β₂ GPI selectively inhibits ristocetin-induced aggregation in PRP. Aggregation of PRP was induced with various concentrations of ADP (0-5.0 μM; A), collagen (0-2.0 μg/mL; B), or ristocetin (0.8-1.3 mg/mL; C) in the absence (■) or presence (●) of β₂ GPI (2.2 μM), monoclonal anti-β₂ GPI antibody 1F12 without LA activity (□; 2.2 μM), or monoclonal anti-β₂ GPI antibody 27G7 with LA activity (2.2 μM; ○). A typical example of each aggregation is shown. All aggregation experiments were performed 3 times using blood of different donors.