MHC-immunized recipients rapidly clear MHC-mismatched platelets. (A-B) Serum from nonimmunized C57BL/6 (H-2^b) recipients (NI) or FVB (H-2^q)-immunized C57BL/6 recipients (I) was incubated with FVB platelets (A) or C57BL/6 (B6) platelets (B) followed by detection of bound antibody by incubation with anti–immunoglobulin G (IgG) and flow cytometric examination (n = 5). (C) Nonimmunized or FVB-immunized C57BL/6 recipients were transfused with C57BL/6.GFP × FVB (FVB^GFP) or C57BL/6.GFP (B6^GFP) platelets followed by flow cytometric examination 24 hours later (gate = percentage of total platelets). (D-E) Percentage of FVB^GFP (D) or B6^GFP (E) platelets remaining, normalized to nonimmunized recipients, as indicated at various time points posttransfusion into nonimmunized (NI) or FVB-immunized (I) C57BL/6 recipients (n = 5). Significance was determined in panels A, B, D, and E by Student t test (**P ≤ .01, ****P ≤ .0001). MFI, mean fluorescence intensity; ns, no significance; plts, platelets; SSC, side scatter.