Inhibition of FcγRIIa-mediated calcium mobilization mediated by anti-PECAM-1 Fab fragments. Calcium mobilization was measured after FcγRIIa cross-linking of Fura-2 labeled washed platelets in the presence and absence of coengagement of CD151 and PECAM-1. The Fura-2 ratio was determined using a luminescence spectrometer and recorded for 500 seconds.