The in vivo effect of anti-PF4 antibodies. (A) Same as in Figure 4, but for change in platelet count in WT animals injected with either F(ab′)2 fragments prepared from IgG from preimmunization serum (•) or from polyclonal anti-mPF4 antibodies (♦). *P ≤ .05. (B) Same as in panel A, but for change in platelet count from baseline of WT animals (•) versus littermate hPF4×6⁺ animals injected with either F(ab′)2 fragments from either anti-hPF4 (♦) or preimmune IgG (◇). For panels A and B, *P < .03 and **P < .003 comparing mice treated with preimmune IgG to the mice treated with anti-PF4 antibody. Each point in this figure represents the mean of 6 to 12 animals plus or minus 1 SD.