Updated project metadata. Effective protection from viral infections depends on the antibody-specific isotype. Here, we found that the protein kinase, DYRK1A, is essential for B cell-mediated protection from viral infection and vaccination through regulation of class switch recombination (CSR). Dyrk1a-deficient B cells were impaired in CSR activity in vivo and in vitro. Phospho-proteomic screens and kinase-activity assays identified MSH6, a DNA mismatch repair protein, as a direct substrate for DYRK1A, and deletion of a single phosphorylation site attenuated CSR. After CSR and germinal center seeding, DYRK1A was required for proper clonal expansion of antigen-specific B cells through attenuation of proliferation. These findings reveal DYRK1A-mediated biological mechanisms of antibody-mediated immune responses that may be used for manipulation in antibody-mediated autoimmunity