Immunohistochemical analysis of human fetal bone sections engrafted with WM cells. A fetal bone engrafted with IgMκ WM cells was retrieved from murine host after detection of rising levels of human paraprotein in mouse serum and examined. (A) Staining with anti-CD20 monoclonal antibody (mAb; original magnification, × 200), (B) anti-IgM polyclonal Ab (original magnification, × 200), and (C) anti-κ mAb (original magnification, × 200), demonstrates predominantly cytoplasmic reactivity within the plasmacytic component of the tumor only, (D) anti-IgG mAb (original magnification, × 400), and (E) anti-λ mAb (original magnification, × 200).