Collagenous gastritis: current understanding of a rare immune-mediated gastropathy across pediatric and adult phenotypes-from pathogenesis to therapeutic strategies.

Collagenous gastritis (CG) is a rare chronic inflammatory disorder defined histologically by a subepithelial collagen band exceeding 10 μm, together with a chronic inflammatory infiltrate within the lamina propria. First described in 1989, CG presents with a striking age-stratified dichotomy. The pediatric phenotype is dominated by treatment-refractory iron-deficiency anemia and chronic abdominal pain, with disease usually confined to the stomach. The adult phenotype is dominated by chronic watery diarrhea and frequently coexists with collagenous colitis as part of a broader collagenous gastroenteropathy spectrum. The pathogenesis is widely held to be immune-mediated. Strong associations exist with autoimmune conditions, including celiac disease, common variable immunodeficiency (CVID), and systemic lupus erythematosus (SLE). Recent gene-expression and single-cell studies have identified mixed T-helper 1 (Th1) and T-helper 2 (Th2) cytokine profiles in gastric tissue, together with α4β7-mediated mucosal homing of activated CD4+ T cells, suggesting complex immune dysregulation rather than a primary disorder of collagen biosynthesis. Implicated triggers include certain medications-notably olmesartan-and, more speculatively, infectious agents; a single case report has increased the possibility of Epstein-Barr virus (EBV) reactivation. A single proteomics study has identified reduced epidermal growth factor (EGF) expression as a candidate biomarker, suggesting impaired mucosal repair, although further validation is needed. Diagnosis demands a high index of clinical suspicion and rests on histopathological examination of multi-site gastric biopsies since endoscopic appearances range from normal mucosa to characteristic nodular patterns. Management is empirical and individualized: symptomatic support with proton pump inhibitors (PPIs) and iron supplementation, anti-inflammatory therapy with topically targeted budesonide, dietary intervention in selected patients, and emerging mechanism-targeted approaches including α4β7 blockade. This narrative review synthesizes 101 articles published between 1989 and 31 August 2025, describing approximately 730 histopathologically confirmed CG cases (≈40% pediatric, ≈60% adult; overall female-to-male ratio ≈2.1:1). Replicated findings are distinguished throughout from those based on single reports, with preliminary observations explicitly flagged as hypothesis-generating.