A case of concurrent cold agglutinin disease and C3 glomerulonephritis requiring differentiation from other iatrogenic immunodeficiency-associated lymphoproliferative disorder in a patient with rheumatoid arthritis and Sjögren's disease.

We report a case of cold agglutinin disease (CAD) considered as other iatrogenic immunodeficiency-associated lymphoproliferative disorder (OIIA-LPD) in a patient with rheumatoid arthritis (RA) and Sjögren's disease. A 69-year-old woman developed hemolytic anemia and renal dysfunction after long-term methotrexate and infliximab therapy. Laboratory findings demonstrated monoclonal IgM-κ paraproteinemia and a high cold agglutinin titer, confirming CAD, while imaging studies and bone marrow evaluation revealed no evidence of overt malignancy. Renal biopsy revealed membranoproliferative glomerulonephritis-like lesions with C3-dominant deposition, consistent with C3 glomerulonephritis (C3GN). These findings suggest that both CAD and C3GN may represent manifestations of OIIA-LPD associated with long-term immunosuppressive therapy. Rituximab combined with glucocorticoids led to a prompt and marked improvement in anemia and renal lesions, resulting in sustained remission of both CAD and C3GN while maintaining RA remission. This case suggests the dual pathogenesis of B-cell dysregulation and complement activation and indicates that B-cell-targeted therapy may contribute to controlling both abnormalities. Accumulation of similar cases will be essential to refine disease classification and to optimize therapeutic strategies for immune-mediated overlap disorders.