Which of the following is true of the amplification loop in complement activation?

• A. It is only found in the alternative pathway.
• B. The membrane attack unit is amplified.
• C. C3b is the product that is increased.
• D. Increasing amounts of C1qrs are produced.

Answer: (C)

The amplification loop in complement activation is a positive feedback that boosts C3b deposition on the target surface. Once a C3 convertase cleaves C3 into C3a and C3b, the C3b that is produced can associate with the convertase or surface to form more C3 convertase. In the classical/lectin pathway, adding C3b helps generate the C5 convertase, while in the alternative pathway, C3b binds factor B and D to make additional C3 convertases. This cycle repeats, steadily increasing the amount of C3b on the surface, which enhances opsonization and drives progression toward the terminal membrane attack complex. So, the product that is increased by the amplification loop is C3b. It’s not limited to the alternative pathway, nor does it directly increase C1qrs production or merely amplify the membrane attack complex itself.