Normal FH reduces C3 and C9 binding and CD40L and CD62P expression after exposure to patient serum. Incubation of patient sera with normal washed platelets (patients 1 [♦], 2 [▴], 3 [●], and 4 [◇], from citrated tubes; other platelet donors than in Figure 4) led to C3 and C9 deposition as well as CD40L and CD62P expression. Preincubation of washed platelets with normal FH before addition of patient serum reduced C3 and C9 binding (A,B) as well as expression of CD40L and CD62P (C,D) on normal platelets. Normal serum added to normal platelets (○) led to minimal C3 and C9 binding or CD40L and CD62P expression on the platelet surface; this pattern was not altered in the presence of normal FH. Results were carried out 3 times with reproducible results. *P < .001, when comparing platelets incubated with patient serum with and without normal FH and when comparing patient sera with control sera. Results are expressed as median plus or minus SD.