What is the C3 convertase of the classical pathway?

The C3 convertase of the classical pathway is indeed C4b2a. In the context of the immune response, the classical pathway of complement activation is initiated by antibodies binding to antigens, which then recruits components of the complement system.

In this pathway, C1 is activated by the presence of antibody-antigen complexes and subsequently cleaves C4 into C4a and C4b. C4b then binds to the surface of a pathogen or the antibody itself and interacts with C2. This interaction leads to the cleavage of C2 by C1, resulting in the generation of C2a and C2b. However, it is C2a that remains bound to C4b, forming the complex C4b2a, which acts as the C3 convertase of the classical pathway. This complex is crucial for the subsequent cleavage of C3 into C3a and C3b, leading to opsonization, cell lysis, and an amplified immune response.

Understanding this mechanism is essential, as it highlights how the immune system can precisely target pathogens. The other choices represent different components or forms associated with the complement system but do not function as the C3 convertase of the classical pathway.