Analysis of IgG1, IgG2a, IgG2b, and IgG3 isotypes in WT, CD8-depleted, NK-depleted, and CD8/NK-depleted mice. Donor cells were incubated with a 1:50 dilution of recipient sera obtained after the fifth platelet transfusion, labeled with the indicated FITC-goat antimouse IgG isotype antibody, and analyzed by flow cytometry. Fold increase was calculated by the following formula: percentage of donor cells positive with test serum/percentage of donor cells positive with prebleed test serum. The results are expressed as the mean (± SD) of the fold increases in 20 mice from each group. • indicates WT; □, CD8-depleted; ▦, NK-depleted; ▧, CD8/NK-depleted. Significance values between WT and CD8-depleted mice are indicated (^*P = .04; ^**P = .009).