Which of the following is the soluble form of C3 convertase of the alternative pathway of complement activation?

Question

a. iC3
b. iC3b
c. C3b
d. iC3Bb
e. C3bBb.

Answer 1

A. The cleavage of C3 into C3a and C3b and the covalent bonding of C3b to the pathogen surface is called complement fixation, and is the reaction on which the alternative, lectin, and classical pathways of complement activation converge.
B. The enzyme responsible for cleaving C3 into C3a and C3b is called C3 convertase, and it differs in composition depending on the particular complement pathway. The classical and lectin pathways use the classical C3 convertase (C4b2a), whereas the alternative pathway uses the alternative convertase (C3bBb).
C. C3 is the most abundant complement component in the plasma and circulates as a zymogen, an inactive enzyme. When cleaved into C3a and C3b, three different effector mechanisms are armed: (1) C3b binds to and tags pathogens for destruction by phagocytes through binding to a C3b receptor, CR1; (2) C3b contributes to a multicomponent enzyme, C5 convertase, that catalyzes the assembly of the terminal complement components and the formation of the membrane-attack complex; and (3) C3a is an inflammatory mediator that serves as a chemoattractant and recruits inflammatory cells to the infection site.