O What is the difference between complement activation by antigen-IgM complexes and antigen- IgG complexes?

the difference between complement activation by antigen-IgM complexes and antigen-IgG complexes accounts for thae fact that a pentameric IgM bound to an antigen on the surface of the target cell takes up the "staple"conformation. in this conformation 3 binding sites for C1q are exposed.

In contrast, an IgG molecule has one C1q binding site in the C_H2 domain of Fc region. A strong C1q binding is achieved when 2 IgG molecules are close i.e. within 30-40nm of each other on the surface of the target site,in turn providing two attachment sites for C1q.

due to this difference a single IgM molecule is responsible to activate the complement system while nearly 1000 molecules of IgG is required to do so as it then ensures whether any 2 IgG molecule is close enough on the surface of the cell to initiate C1q binding.