rTT.C-specific CD27high PBMCs express intracellular IgG. Phenotype of rTT.C-specific PBMCs 6 days after TT booster immunization. Formaldehyde-fixed PBMCs were gated (A) and stained with anti-CD27 Cy5 mAb, biotinylated anti-CD19 mAb/SA PerCP, anti–human IgG-FITC, rTT.C-Dig, and anti-Dig PE in the presence (C-D, F-G) or absence (B, E) of saponin. Expression of intracellular IgG, surface CD27, and surface CD19 of rTT.C-specific PBMCs, gated as indicated in panels C and F, is shown in panels D and G, respectively.