Antineutrophil Cytoplasmic Antibody-Associated Vasculitis Overlapping with IgG4-Related Disease Accompanied by Mesangial Immunoglobulin a Deposition: A Case Report.

Background: Antineutrophil cytoplasmic antibody-associated vasculitis (AAV) and immunoglobulin G4-related disease (IgG4-RD) are both systemic autoimmune disorders that can involve multiple organs; however, they exhibit distinct differences in pathogenesis, clinical manifestations, and histopathological features. Emerging evidence suggests that, in rare instances, AAV and IgG4-RD may coexist in the same patient, constituting an immunological overlap syndrome. Case Presentation: We reported the case of a 67-year-old woman who presented with proteinuria, hematuria, and impaired renal function. After comprehensive laboratory investigations, imaging studies, and renal biopsy, the patient fulfilled the diagnostic criteria for both AAV and IgG4-RD. Renal biopsy revealed necrotizing crescentic glomerulonephritis with marked IgG4-positive plasma cell infiltration and mesangial immunoglobulin A (IgA) deposition. The patient was treated with systemic glucocorticoids combined with cyclophosphamide, leading to significant improvement in renal function. Conclusion: The coexistence of AAV and IgG4-RD with concurrent IgA deposition is exceedingly uncommon in clinical practice. This case highlights the complexity of immune-mediated overlap syndromes and underscores the importance of careful pathological assessment, which may inform diagnostic evaluation and treatment strategies.