High-affinity Fc receptor for immunoglobulin M (IgM), both secreted and membrane-bound IgM. Primarily regulates IgM transport and homeostasis. Primarily regulates IgM transport and homeostasis. In lymphoid cells, enables exocytosis of membrane-bound IgM on the plasma membrane as well as endocytosis of IgM-antigen complexes toward lysosomes for degradation. In mucosal epithelium, mediates retrotranscytosis of antigen-IgM complexes across mucosal M cells toward antigen-presenting cells in mucosal lymphoid tissues. Triggers costimulatory signaling and mediates most of IgM effector functions involved in B cell development and primary immune response to infection. Likely limits tonic IgM BCR signaling to self-antigens for proper negative selection of autoreactive B cells in the bone marrow and for the maintenance of regulatory B cell pool in peripheral lymphoid organs. Mediates antibody responses to T cell-dependent and T cell-independent antigens and promotes induction of an efficient neutralizing IgG response. Engages in cross-talk with antigen-receptor signaling via the non-canonical NF-kappa-B, MAP kinases and calcium signaling pathways.