Slight influence of MV infection on the KLH-DNP-specific B cell response. (A) Spleen cells from animals (infected 4 days previously or noninfected) immunized with 100 μg of KLH-DNP were analyzed for the numbers of B cells secreting IgM and IgG specific for KLH-DNP from days 4, 14, and 21 (four animals per group) by ELISPOT. The number of B cells per spleen was estimated with a cross-reactive serum (rabbit anti-rat immunoglobulins) and the number of cells secreting KLH-DNP-specific antibodies were expressed per 1,000 B cells in the spleen. There was no difference in the number of B cells secreting IgM and IgG specific for KLH-DNP between infected and control animals. Animals immunized with 1 mg of KLH-DNP were tested on day 28 (nine animals per group). The high dose of antigen led to an increase in IgM- but not IgG-secreting B cells, and the difference between infected and noninfected animals was weakly significant (P > 0.04). (B) Sera taken from the same animals on days 4, 14, 21, and 28 were tested at a 1:200 dilution in an ELISA assay for KLH-DNP-specific IgG and expressed as OD ± SD. The only difference between infected and noninfected animals was seen on day 14 (P < 0.006).