In the classical pathway, C3 convertase is formed by which of the following?

In the classical pathway, C3 convertase is formed by which of the following?

• A. C3bBb
• B. C1qrs
• C. C4b2a
• D. C3bD

Answer: (C)

In the classical pathway, the C3 convertase is the C4b2a complex. After C1 recognizes antibody-bound antigen, C1s cleaves C4 into C4b and C4a and C2 into C2a and C2b. C4b binds to the surface and combines with C2a to form the enzyme complex C4b2a, which then cleaves C3 into C3a and C3b, driving the cascade forward. The other options correspond to different components or pathways: C3bBb is the C3 convertase of the alternative pathway, C1qrs is the initiator complex of the classical pathway, and C3bD is not a standard convertase. Thus, C4b2a is the correct C3 convertase for the classical pathway.

In the classical pathway, the C3 convertase is the C4b2a complex. After C1 recognizes antibody-bound antigen, C1s cleaves C4 into C4b and C4a and C2 into C2a and C2b. C4b binds to the surface and combines with C2a to form the enzyme complex C4b2a, which then cleaves C3 into C3a and C3b, driving the cascade forward. The other options correspond to different components or pathways: C3bBb is the C3 convertase of the alternative pathway, C1qrs is the initiator complex of the classical pathway, and C3bD is not a standard convertase. Thus, C4b2a is the correct C3 convertase for the classical pathway.