Splenic MZP frequencies decline with age in spleen and blood. (A) Frequencies (expressed as percentages of total CD19⁺ B cells) of Dump⁻CD19⁺-gated IgD⁺CD27⁻MTG⁻MEM55⁺ (MZP) cells in spleen (top) and blood (bottom) from children and adults. (B) The same analysis performed for MTG⁺MEM55⁺ cells in spleen (top) and blood (bottom) for children and adults. Age range for children’s spleen samples: 4–9 yr; for blood samples: 1–9 yr. Mean values ± SEM are indicated on the graphs. ***, P < 0.001; **, P < 0.01; *, P < 0.05; Mann-Whitney statistical test.