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Serum IgM is in a planar form in which the complement binding sites in the Fc region are not accessible. Only after binding to antigen does IgM assume a conformation in which the complement-binding sites are accesible
Among all the antibodies IgM antibody is the largest antibody. It has planar conformation. To activate complement pathways, complement particles needs to get attached to the Fc portion of antibodies. But, due to the planar structure, the Fc region is not accessible.