The Immunoglobulin M (IgM) Fc5μ fragment, comprising the constant region (Cμ2-Cμ4 domains) of the μ heavy chain, mediates critical immune functions despite lacking antigen-binding capacity. This fragment drives complement fixation via C1q binding, with studies showing its pentameric form in Waldenström macroglobulinemia exhibits 19-fold higher complement activation than intact IgM. Structurally, the Fc5μ's conserved N-glycosylation at Asn-297 stabilizes its conformation and enables interactions with Fc receptors (FcμR) on B cells, T cells, and NK cells, modulating immune responses like phagocytosis and antibody-dependent cytotoxicity. While IgM typically lacks placental transfer due to its size, Fc5μ's interaction with FcRn-a receptor critical for IgG transcytosis-hints at nuanced roles in maternal-fetal immunity, though this requires further elucidation.