Rituximab (anti-CD20) induces B-cell apoptosis (A, right). The rapid response seen in some patients with ITP may reflect competition between clearance of antibody-coated B cells and antibody-coated platelets (P) by Fcγ receptors (cylinder) expressed on macrophages (M). Phagocytosis of anti-CD20 – coated B cells (A, left) may augment apoptosis mediated by complement (C) and natural killer (NK) cells (right). Illustration by Paulette Dennis.