Animals deficient in complement components C1q, C4, C3, and CR1/2 have severely impaired antibody responses. C1q is primarily activated by antibody-antigen complexes. Antigen-specific IgM in complex with an antigen is able to enhance the antibody response against that antigen. This is dependent on the ability of IgM to activate complement. Naïve mice have very low amounts of specific antibodies and therefore it is surprising that classical pathway activation plays a role for primary antibody responses. It was hypothesized that natural IgM, present in naïve mice, would bind an antigen with enough affinity to activate C1q. To test this, a knock-in mouse strain, Cm13, with a point mutation in m heavy chain, making its IgM unable to activate complement was constructed. Surprisingly, the antibody responses in Cm13 were normal. Puzzled by the finding that the ability of IgM to activate complement was required only for some effects, the immunization protocol was changed to mimic an infectious scenario. With this regime, Cm13 mice had an impaired antibody response compared to wildtype (WT) mice. The antibody response in WT mice to these repeated low-dose immunizations was also enhanced. These observations suggest that IgM-mediated enhancement indeed plays a physiological role in initiation of early antibody responses. IgM-mediated enhancement cannot however compensate for the dependecy of T-cell help. Although IgM from WT mice enhanced the antibody response, the T-cell response was not enhanced. The connection between classical pathway activation and CR1/2 is thought to be generation of ligands for CR1/2. In mice, CR1/2 are expressed on B cells and follicular dendritic cells (FDC). Although CR1/2 are crucial for a normal antibody response, the molecular mechanism(s) are not understood. To investigate whether CR1/2 must be expressed on B-cells or FDC to generate a normal antibody response, chimeric mice between WT and CR1/2-deficient mice were constructed. The results show that CR1/2+ FDC were crucial for the generation of antibody responses. In the presence of CR1/2+ FDC, both CR1/2+ and CR1/2- B cells were equally good antibody producers. However, for an optimally enhanced antibody response against IgM-antigen complexes, both B cells and FDC needed to express CR1/2.