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Complement System — MFDS Part 1 MCQ

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HardPhysiologyComplement SystemMFDS Part 1

Activation of complement on a bacterial surface generates C3 and C5 cleavage products. Which option correctly matches each complement effector with its principal function?

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Correct answer: D — C3b—opsonisation; C5a—leucocyte chemotaxis; C5b-9—membrane lysis

Option D is correct. C3 convertase generates C3b, which is deposited on microbial surfaces and promotes recognition and phagocytosis through complement receptors. Subsequent C5 cleavage releases C5a, a potent inflammatory mediator and leucocyte chemoattractant. C5b initiates assembly with C6, C7, C8 and C9 to form the C5b-9 membrane attack complex, which creates membrane pores and can lyse susceptible targets. The other options interchange these functions: C3a and C5a are soluble anaphylatoxins rather than opsonins or lytic pores; C5b initiates terminal-complex assembly rather than chemotaxis; and membrane lysis is mediated by assembled C5b-9, not by C3b or C5a.

Reference: Jayaraman A, Walachowski S, Bosmann M. The complement system: A key player in the host response to infections. European Journal of Immunology. 2024;54(11):e2350814. Figure 1 and accompanying text. https://pubmed.ncbi.nlm.nih.gov/39188171/