Immunoblot of SDS-soluble platelet proteins subjected to SDS-PAGE (6.5% polyacrylamide gel) under nonreduced conditions and transferred to nitrocellulose membrane. For each sample, 150 μg total platelet protein was loaded. An anti–HPA-1a serum was used as a positive control (A), and the mother's serum was tested (B). Lanes: (1) HPA-1–positive platelets, (2) HPA-1–negative platelets, (3) father's platelets, (4) mother's platelets, (5) supernatant recovered after α-chymotrypsin treatment and sedimentation of the father's platelets, and (6) pellet after α-chymotrypsin treatment and centrifugation of the father's platelets. The band at ≈160 kD represents platelet-associated IgG.