Old mice with a rejuvenated peripheral repertoire mount an increased anti-NP IgG1 response. Old C57Bl6 WT mice (22 months, prescreened to have < 5% AA4.1⁺ B cells in peripheral blood as an indication for an “old-like” B-cell phenotype) were subjected to one round of B-cell depletion and were immunized intraperitoneally with NP-CGG 70 days later. Before immunization, these mice were bled to confirm reconstitution of >80% of B220⁺ in peripheral blood. Old, age-matched, and young C57Bl6 mice (4 months) that were untreated for B-cell depletion were used as controls and immunized with NP-CGG at the same time. The amount of anti–NP-specific IgG1 antibodies in the serum was determined by enzyme-linked immunosorbent assay 7 days later, using an IgG1 standard curve for reference. The plot shows titers of anti-NP IgG1 antibodies of individual mice (old depleted, n = 9; old untreated, n = 11; young untreated, n = 7; and naive, n = 4) collected from 4 different experiments and run simultaneously, and the mean and SEs of each group. Student t test was conducted to examine statistical significance between antibody titers of different groups.