Complement activation by KKO and HIT ULICs occurs in plasma and WB. Plasma (A,C) or WB (B,D) were incubated with KKO or ISO (A,B) or anti-PF4/heparin IgG from patients with (“HIT IgG”; n = 6) or without (“αPF4/H IgG”, n = 3) HIT or control IgG either in buffer or with PF4/heparin (H) as shown on the x-axis. Complement degradation product C3c was measured. Results shown are representative of 3 independent experiments. ***P < .0001, with all other conditions (A,B) and *P < .05 (C,D) (1-way analysis of variance with Tukey’s multiple comparison test). ns, not significant.