An IgG autoantibody against the ALTERNATIVE PATHWAY C3 CONVERTASE, found in serum of patients with MESANGIOCAPILLARY glomerulonephritis. The binding of this autoantibody to C3bBb stabilizes the enzyme thus reduces the actions of C3b inactivators (complement factor h; complement factor i). This abnormally stabilized enzyme induces a continuous complement activation and generation of C3b thereby promoting the assembly of MEMBRANE ATTACK COMPLEX and cytolysis.