The function of FCMR (Fc mu receptor, Ensembl gene identifier ENSG00000162894) is as follows. High-affinity Fc receptor for immunoglobulin M (IgM), both secreted and membrane-bound IgM (PubMed:19858324, PubMed:22675200, PubMed:36949194, PubMed:37095205). 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 (PubMed:21908732, PubMed:28230186). 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 (PubMed:19858324, PubMed:22675200, PubMed:25601920, PubMed:30840890). {ECO:0000269|PubMed:19858324, ECO:0000269|PubMed:21908732, ECO:0000269|PubMed:22675200, ECO:0000269|PubMed:25601920, ECO:0000269|PubMed:28230186, ECO:0000269|PubMed:30840890, ECO:0000269|PubMed:36949194, ECO:0000269|PubMed:37095205}.