This study investigates the mechanisms underlying barrier dysfunction in active Crohn's disease, focusing on tight junctions. Biopsy specimens from the sigmoid colon of patients with mild to moderately active or inactive Crohn's disease were analyzed using Ussing chambers, freeze fracture electron microscopy, immunohistochemistry, and Western blotting. The results show that active Crohn's disease is associated with impaired intestinal barrier function, characterized by reduced epithelial resistance. Freeze fracture electron microscopy revealed reduced and discontinuous tight junction strands, with downregulation and redistribution of sealing claudins 5 and 8, while the pore-forming claudin 2 was strongly upregulated. Epithelial apoptosis was also significantly increased in active Crohn's disease. These findings suggest that altered tight junction structure and increased epithelial apoptosis contribute to barrier dysfunction in mild to moderately active Crohn's disease. The study highlights the importance of claudin 2 upregulation and the downregulation of sealing claudins in the pathogenesis of Crohn's disease.This study investigates the mechanisms underlying barrier dysfunction in active Crohn's disease, focusing on tight junctions. Biopsy specimens from the sigmoid colon of patients with mild to moderately active or inactive Crohn's disease were analyzed using Ussing chambers, freeze fracture electron microscopy, immunohistochemistry, and Western blotting. The results show that active Crohn's disease is associated with impaired intestinal barrier function, characterized by reduced epithelial resistance. Freeze fracture electron microscopy revealed reduced and discontinuous tight junction strands, with downregulation and redistribution of sealing claudins 5 and 8, while the pore-forming claudin 2 was strongly upregulated. Epithelial apoptosis was also significantly increased in active Crohn's disease. These findings suggest that altered tight junction structure and increased epithelial apoptosis contribute to barrier dysfunction in mild to moderately active Crohn's disease. The study highlights the importance of claudin 2 upregulation and the downregulation of sealing claudins in the pathogenesis of Crohn's disease.