Activation of GPIIb-IIIa using flow cytometry. Binding of antibody CD41a (which binds to the GPIIb-IIIa complex) and PAC-1 (which binds to the activated GPIIb-IIIa complex) in the patient, brother, mother, and sister. CD41a binding was normal in all subjects, indicating that the GPIIb-IIIa complexes were present in normal numbers. PAC-1 binding in response to PAF and U46619 was diminished in the patient but normal in the family members. For PAC-1 binding, the open peak represents the resting state; the filled peak is after activation.