Crohn's disease (CD) of the ileum is associated with reduced expression of Paneth cell (PC) α-defensins, which are antimicrobial peptides critical for intestinal immunity. This study shows that ileal CD patients have decreased antibacterial activity in their mucosal extracts, along with reduced expression of PC α-defensins HD5 and HD6, while other PC products remain unchanged or increase. The decrease in α-defensins is independent of inflammation severity and not observed in other IBD forms like colonic CD or ulcerative colitis. Functional studies using a transgenic mouse model revealed that reduced HD5 expression alters the luminal microbiota, suggesting that PC α-defensin deficiency may compromise innate immunity in ileal CD, contributing to disease initiation and progression. PCs are key producers of antimicrobial peptides, including α-defensins, in the small intestine. NOD2, an intracellular receptor for bacterial peptidoglycan, is expressed in PCs and is associated with CD localization in the small intestine. NOD2 mutations are linked to reduced α-defensin expression and increased susceptibility to bacterial infections. The study also found that CD patients with NOD2/CARD15 mutations had significantly reduced HD5 and HD6 mRNA levels compared to non-IBD controls. These findings suggest that α-defensin deficiency in ileal CD may be a key factor in disease pathogenesis by impairing mucosal defenses and allowing bacterial colonization, leading to chronic inflammation. The study highlights the role of PC α-defensins in maintaining intestinal homeostasis and preventing microbial overgrowth. Deficiencies in these peptides may contribute to the development of ileal CD by compromising the mucosal barrier and triggering immune responses. The results support the hypothesis that innate immune defects, particularly in PC α-defensins, play a central role in CD pathophysiology. This understanding may lead to new therapeutic strategies aimed at restoring host-microbe balance and enhancing innate immunity in IBD patients.Crohn's disease (CD) of the ileum is associated with reduced expression of Paneth cell (PC) α-defensins, which are antimicrobial peptides critical for intestinal immunity. This study shows that ileal CD patients have decreased antibacterial activity in their mucosal extracts, along with reduced expression of PC α-defensins HD5 and HD6, while other PC products remain unchanged or increase. The decrease in α-defensins is independent of inflammation severity and not observed in other IBD forms like colonic CD or ulcerative colitis. Functional studies using a transgenic mouse model revealed that reduced HD5 expression alters the luminal microbiota, suggesting that PC α-defensin deficiency may compromise innate immunity in ileal CD, contributing to disease initiation and progression. PCs are key producers of antimicrobial peptides, including α-defensins, in the small intestine. NOD2, an intracellular receptor for bacterial peptidoglycan, is expressed in PCs and is associated with CD localization in the small intestine. NOD2 mutations are linked to reduced α-defensin expression and increased susceptibility to bacterial infections. The study also found that CD patients with NOD2/CARD15 mutations had significantly reduced HD5 and HD6 mRNA levels compared to non-IBD controls. These findings suggest that α-defensin deficiency in ileal CD may be a key factor in disease pathogenesis by impairing mucosal defenses and allowing bacterial colonization, leading to chronic inflammation. The study highlights the role of PC α-defensins in maintaining intestinal homeostasis and preventing microbial overgrowth. Deficiencies in these peptides may contribute to the development of ileal CD by compromising the mucosal barrier and triggering immune responses. The results support the hypothesis that innate immune defects, particularly in PC α-defensins, play a central role in CD pathophysiology. This understanding may lead to new therapeutic strategies aimed at restoring host-microbe balance and enhancing innate immunity in IBD patients.