Platelet activation by HIT IgG. (A) Squares represent IgG isolated from 4 HIT plasmas incubated with human platelets that had been exposed to different amounts of hPF4. Filled circles indicate simultaneously studied KKO; and open circles, simultaneously studied isoimmune control TRA. (B) Details are the same as for panel A using a commercial control IgG preparation (open diamonds) or 4 preparations of IgG from healthy controls (filled and gray diamonds). The mean value is shown for each patient studied on 3 to 5 separate occasions. Each experiment was performed in duplicate. For clarity, standard deviations are not shown.