Question

Why does it take more molecules of IgG than IgM to activate the classical complement pathway

Answer 1

Complement classic pathway :-
• This is system of some proteins which is activated by antigen antibody reaction and leads to lysis of pathogens.
Steps -
• Firstly, antibodies binds to antigen and then this antigen antibody complex binds to C1 protein of complement system and this pathway activates.

• Mainly two types of antibodies are involved in this pathway - IgG and IgM.
• More number of IgG are required for complement activation with compared to IgM because IgM is monomer and can bind to only two antigens at a time whereas IgM is a pentameric antibody which can bind to ten antigens at a time.
• Hence, because of more antigen binding sites, less number of IgM antibodies are required for complement fixation. Approx 5 times less amount of IgM with compared to IgG.