What are the functions of IgM antibodies?

The main function of IgM antibodies is the control of B-cell activation. IgM antibodies are the prime mediators of the primary immune response. They are the first type of antibody produced by B-cells in response to initial exposure to a pathogenic antigen. IgM are quick to recognize antigens and initiate an immune response either by directly killing the pathogens or clearing novel antigens. 

The large size of IgM antibodies give them excellent binding avidity, allowing them to identify and pick up even trace amounts of infection to be marked for recognition by phagocytes. Its unparalleled efficiency in recognizing and binding with antigen immediately is of particular importance as it gives the body time to synthesize sufficient quantities of IgG antibodies, which take over the immune response a few days after the pathogen invasion.