<prompt> The most avidly complement fixing antibody is -IgAIgGIgMIgE  <response>: IgG is the main serum immunoglobulin consisting of 80 of the total. equally distributed between intravascular and extravascular compaments. IgG paicipates in most immunological reactions such as complement fixation, precipitation a, d neutralization toxins and virus. REF:Anathanarayan panikers 9th edition, pg no:97 p 