Complement biological functions MCQs With Answer

Introduction: This quiz set on “Complement biological functions MCQs With Answer” is designed for M.Pharm students to deepen their understanding of the complement system’s roles in immunity, pathology and therapeutics. The complement cascade bridges innate and adaptive responses, mediating opsonization, inflammation, membrane attack complex formation, immune complex clearance, and modulation of B‑cell responses. These questions explore molecular mechanisms, pathway activation, regulatory proteins, clinical consequences of deficiencies, laboratory assessment, and modern pharmacologic interventions targeting complement. Working through these MCQs will strengthen mechanistic insight and clinical reasoning useful for immunopharmacology, biologic drug development, and patient management where complement activity is relevant.

Q1. Which complement component is primarily responsible for opsonization that enhances phagocytosis?

• C3b
• C5a
• C1q
• C9

Correct Answer: C3b

Q2. Which fragment acts as a potent chemoattractant recruiting neutrophils during complement activation?

• C3a
• C5a
• C4b
• C1r

Correct Answer: C5a

Q3. The membrane attack complex (MAC) is formed by assembly of which terminal components?

• C5b, C6, C7, C8 and multiple C9
• C1q, C1r, C1s, C2 and C4
• C3a, C3b, factor B and factor D
• C4a, C2b, properdin and C9

Correct Answer: C5b, C6, C7, C8 and multiple C9

Q4. Which pathway of complement is initiated by antigen‑antibody complexes?

• Lectin pathway
• Alternative pathway
• Classical pathway
• Properdin‑mediated pathway

Correct Answer: Classical pathway

Q5. Which serum protein regulates initiation of the classical pathway by inhibiting C1 complex activity?

• Factor H
• C1 inhibitor (C1‑INH)
• Factor I
• CD59

Correct Answer: C1 inhibitor (C1‑INH)

Q6. Deficiency of late complement components C5–C9 most commonly leads to increased susceptibility to which infection?

• Streptococcus pneumoniae
• Neisseria meningitidis and Neisseria gonorrhoeae
• Mycobacterium tuberculosis
• Herpes simplex virus

Correct Answer: Neisseria meningitidis and Neisseria gonorrhoeae

Q7. Which complement receptor on B cells lowers the threshold for B cell activation by binding C3d‑tagged antigens?

• CR1 (CD35)
• CR2 (CD21)
• CR3 (CD11b/CD18)
• CR4 (CD11c/CD18)

Correct Answer: CR2 (CD21)

Q8. Properdin has a unique role among complement factors because it:

• Cleaves C3 into C3a and C3b
• Stabilizes the alternative pathway C3 convertase
• Inhibits formation of the MAC
• Directly opsonizes bacteria for phagocytosis

Correct Answer: Stabilizes the alternative pathway C3 convertase

Q9. Complement factor H deficiency or dysfunction is most directly associated with which condition?

• Atypical hemolytic uremic syndrome (aHUS) and age‑related macular degeneration risk
• Severe combined immunodeficiency (SCID)
• Chronic granulomatous disease
• Hyper IgM syndrome

Correct Answer: Atypical hemolytic uremic syndrome (aHUS) and age‑related macular degeneration risk

Q10. Which complement split product is primarily responsible for increased vascular permeability and smooth muscle contraction (anaphylatoxin activity)?

• C3b
• C3a
• C4b
• C5b

Correct Answer: C3a

Q11. Eculizumab, a monoclonal antibody drug, exerts its therapeutic effect by targeting which complement component?

• C3
• C5
• Factor B
• Properdin

Correct Answer: C5

Q12. In laboratory testing, consumption of complement (low CH50) most commonly indicates:

• Ongoing complement activation or deficiency of classical pathway components
• Primary B cell deficiency
• High serum albumin levels
• Excessive production of clotting factors

Correct Answer: Ongoing complement activation or deficiency of classical pathway components

Q13. Which membrane regulatory protein prevents insertion of MAC into host cell membranes?

• Decay‑accelerating factor (DAF, CD55)
• Membrane cofactor protein (MCP, CD46)
• CD59 (protectin)
• Properdin

Correct Answer: CD59 (protectin)

Q14. Complement activation‑related pseudoallergy (CARPA) is a clinically important reaction in which context?

• Autoimmune destruction of red blood cells
• Infusion or intravenous administration of nanoparticle or liposomal drugs triggering complement activation
• Chronic viral infections where complement is depleted
• Genetic absence of C3

Correct Answer: Infusion or intravenous administration of nanoparticle or liposomal drugs triggering complement activation

Q15. The lectin pathway is activated by recognition of carbohydrate patterns via which initiator molecules?

• Mannose‑binding lectin (MBL) and ficolins
• Properdin and factor P
• C1q and C1r
• Factor H and factor I

Correct Answer: Mannose‑binding lectin (MBL) and ficolins

Q16. Which complement activity is most important for clearance of circulating immune complexes by erythrocytes and delivery to the spleen and liver?

• C3b binding to CR1 on erythrocytes
• C5b‑9 formation on immune complex surface
• C3a induced chemotaxis
• Factor D mediated proteolysis

Correct Answer: C3b binding to CR1 on erythrocytes

Q17. Factor I requires which cofactors to cleave C3b and C4b regulatory fragments?

• Properdin and factor B
• Factor H, CR1 and MCP (CD46)
• C1 inhibitor and C4b‑binding protein
• CD59 and DAF

Correct Answer: Factor H, CR1 and MCP (CD46)

Q18. Which experimental assay measures the functional activity of the classical complement pathway?

• CH50 assay
• AH50 assay
• ELISA for properdin
• Western blot for C9

Correct Answer: CH50 assay

Q19. Complement receptor 3 (CR3, CD11b/CD18) primarily mediates which function?

• Inhibition of C1 complex
• Phagocytosis of iC3b‑opsonized particles and leukocyte adhesion
• Stabilization of C3 convertase
• Direct cleavage of C5 into C5a and C5b

Correct Answer: Phagocytosis of iC3b‑opsonized particles and leukocyte adhesion

Q20. In transplantation immunology, complement contributes to hyperacute rejection primarily by:

• Recruiting regulatory T cells to the graft
• Generating anaphylatoxins that block coagulation
• Antibody‑mediated activation of complement causing endothelial injury and thrombosis
• Enhancing graft tolerance through C3d‑mediated B cell suppression

Correct Answer: Antibody‑mediated activation of complement causing endothelial injury and thrombosis

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