Flow cytometric analysis of CD4 expression on human peripheral blood lymphocytes. Human whole blood was stained with the BD Horizon™ BV510 Mouse Anti-Human CD4 antibody (Cat. No. 562970/562971; solid line histogram) or with a BD Horizon™ BV510 Mouse IgG1, κ Isotype Control (Cat. No. 562946; dashed line histogram). The erythrocytes were lysed with BD FACS™ Lysing Solution (Cat. No. 349202). The fluorescence histograms were derived from events with the forward and side light-scatter characteristics of viable lymphocytes. Flow cytometry was performed using a BD™ LSR II Flow Cytometer System.