Discussion
IgG and IgM activate compliment, IgM is better activator the IgG because of :
Answer: C
IgM is the better activator of complement because of its pentavalent structure.
The classical pathway starts when C1q binds to at least two antibody Fc regions close together on an antigen. IgM is a pentamer of five antibody units joined by a J chain, so a single molecule bound to a cell surface already offers several Fc regions to C1q. IgG is a monomer, and two IgG molecules must happen to bind next to each other to do the same.
IgM has a much higher molecular weight than IgG (about 900 kDa against 150 kDa), not a lower one, and lower concentration would make it less effective, not more. The secretory form is a feature of IgA and has nothing to do with complement activation.
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