Bone marrow biopsy from a patient with new-onset systemic JIA. (a) H&E staining showing hypercellular bone marrow with prominent accumulations of nucleated erythroblasts (arrows). (b) Immunostaining with monoclonal antibodies specific for CD163. Brown staining identifies CD163+ cells (some are hemophagocytic). CD163+ macrophages are very rare in a normal bone marrow (not shown).