations of European ancestry and are essentially monomorphic in East Asian cohorts. In those cohorts TNFSF15 is the dominant CD locus, with an effect size exceeding that of NOD2 in European-ancestry populations.[4]
Major Crohn’s disease susceptibility genes and pathways
Gene / locus Pathway Clinically relevant features
NOD2 (CARD15) Innate sensing of bacterial muramyl dipeptide; Paneth cell antimicrobial function First CD gene identified.[5][6] CD-associated variants are loss-of-function. Risk rises approximately 10–40-fold with homozygosity or compound heterozygosity, with incomplete penetrance. Most strongly associated with ileal and stricturing disease.[7][8]
ATG16L1 (T300A), IRGM, LRRK2 Autophagy and clearance of intracellular bacteria NOD2 recruits ATG16L1 to sites of bacterial entry. CD-associated variants in either gene disrupt this step, making defective autophagy a convergent mechanism.[7] ATG16L1 T300A predisposes to ileal CD independently of NOD2.[9]
XBP1, ORMDL3 Endoplasmic reticulum (unfolded protein response) stress ER stress in Paneth and epithelial cells converges with the NOD2 and autophagy pathways.[7]
IL23R, IL12B, STAT3, JAK2 IL-23/Th17 axis Implicates hyperactive effector T-cell responses.[4]
TNFSF15 TL1A signaling Dominant CD locus in East Asian populations.[4]
Schematic of the NOD2 (CARD15) gene. Loss-of-function variants are associated with ileal and stricturing Crohn’s disease.

Immune Dysregulation

  • CD is characterized by a Th1/Th17-skewed mucosal response with impaired regulatory T cell activity. Inflamed mucosa shows elevated TNF-α, IFN-γ, IL-1, IL-6, IL-12, IL-23, and IL-17.[1]
  • IL-12 and IL-23, which share the p40 subunit, are produced by macrophages and dendritic cells and act as upstream drivers. IL-23 is especially important in sustaining pathogenic Th17 responses.[4]
  • Whether immune abnormalities are primary or secondary to barrier breakdown remains unresolved. Many susceptibility genes point to impaired innate handling of commensal bacteria rather than classic autoimmunity.[2][10]

Gut Microbiome

  • CD is associated with dysbiosis:[1]
    • reduced microbial diversity, with fewer Firmicutes and Bacteroides and more Proteobacteria and Actinobacteria
    • depletion of butyrate producers such as Faecalibacterium prausnitzii and Roseburia spp.
    • enrichment of adherent-invasive Escherichia coli (AIEC)
  • No single causative organism has been identified. The implicated microbes are mostly pathobionts: commensals that become pathogenic under disease-promoting host conditions. They may perpetuate inflammation rather than initiate it.[3][11]
  • Gene–microbe interaction is central. Loss-of-function NOD2 and autophagy variants allow pathobionts to accumulate and penetrate the mucosa, driving T-cell–mediated inflammation. Examples include AIEC adhesion via aberrant epithelial CEACAM6 expression, and Mucispirillum schaedleri in Nod2/Cybb-deficient models.[10][12]

Environmental Factors

Environmental exposures (the exposome) act across the life course, with particular influence during early-life windows of microbiome and immune maturation.[13]

Environmental factors associated with Crohn’s disease
Exposure Direction
Cigarette smoking Increases risk
Antibiotic exposure, especially in early childhood Increases risk
High ultra-processed food intake Increases risk
Diet high in fiber, vegetables, and fish Protective
Physical activity Protective
Breastfeeding; early-life greenspace exposure Protective
Industrialization, urbanization, and migration Increases risk
References

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Dolinger M, Torres J, Vermeire S (2024). “Crohn’s disease”. Lancet. 403 (10432): 1177–1191. doi:10.1016/S0140-6736(23)02586-2. PMID 38437854 Check |pmid= value (help).
  2. ↑ 2.0 2.1 Cushing K, Higgins PDR (2021). “Management of Crohn disease: a review”. JAMA. 325 (1): 69–80. doi:10.1001/jama.2020.18936. PMID 33399844 Check |pmid= value (help).
  3. ↑ 3.0 3.1 Kennedy MS, Chang EB (2025). “Emerging concepts and shifting paradigms for understanding the microbial basis of inflammatory bowel diseases”. J Clin Invest. 135 (17): e193969. doi:10.1172/JCI193969.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Roda G, Chien Ng S, Kotze PG, Argollo M, Panaccione R, Spinelli A, Kaser A, Peyrin-Biroulet L, Danese S (2020). “Crohn’s disease”. Nat Rev Dis Primers. 6 (1): 22. doi:10.1038/s41572-020-0156-2. PMID 32242028 Check |pmid= value (help).
  5. ↑ Hugot JP, Chamaillard M, Zouali H, Lesage S, Cézard JP, Belaiche J; et al. (2001). “Association of NOD2 leucine-rich repeat variants with susceptibility to Crohn’s disease”. Nature. 411 (6837): 599–603. doi:10.1038/35079107. PMID 11385576.
  6. ↑ Ogura Y, Bonen DK, Inohara N, Nicolae DL, Chen FF, Ramos R; et al. (2001). “A frameshift mutation in NOD2 associated with susceptibility to Crohn’s disease”. Nature. 411 (6837): 603–606. doi:10.1038/35079114.
  7. ↑ 7.0 7.1 7.2 Fritz T, Niederreiter L, Adolph T, Blumberg RS, Kaser A (2011). “Crohn’s disease: NOD2, autophagy and ER stress converge”. Gut. 60 (11): 1580–1588. doi:10.1136/gut.2009.206466. PMID 21252204.
  8. ↑ Cuthbert AP, Fisher SA, Mirza MM, King K, Hampe J, Croucher PJ; et al. (2002). “The contribution of NOD2 gene mutations to the risk and site of disease in inflammatory bowel disease”. Gastroenterology. 122 (4): 867–874. PMID 11910337.
  9. ↑ Prescott NJ, Fisher SA, Franke A, Hampe J, Onnie CM, Soars D; et al. (2007). “A nonsynonymous SNP in ATG16L1 predisposes to ileal Crohn’s disease and is independent of CARD15 and IBD5”. Gastroenterology. 132 (5): 1665–1671. PMID 17484864.
  10. ↑ 10.0 10.1 Caruso R, Lo BC, Núñez G (2020). “Host-microbiota interactions in inflammatory bowel disease”. Nat Rev Immunol. 20 (7): 411–426. doi:10.1038/s41577-019-0268-7. PMID 32005980 Check |pmid= value (help).
  11. ↑ Gilliland A, Chan JJ, De Wolfe TJ, Yang H, Vallance BA (2024). “Pathobionts in inflammatory bowel disease: origins, underlying mechanisms, and implications for clinical care”. Gastroenterology. 166 (1): 44–58. doi:10.1053/j.gastro.2023.09.019. PMID 37734419 Check |pmid= value (help).
  12. ↑ “Gene-environment interactions shape the host-microbial interface in inflammatory bowel disease”. Nat Immunol. 2025. doi:10.1038/s41590-025-02197-5. PMID 40528029 Check |pmid= value (help).
  13. ↑ Agrawal M, Midya V, Vianello A, Valvi D, Kaplan GG, Colombel JF (2026). “The exposome and inflammatory bowel disease: a framework for primary and primordial prevention”. Nat Rev Gastroenterol Hepatol. doi:10.1038/s41575-026-01260-2.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Fahad Hasan, M.D.[2] Aditya Ganti M.B.B.S. [3]