The role of antibodies in platelet aggregation to Sk.

(A) Sk (500 U/mL) combined with a subthreshold concentration of ADP (2 μM) results in irreversible platelet aggregation. This is inhibited by 5 μg/mL of the Fc receptor-blocking antibody IV.3. (B) In individuals who fail to aggregate to low concentrations of Sk and ADP, the addition of 50 μL purified anti-Sk antibodies results in an aggregation response. This is also sensitive to inhibition by IV.3. (C) Whole-plasma IgG from a responsive donor (donor 1) but not from a nonresponsive donor (donor 2) can confer sensitivity to Sk on PRP from a third donor. These results are representative of tracings from at least 5 (A and B) or 2 (C) individuals.