IgM is the first antibody produced in a primary immune response and circulates mainly as a high-avidity pentamer. It efficiently activates the classical complement pathway and is also expressed on naïve B cells as part of the B-cell receptor. Anti-human IgM monoclonal antibodies specifically recognize the μ heavy chain and are widely used in flow cytometry and immunofluorescence to identify IgM+ B cells in studies of B-cell maturation, immune function, and related disorders.