What Is Collagenous Gastritis?
Collagenous gastritis is a rarely reported stomach disorder characterized by the deposition of a subepithelial collagen band and accompanying inflammatory infiltrate in the stomach wall.
A subepithelial collagen band means that collagen, a tough fibrous protein, has abnormally accumulated under the surface layer of epithelial cells which form the mucosa lining the stomach.
The surface mucosa may remain intact and not show lymphocytosis and/or gastritis on endoscopic examination or it may become stripped off and bleed from entrapped capillaries. Biopsy is necessary to diagnose collagenous gastritis.1 
Q: Why does collagen become deposited under the surface lining of the stomach?
A: It is thought that collagen deposition is preceded by inflammation and tissue damage. However, the natural history and pathogenesis of collagenous gastritis remain unclear.2 
What Is Collagenous Gastritis In Celiac Disease and/or Gluten Sensitivity?
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Jain R, Chetty R. Collagenous gastritis. Int J Surg Pathol. 2010 Dec;18(6):534-6. doi: 10.1177/1066896908329588. Epub 2008 Dec 22. ↩ 
Leung ST, Chandan VS, Murray JA, Wu TT. Collagenous gastritis: histopathologic features and association with other gastrointestinal diseases. Am J Surg Pathol. 2009 May;33(5):788-98. doi: 10.1097/PAS.0b013e318196a67f. ↩