<prompt> The Fc piece of which immunoglobulin fixes complement 1 -IgAIgGIgMIgE  <response>: Ans. is c b i.e., IgM IgG o Make it very clear in your mind that both IgM and IgG fix C1. CH2 of IgG and CH4 of IgM bind to C1q portion of C1---Ananthanarayan Clq portion of Cl is an aggregate of 18 polypeptides that binds to the Fc portion of IgM and IgG--LANGE microbiologyo However, if you will have to choose one option, IgM is the best because IgM is more effective than IgG for complement fixation. 