Human platelets expressing decreased FcγRIIa fail to spread on immobilized fibrinogen. (A-C) Flow cytometric analysis of washed platelets from a patient with an autoantibody specific for GPVI, demonstrating approximately 60% reduced expression of FcγRIIa (as reported by the binding if mAb IV.3), but normal expression of αIIbβ3 and the GPIb complex. (D,E) Washed patient platelets or control platelets from a healthy donor were allowed to spread on glass slides that had been coated with 100 μg/mL fibrinogen. The degree of platelet spreading was analyzed at both 30 and 60 minutes. Note that the patient's platelets failed to spread. Data are the mean plus or minus SEM.