Inhibition of HIT IgG binding to platelets by anti-PF4.

Platelets that were activated by collagen plus heparin were resuspended in PBS containing 2% BSA, 6 mg/mL human IgG, 10% ETP, and 50 μg/mL IV.3. Platelets (60 × 10⁶) were blocked for 30 minutes with various concentrations of affinity-purified sheep anti-PF4 IgG before HIT ¹²⁵I-IgG tracer was added for 10 minutes. All incubations were performed at 0°C to minimize further platelet activation. After centrifugation and 2 washes with PBS, free¹²⁵I-IgG was removed. Binding of HIT IgG from patient 1 (▪) and patient 2 (•) is expressed as a percentage of the total radioactivity added because the IgG concentration of the¹²⁵I-IgG tracer alone cannot be accurately measured. Background binding of normal ¹²⁵I-IgG was only 0.05%.