Delayed bone marrow plasma cell response in c-mpl^−/− mice. Wild-type and c-mpl^−/− mice were immunized with NP-KLH that induces a predominant IgG1 response. NP-specific IgG1-ASCs in bone marrow were quantified at day 12 and day 28 after primary and day 5 and day 21 after secondary immunization by ELISPOT. Frequencies of NP-IgG1-ASCs in the spleen were normal in the c-mpl^−/− mice (supplemental Figure 5). Data are mean ± SD for groups of 3 to 5 mice (statistics: unpaired t test).