C5b initiates the membrane attack complex (MAC) in the complement system, making it the correct answer to this immunology question. This process is key in innate immunity for lysing pathogens. Understanding each option clarifies why C5b stands out.

Option Breakdown

• (A) C3a: Acts as an anaphylatoxin, promoting inflammation by triggering mast cell degranulation and increasing vascular permeability, but plays no role in MAC initiation.​

• (B) C3b: Serves as an opsonin for phagocytosis and helps form C5 convertase, yet does not start MAC assembly.​

• (C) C5a: Functions as a potent chemoattractant for neutrophils and an anaphylatoxin, similar to C3a, without involvement in MAC formation.​

• (D) C5b: Correct choice; C5 convertase cleaves C5 into C5a and C5b, with C5b binding C6, then C7, to nucleate MAC on target membranes.​

MAC Formation Steps

Complement pathways converge at C5 cleavage, generating C5b.
C5b recruits C6 and C7, forming C5b-7 that inserts into lipid bilayers.
C8 and multiple C9 molecules polymerize to create lytic pores.​

Exam Relevance

This question tests terminal complement pathway knowledge, common in molecular biology and immunology curricula. C5b’s role distinguishes it from earlier effectors like C3 fragments. For students in biotechnology, mastering MAC aids in studying immune therapeutics and pathogen defense.​