Q.1 Which of the following is the central component of the complement system that forms the membrane attack complex?
C1
C3
C5b
C9
Explanation - C5b initiates the assembly of the membrane attack complex (MAC) by binding sequentially to C6, C7, C8, and multiple C9 molecules.
Correct answer is: C5b
Q.2 Which pathway of the complement system is triggered by antibodies bound to antigens?
Classical pathway
Alternative pathway
Lectin pathway
All of the above
Explanation - The classical pathway is activated when C1 binds to antibodies (IgG or IgM) that are attached to antigens.
Correct answer is: Classical pathway
Q.3 Which complement component is primarily responsible for opsonization of pathogens?
C3b
C5a
C9
C1q
Explanation - C3b binds to microbial surfaces and enhances phagocytosis by serving as an opsonin.
Correct answer is: C3b
Q.4 Which of the following is a potent anaphylatoxin released during complement activation?
C3b
C5b
C3a
C6
Explanation - C3a and C5a are anaphylatoxins that increase vascular permeability and attract immune cells to the site of infection.
Correct answer is: C3a
Q.5 Which pathway of complement activation is antibody-independent and initiated by microbial surfaces?
Classical pathway
Lectin pathway
Alternative pathway
None of the above
Explanation - The alternative pathway is activated directly by pathogen surfaces without the need for antibodies.
Correct answer is: Alternative pathway
Q.6 Mannose-binding lectin (MBL) activates which complement pathway?
Classical pathway
Lectin pathway
Alternative pathway
All pathways
Explanation - MBL binds to carbohydrate patterns on pathogens, activating the lectin pathway through MASPs (MBL-associated serine proteases).
Correct answer is: Lectin pathway
Q.7 C5a is known to perform which of the following functions?
Chemoattractant for neutrophils
Directly forming MAC
Inactivating C3b
Binding antibodies
Explanation - C5a is a potent chemoattractant and recruits immune cells like neutrophils to sites of infection.
Correct answer is: Chemoattractant for neutrophils
Q.8 Which regulatory protein prevents complement activation on host cells?
Factor H
C3b
C9
C5a
Explanation - Factor H binds to host cell surfaces and inactivates C3b to prevent accidental complement-mediated damage.
Correct answer is: Factor H
Q.9 Which component directly forms the pore in the membrane attack complex?
C5b
C7
C8
C9
Explanation - Multiple C9 molecules polymerize to form the transmembrane channel of the MAC, causing cell lysis.
Correct answer is: C9
Q.10 Which of the following complement fragments is primarily involved in inflammation?
C3b
C5a
C6
C9
Explanation - C5a is a strong proinflammatory mediator that induces chemotaxis, histamine release, and increased vascular permeability.
Correct answer is: C5a
Q.11 Which enzyme complex converts C3 into C3a and C3b?
C3 convertase
C5 convertase
C1 complex
C9 polymerase
Explanation - C3 convertase cleaves C3 into C3a (anaphylatoxin) and C3b (opsonin) in all pathways of complement activation.
Correct answer is: C3 convertase
Q.12 Properdin stabilizes which complement component?
C3bBb
C1q
C5b-9
C4b2a
Explanation - Properdin stabilizes the alternative pathway C3 convertase (C3bBb), enhancing its activity.
Correct answer is: C3bBb
Q.13 Which of the following pathways is triggered by lectin binding to mannose on microbial surfaces?
Classical
Lectin
Alternative
None
Explanation - The lectin pathway is activated when mannose-binding lectin (MBL) binds to carbohydrates on microbes, initiating complement activation.
Correct answer is: Lectin
Q.14 Which complement component is first cleaved in the classical pathway?
C1
C2
C3
C4
Explanation - Upon activation of the classical pathway, C1s cleaves C4 into C4a and C4b, initiating formation of the C3 convertase.
Correct answer is: C4
Q.15 Which complement protein acts as an opsonin?
C3b
C5a
C9
C1q
Explanation - C3b binds to pathogens and facilitates their recognition and phagocytosis by immune cells.
Correct answer is: C3b
Q.16 Deficiency in C3 leads to increased susceptibility to:
Viral infections
Bacterial infections
Autoimmune diseases
Fungal infections
Explanation - C3 is central to complement activation, opsonization, and MAC formation; its deficiency mainly increases susceptibility to bacterial infections.
Correct answer is: Bacterial infections
Q.17 Which of the following is NOT a function of the complement system?
Opsonization
Cell lysis
Antibody production
Inflammation
Explanation - Complement enhances immune defense but does not produce antibodies; that is the role of B cells.
Correct answer is: Antibody production
Q.18 Which component is involved in formation of C5 convertase in the classical pathway?
C4b2a3b
C3bBb
C1q
C9
Explanation - C4b2a (C3 convertase) binds C3b to form C5 convertase (C4b2a3b), which cleaves C5 into C5a and C5b.
Correct answer is: C4b2a3b
Q.19 Which complement pathway can be triggered by microbial endotoxins without antibodies?
Classical pathway
Alternative pathway
Lectin pathway
All pathways
Explanation - The alternative pathway is activated directly by microbial surfaces such as endotoxins, independent of antibodies.
Correct answer is: Alternative pathway
Q.20 C1q binds to which part of an antibody to initiate classical pathway?
Fc region
Fab region
Hinge region
Variable region
Explanation - C1q binds to the Fc portion of IgG or IgM antibodies that are bound to antigens to start the classical pathway.
Correct answer is: Fc region
Q.21 Which complement fragment attracts phagocytes to the site of infection?
C3b
C5a
C9
C6
Explanation - C5a functions as a chemoattractant, directing neutrophils and other phagocytes to the infection site.
Correct answer is: C5a
Q.22 Which regulatory protein accelerates the decay of C3 convertase?
Decay-accelerating factor (DAF)
C1 inhibitor
Factor B
Properdin
Explanation - DAF prevents overactivation of complement by dissociating C3 convertase complexes on host cells.
Correct answer is: Decay-accelerating factor (DAF)
Q.23 Which complement pathway involves MASPs (MBL-associated serine proteases)?
Classical
Lectin
Alternative
Terminal
Explanation - MASPs associated with mannose-binding lectin cleave C4 and C2 to form C3 convertase in the lectin pathway.
Correct answer is: Lectin
Q.24 Which complement component deficiency is associated with recurrent Neisseria infections?
C5-C9
C1q
C3
C4
Explanation - Deficiencies in terminal components (C5-C9) impair MAC formation, leading to susceptibility to Neisseria infections.
Correct answer is: C5-C9
Q.25 Which complement fragment is primarily responsible for triggering mast cell degranulation?
C3a
C3b
C4b
C9
Explanation - C3a is an anaphylatoxin that induces histamine release from mast cells, contributing to inflammation.
Correct answer is: C3a