Passive transfer of cell-free serum from HEL/CFA-immunized mice is sufficient to induce HEL antigen loss from transfused mHEL RBCs, but antigen loss does not occur in vitro. A mixture of leukoreduced CM-DiI–labeled mHEL RBCs and DiO-labeled C57BL/6 RBCs was transfused into mice that had previously received an intravenous injection of HEL/CFA immune serum (B,G), OVA/CFA immune serum (C,H), or no injection (D,I). The mixture of RBCs was also incubated with HEL/CFA or OVA/CFA immune serum in vitro (E-K). Two days after transfusion, peripheral blood was harvested. Antibody coating of RBCs was measured by gating on transfused mHEL (CM-DiI⁺) or C57BL/6 (DiO⁺) cells and comparing staining with anti–mouse Ig. Remaining HEL antigen was measured by staining with anti-HEL. The experiments shown in this figure have been reproduced in at least 3 separate experiments. The data presented in this figure are representative results.