In classical pathway how can complement bind only to Ab complexed with Ag ?

[whaleshark]

Advertisement - Members don't see this ad

In complement fixation test, we use mechanism that complement binds to Fc of Ab complexed with Ag so that RBC lysis wouldn't occur.
however, in normal blood there are Ab and complement together and they do not react each other (not bind)
and only at the time Ag-Ab complexes form, the complement will start to bind them. why it happens? change of structure in Fc??