MKI67 and infection: We also detected a small decrease in the frequency of Ki-67-positive proliferating B cells in old mice at days 6 and 8 after infection (3.5% to 2.3% at day 6 and 5.5% to 4.2% at day 8, P < 0.05, Fig 2M), which could reflect delays in the germinal center reaction observed in the old DLN (Fig 2A and 2C).