Reactivity of sera from group 2 with L αβ cells before and after transfection with GPIX cDNA. L αβ cells were transfected with GPIX cDNA using the GIBCO-BRL lipofectAMINE kit as described in Materials and Methods. The cells were labeled with primary antibody (either anti-GPIX MoAb FMC25 [A and B], or patient sera from group 2 plus quinine [C and D]) followed by a FITC-conjugated secondary antibody and examined by flow cytometry. L αβ cells did not react with anti-GPIX MoAb before transfection with GPIX cDNA (A) but bound anti-GPIX MoAb after GPIX cDNA transfection (B). Patient sera from group 2 did not react with L αβ cells before transfection with GPIX cDNA (C) but were able to bind to L αβ cells expressing GPIX on their surfaces after transfection with GPIX cDNA (D). The graphs are representative of the results observed with three different patients' sera. The solid peak in each graph represents the negative control.