Frequency and phenotype of circulating CD4/CD8 double-positive cells in healthy blood donors. (A) Frequency of CD4⁺CD8⁺ cells in 10 healthy blood donors (HD). The percentage of CD4^highCD8^low and CD4^lowCD8^high double-positive cells in the PBMC population is indicated. (B) Percentage of naive, effector, and memory cells within the CD4^highCD8^low and CD4^lowCD8^high double-positive cell populations of 10 healthy blood donors. CCR7⁺CD45RO⁺ indicates central memory cells; and CCR7^-CD45RO⁺, effector memory cells. Error bars indicate SD. (C) Phenotype of CD4 and CD8 single-positive cells and CD4⁺CD8⁺ cells (DP). The percentage of cells with the indicated marker is shown for each subpopulation. The solid line indicates the mean percentage of CD4 and CD8 single-positive and CD4⁺CD8⁺ cells with the indicated surface marker. Significant differences (P ≤ .05; Student t test) are indicated.