Aggregation and P-selectin expression of normal platelets exposed to plasma from an aHUS patient with CFH autoantibodies. Platelet optical aggregometry tracings of normal washed platelets incubated with ABO-matched normal plasma (A) or with plasma from aHUS patient 2 (B). Platelets were either preincubated with CFH (tracings a-b) or with buffer (tracings c-d) before measuring aggregation. Flow cytometry histograms showing P-selectin expression of normal washed platelets incubated with autologous plasma (C), ABO-matched normal plasma (D), plasma from an aHUS patient (E), or autologous plasma stimulated with U46619 (F). Incubation with aHUS plasma does not induce P-selectin expression (compare C-E), whereas stimulation with U46619 does (F).