A antigen on platelet membranes by donor, A₁/A₂ subgroup, age, sex, and Le (b+) or secretor phenotype. (A) Platelets were double stained with PE-CD41, a platelet-specific marker, and FITC-labeled HPA lectin, which recognizes the blood group A antigen. The results were reported as the mean percentage of CD41⁺ platelets that were also HPA⁺. Shown are sample histograms from an A₁ (B), A₂ (C), ABH-high expresser (D, HPX), and group O (E) donors. Horizontal brackets (B-E) indicate HPA⁺ fluorescence. Cells left of bracketed area were considered HPA-negative. (F-G) HPA staining in all group A, A₁, and A₂ donors by Le (b+) and Le (b-) phenotype, respectively. (H) The percent HPA⁺ platelets for the first (S1) and second (S2) samples plotted for each individual donor, where y = 0.5 + 0.34x, R = 0.93. (I) Expression of A antigen on platelet membranes in group A₁ donors, relative to donor age and sex. The analysis was restricted to A₁ donors only, and reported as percent HPA⁺ platelets (X ± SD) by decade in female (▨) and male (▪) donors. There was a small but significant increase in HPA staining in female donors after the fifth decade (P = .03).