Effect of blocking platelet Fc receptor on the binding of HIT IgG during platelet aggregation.

Platelet-rich plasma (300 × 10⁶ platelets/mL) was mixed with the monoclonal antibody IV.3 (anti-FcγRII, 50 μg/mL), HIT IgG (12 μg/mL), and ¹²⁵I-IgG tracer in a platelet aggregometer for 12 minutes before heparin (0.5 U/mL) was added. Aggregation was induced by the addition of collagen (4 μg/mL) 12 minutes after heparin was added. The reaction was sampled 2 minutes before heparin, 10 minutes after heparin (when full aggregation would have occurred without IV.3), and when the collagen-induced aggregation was complete. Samples were centrifuged through a 17% sucrose cushion to separate platelets from unbound IgG. Specific binding of patient 1 (▪) and patient 2 (•) was quantitated by the inclusion of the same HIT ¹²⁵I-IgG tracer, whereas nonspecific binding (▵) was measured by the binding of normal ¹²⁵I-IgG. The gray trace shows a typical aggregation profile. IgG bound is the mean of 2 experiments.