RDD LAD with increased IgG4+ plasma cells. Top left: H&E, 2 ×  magnification. Left axillary lymph node biopsy from a patient with RDD disease. The lymph node architecture is preserved, but there are distended sinuses with lesional histiocytes (top left image, arrows). Top right: H&E, 40 ×  magnification. Histiocytes with ample eosinophilic cytoplasm and nuclei containing prominent nucleoli characterize the lesion. Many contain engulfed inflammatory cells (emperipolesis), such as those labeled with a black arrow. Occasional background plasma cells are identified (blue arrow). Bottom left: IgG immunohistochemical stain, 40 ×  magnification. The higher magnification of Figure 4 highlights the many plasma cells positive for IgG. Bottom right: IgG4 immunohistochemical stain, 40  ×  magnification. The higher magnification of Figure 4 highlights the scattered plasma cells positive for IgG4. The IgG4/IgG ratio is not increased (14%). Figure courtesy of Collin Pryma.