Enhanced humoral response to NP-KLH in 4.1R^−/− mice. Ten 4.1R^+/+ mice and 10 4.1R^−/− mice were immunized with NP-KLH. The concentrations of anti-NP–specific IgM at day 10 after primary immunization and IgG1 at day 23 after primary immunization (10 days after a booster injection on day 13) were measured. Error bars indicate SD.