Question

How does C1q activation lead to the assembly of the C3 convertase?

a) Recruitment of phagocytes
b) Cleavage of C4 and C2
c) Disruption of thioester bonds
d) Inhibition of MASP-1/2

Answer 1

C1q activation leads to the assembly of the C3 convertase by causing the cleavage of complement components C4 and C2, which combine to form the C3 convertase.

Step-by-step explanation:

The activation of C1q leads to the assembly of C3 convertase through the cleavage of complement components C4 and C2. When C1q binds to an antigen-antibody complex on the surface of a pathogen, it triggers a cascade of reactions. After binding, C1q activates C1r and C1s, which then split C4 into C4a and C4b, and C2 into C2a and C2b. The fragments C4b and C2b combine to form the C3 convertase of the classical pathway. This enzyme is crucial for the complement system as it cleaves C3 into C3a and C3b, advancing the complement cascade that leads to pathogen destruction through the formation of the membrane-attack complex (MAC) and opsonization, which promotes phagocytosis.