The effects of intravenous immunization against influenza on FVIII immunogenicity. Mice were immunized intravenously with a scaled standard human dose (4.8 μg/kg) of influenza vaccine within 24 hours of the first of 7 rhFVIII (6 IU, ∼200 IU/kg) infusions. Blood was collected by cardiac puncture 4 weeks later, and plasma was isolated by centrifugation. Plasma samples were assessed for the presence of FVIII-specific IgG (A) and inhibitors (B). Total cohort sizes are indicated above each bar. (C) Comparison of titers of influenza- and FVIII-specific IgG among FVIII responders. (D) Comparison of inhibitory activity among FVIII responders. The horizontal lines and error bars represent the mean and SEM.