Factor H is constitutively present on mouse platelets but not erythrocytes. Although FACS analysis detected reactivity to anti–human factor H antibodies on the mouse platelets (A) and erythrocytes (B), Western blot analysis confirmed fH association with platelets but not with erythrocytes (C right). Factor H in human and mouse sera were used as a positive control for antibody reactivity (C left), and GAPDH signal was used as a loading control for platelet and erythrocyte proteins. The positions of molecular weight markers (in kilodaltons) were indicated on the right side of panel C.