Complement C1q needs to bind to more than one immunoglobulin molecule to be activated

All antibody isotypes, once bound to their target, have the capacity to activate complement C1q

IgM, with its pentamer structure and 10 antigen binding sites, is the only isotype that can effectively activate the complement system

Once complement C1q is bound to the surface of a pathogen, it can easily activate IgG and IgM