Serum bactericidal activity against Hib. (A) Titer (reciprocal of the serum dilution capable of killing 50% of bacterial cells compared with complement controls) of serum bactericidal activity against Hib post vaccinations in ITP patients treated with rituximab (n = 14) or placebo (n = 6) (*P < .05; bars represent SEM). (B) Vaccine responders, defined as patients who achieved a fourfold increase in functional Hib killing in the first month after vaccinations.