IgG1 is the predominant antibody that binds to mHEL RBCs in HEL/CFA-immunized mice. C57BL/6 mice were immunized with HEL/CFA. Two weeks after immunization, serum was harvested. The titer, isotype, and subtype of anti-HEL antibodies were determined by HEL-specific ELISA (A-B). The capacity of each subtype to bind to mHEL RBCs was determined by incubating serum from HEL/CFA-immunized mice, at a dilution of 1:50, with RBCs from either mHEL or C57BL/6 mice followed by IgG subtype-specific antibodies. Binding was measured by flow cytometry (C). Solid line indicates mHEL RBCs; and dotted line, B6 RBCs. Analysis of this type was performed on all groups of immunized mice in all subsequent experiments to confirm similar patterns of immunization. Essentially identical results were obtained in all animals. The experiments shown in this figure have been reproduced in at least 3 separate experiments. The data presented in this figure are representative results. O.D. indicates optical density.