In the classical complement pathway, what is the minimum antibody requirement for activation?

• A. Activation requires antibody binding with one IgM or two IgG molecules.
• B. C3 is activated by binding C-reactive protein.
• C. C1q is activated by the presence of a single Fab region.
• D. The pathway is triggered by the presence of C4.

Answer: (A)

Activation of the classical complement pathway hinges on antibody-bound antigen presenting Fc regions that C1q can bind. Because of its pentameric IgM structure, a single IgM bound to antigen provides multiple Fc sites in the right arrangement, allowing C1q to bind and initiate the cascade with one molecule of IgM. In contrast, IgG antibodies require two molecules bound in close proximity so their Fc regions are available to engage C1q, enabling activation.

Once C1q binds, it activates C1r and C1s, leading to cleavage of C4 and C2 and formation of the C3 convertase, which then drives the rest of the cascade. The statement about activation by C-reactive protein or by a single Fab region misstates the trigger, and the idea that the pathway is triggered simply by the presence of C4 is not correct, since C4 activation occurs downstream after C1q engagement.