In vivo platelet tracking using flow cytometry. (A) Flow cytometric data showing sensitized platelets in peripheral blood. Sensitized platelets can be detected with an FITC-labeled anti-human IgG antibody in the peripheral blood after re-injection. Platelet sensitized with B2G1Δnab (top panel) are detected up to 1 hour post-reinjection but after 24 hours the antibody has redistributed to the whole platelet population as evidenced by the shoulder on the histogram and increased MFI for the whole platelet population. In contrast, when platelets are sensitized with a combination of 75% B2G1Δnab/25% B2G1, the majority of platelets are rapidly cleared from circulation after 60 minutes. (B) Correlation between flow cytometry and radiolabeling studies data in 4 volunteers. The percentage of platelets (normalized at 100% at the 5-minute time point) remaining in circulation in the first hour, measured either by flow cytometry or radiolabeling, is consistent between the 2 methods. Over 75% of platelets sensitized with B2G1Δnab only remain in circulation (n = 1) while platelets sensitized with 75% B2G1Δnab/25% B2G1 show a sharp decrease in the first hour down to below 25%.