A Rare Case of Gastritis with Prominent IgG4-Positive Plasmacytosis Mimicking IgG4-Related Gastrointestinal Disease.

INTRODUCTION: IgG4-related disease (IgG4-RD) is a systemic fibroinflammatory condition characterized by dense infiltration of IgG4-positive plasma cells across multiple organs, with well-established comprehensive diagnostic criteria and organ-specific criteria. However, diagnostic standards for IgG4-related disease involving the digestive tract remain relatively new and less clearly defined. We report a case suspicious for IgG4-related gastrointestinal disease (IgG4-GID). Although a definitive diagnosis could not be established, the clinical course of the patient and our assessment based on the most up-to-date evidence warrant attention. CASE PRESENTATION: A 40-year-old man was referred to our hospital after esophagogastroduodenoscopy revealed irregular nodules in the gastric mucosa, raising the suspicion of scirrhous-type gastric cancer. He had no significant medical history and had undergone successful eradication of Helicobacter pylori infection 5 years previously. Endoscopic examination revealed a faded shallow depression, irregular nodules, and converging folds in the gastric body. Magnifying endoscopy with narrow band imaging demonstrated a tree-like appearance with several irregular vessels, raising the suspicion of mucosa-associated lymphoid tissue (MALT) lymphoma, and a biopsy was performed. Laboratory tests showed that serum gastrin, anti-gastric parietal cell antibody, and IgG4 levels were within normal ranges. Histopathological examination showed no evidence of MALT lymphoma but revealed bottom-heavy plasmacytosis, and IgG4 immunostaining demonstrated an IgG4/IgG-positive cell ratio exceeding 40%. Endoscopic ultrasound revealed mucosal and submucosal thickening, and computed tomography (CT) showed thickening of the gastric wall, while positron emission tomography-CT showed no abnormal uptake. During the 5-year follow-up, no significant changes were observed. Biopsy also revealed neutrophilic infiltration in the superficial layer. Although IgG4-related gastritis was suspected clinically, neutrophil infiltration is uncommon in IgG4-related gastritis, and further differential diagnosis is required. CONCLUSION: This case highlights the diagnostic challenges of IgG4-rich gastric lesions and underscores the importance of distinguishing them from IgG4-GID and MALT lymphoma. When a definitive diagnosis cannot be established, careful long-term follow-up is warranted.