Anti-idiotypic IgM antibodies. Time kinetics of B cells secreting IgM antibodies binding to F(ab′)₂ fragments of the autologous (•) or isotypic control (○) monoclonal IgG were assessed by ELISPOT. Mean ± SEM (n = 5) for each time point are shown. The increase in the numbers of IgM-secreting cells binding to the autologous IgG was statistically significant at week 8 (P < .05) and week 10 (P < .01) compared with baseline. The difference between autologous and isotypic control monoclonal IgG was statistically significant (P < .05) at all points. Arrows indicate immunization times.