Bone marrow biopsy of lymphoid aggregate rimmed by PCs. (A) The lymphoid aggregate has a regressed germinal center that is Castleman-like (H&E). (B) CD138 positive PCs form a distinctive rim around the lymphoid aggregate. (C) The PCs are monotypic for λ immunoglobulin light chains (D) and negative for κ immunoglobulin light chains by IHC. Photomicrographic images were obtained with an Olympus BX51 microscope equipped with an Olympus DP71 camera and software. Original magnification 20×/0.50 UPlanFL N lens for all panels.