Activation of the Classical Pathway Requires Antigen -Antibody complexes Complement component C1 binds to the Fc region of the Ab(IgM-most potent activator) C1 is a protease that cleaves C2 and C4 to form the C3 convertase of the classical path. Activation of the Classical Pathway Requires Antigen - Antibody complexes Complement component C1 binds to the Fc region of the Ab (IgM- most potent activator) C1 is a protease that cleaves C2 and C4 to form the C3 convertase of the classical path