Which statement accurately reflects how the classical complement pathway is initiated?

Which statement accurately reflects how the classical complement pathway is initiated?

• A. It is initiated by antibody binding to antigen.
• B. It is initiated by bacterial capsule polysaccharides.
• C. It is initiated by properdin.
• D. It is initiated by C3b deposition.

Answer: (A)

Initiation of the classical pathway is triggered by antibody-bound antigen. The C1 complex recognizes the Fc region of antibodies that are attached to an antigen—IgM is especially effective because its structure presents multiple Fc sites—and this binding activates C1r and C1s to start the proteolytic cascade. This cascade then cleaves C4 and C2 to form the C4b2a C3 convertase, which drives the downstream reactions that lead to opsonization, inflammation, and ultimately the membrane attack complex. So antibody binding to antigen best reflects how the classical pathway is initiated. Properdin starts and stabilizes the alternative pathway’s C3 convertase, C3b deposition is a downstream amplification step, and polysaccharides on capsules do not initiate the classical pathway by themselves.

Initiation of the classical pathway is triggered by antibody-bound antigen. The C1 complex recognizes the Fc region of antibodies that are attached to an antigen—IgM is especially effective because its structure presents multiple Fc sites—and this binding activates C1r and C1s to start the proteolytic cascade. This cascade then cleaves C4 and C2 to form the C4b2a C3 convertase, which drives the downstream reactions that lead to opsonization, inflammation, and ultimately the membrane attack complex. So antibody binding to antigen best reflects how the classical pathway is initiated. Properdin starts and stabilizes the alternative pathway’s C3 convertase, C3b deposition is a downstream amplification step, and polysaccharides on capsules do not initiate the classical pathway by themselves.