The correct answer is c3b.
The intrinsic C3 thioester bond of the alternative pathway (AP) is progressively hydrolyzed spontaneously by diverse proteins, lipids, and carbohydrate structures on microbes and other external surfaces [12–14], and this further activates the AP [1, 15]. A blue line pointing to C3 denotes the fluid-phase C3b promoting assembly of AP C3 convertase as the proposed mechanism for the reaction of properdin with a target on the foreign surface [27]. Thick red lines signify amplification mechanisms: Factor H regulates and inhibits the tick-over reaction, a gradual spontaneous activation involving C3(H2O) reacting with factor B. Properdin also promotes self-amplification in the other loop by stabilizing the C3bBb complex.
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