A lymph node lesion of hyper IL-6 syndrome mimicking plasmacytoma, IgG4-related disease, and Castleman disease.

A 72-year-old male patient presented fatigue, anemia, elevated total protein, IgG, IgG4, IL-6, and vascular endothelial growth factor (VEGF) levels. Initial diagnostics suspected multiple myeloma. A plane computed tomography (CT) scan showed pneumonia and the enlargement of generalized lymph nodes. A lymph node biopsy revealed a mix of immature and mature plasma cells, significant IgG4-positive cells, and IL-6-secreting plasma cells, mimicking plasmacytoma, IgG4-related disease, and Castleman disease, ultimately leading to a diagnosis of hyper IL-6 syndrome. Treatment with antibiotics improved the patient's condition without the need for additional therapies, including tocilizumab.