Proliferation of PBMCs fromalloimmunized and unimmunized donorsin response to K peptides is mediated by cells with the CD3⁺CD4⁺ Th phenotype. Flow cytometric analyses of PBMCs from (A) alloimmunized donor 5 or (B) control donor 4 left unstimulated, or stimulated with K peptides 1M(_179-193) or 3M(_181-195) and stained for expression of CD3, CD4, and the activation marker CD71. Tables next to each pair of plots indicate the percentage of cells in quadrants. The results are representative of 5 alloimmunized and 3 control donors. In all cases, > 90% of the increase in the CD71⁺ activated population after peptide stimulation is accounted for by cells that are CD3⁺ and CD4⁺. (C) Flow cytometric analyses of CFSE-stained PBMC from control donor 5 left unstimulated, or stimulated with K peptide 1M_(179-193) and stained for expression of CD4.