NP-specific memory antibody responses are of both IgG and IgM isotypes. Six weeks after priming, mice were boosted with NP-KLH and the levels of IgG (A) and IgM (B) antibody in sera (n = 9) were compared with unboosted controls (n = 5) and controls, which received a booster injection only (n = 6) as measured by ELISA at time points indicated. (C-D) IgG⁺ and IgM⁺ antibody-secreting cells in the spleens (n = 3) were measured by ELISPOT on the days indicated. Controls that received a booster injection only (n = 3) as well as an unboosted control group were included. Horizontal bars represent median values. These data are representative of 2 independent experiments.