Heparin dependence in the binding of HIT IgG to collagen-aggregated platelets.

Platelet-rich plasma (300 × 10⁶ platelets/mL) was mixed with HIT or normal IgG (12 μg/mL) and ¹²⁵I-IgG tracer in a platelet aggregometer for 12 minutes before collagen (4 μg/mL) ± heparin (0.5 U/mL) was added. Platelets were sampled 2 minutes before collagen, 4 minutes after collagen (when platelets were consistently fully aggregated), and 10 minutes after collagen by centrifugation through 17% sucrose. Solid symbols indicate the specific binding of patient 1 (▪) and patient 2 (•) IgG and nonspecific binding of normal IgG (▴) in the presence of heparin. Corresponding open symbols represent binding without heparin. Gray trace shows a typical aggregation profile. Values are the means of 2 experiments.