The B cell immune response to 2,4-dinitrophenyl (DNP) keyhole limpet hemocyanin was compared in antigen-free, germ-free and conventional BALB/c mice. The numbers of total and of DNP-specific IgM-, IgG- and IgA-secreting cells in the spleen were determined by enzyme-linked immunosorbent plaque assays after primary, secondary and hyperimmunization. Three days after primary immunization a peak of DNP-specific IgM-secreting cells was seen in conventional mice only. However, this specific response was accompanied by a rise in the total number of IgM-secreting cells. At day 6 after primary immunization the total number and the frequency of DNP-specific IgG-secreting cells were higher in antigen-free mice, compared to germ-free and conventional mice. After secondary immunization in conventional mice only, a considerable bystander IgG response was seen together with the DNP-specific IgG response. At the end of the secondary response 90% of all IgG-secreting cells were DNP specific in antigen-free mice, while the corresponding figure in germ-free and conventional mice was 63% and 14%, respectively. After hyperimmunization, the absolute number of DNP-specific IgG-secreting cells in the spleen was 5-fold and 11-fold higher in antigen-free mice then in germ-free and conventional mice, respectively. Approximately 48% of all IgG-secreting cells were DNP specific in antigen-free mice,while the corresponding figure in germ-free and conventional mice was 17% and 12%, respectively. The results show that the exogenous antigenic load of animals influences the immune response to newly introduced antigens. The higher absolute and relative numbers of antigen-specific IgG-secreting cells after hyperimmunization in antigen-free mice compared to germ-free and conventional mice may provide a better source for antigen-specific B cells that eventually can be used for hybridoma production.</p
