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 ( , ).
Subcellular locations: Cell membrane, Early endosome membrane, Golgi apparatus, Trans-Golgi network membrane, Lysosome membrane
Continuously recycles between cytoplasmic pool and the plasma membrane to bind as much IgM as possible.
Subcellular locations: Secreted
Detected in the serum.
Expressed by CD19-positive B cells and CD4-positive and CD8-positive T cell populations in primary and secondary lymphoid tissues (at protein level). Among B cell subsets, detected in a subset of bone marrow pro- and pre-B cells, in most follicular and memory B cells and in a small subset of germinal center B cells (at protein level). Expressed at lower levels in CD56-positive NK cells (at protein level) ( , ). Expressed in lymph nodes, lung, thymus and kidneys. Very weak expression detected in spleen, liver, heart, and salivary gland.