<prompt> Complement attaches to immunoglobulin at ?AminoterminalFab regionVariable regionFc fragment  <response>: Ans. is d i.e., Fc fragment . Fc fragment is composed of the carboxyterminal. It determines the biological activity such as complement fixation, placental transfer, skin fixation and catabolic rate. CH-2 of Ig G and CH, of IgM bind to C1q poion of CI. 