PF4/heparin activate complement by classical pathway. (A) Plasma from a healthy donor was incubated with EDTA (10 mM) or EGTA (10 mM) ± MgCl2 (10 mM) or with buffer before incubating with PF4/heparin and complement activation was measured by the antigen-C3c capture ELISA assay. The y-axis shows the complement activation in different incubation conditions. ***P < .0001. (B) Plasma from a healthy donor was incubated with various concentration of anti-C1q antibody, anti-MBL antibody, or control antibody (0-100 μg/mL) before adding PF4/heparin and complement activation by PF4/heparin was determined by the antigen-C3c capture ELISA assay. The y-axis shows the complement activation in presence of various antibodies. *P < .05; **P < .001; ***P < .0001 compared with no antibody added condition. Results are shown from a representative experiment involving 3 donors tested on 3 different occasions.