Immunology: Complement Pathways Exam Prep 2026 – UCF
1. Which molecule acts as the initial recognition component for the classical
complement pathway?
A. C1q
B. C3b
C. Mannose-binding lectin
D. Factor B
Answer: A
Rationale: The classical pathway is initiated when C1q binds to the Fc region of antibodies
(IgM or IgG) bound to an antigen.
2. Which of the following serves as the C3 convertase in the alternative
pathway?
A. C4b2a
B. C1qr2s2
C. C5b6789
D. C3bBb
Answer: D
Rationale: In the alternative pathway, C3b binds Factor B, which is then cleaved by Factor
D to form the C3bBb complex.
,3. Which complement component is common to all three activation pathways?
A. C1q
B. C4
C. Factor D
D. C3
Answer: D
Rationale: C3 is the central component where all three pathways (Classical, Lectin, and
Alternative) converge.
4. The Lectin pathway is initiated by the binding of MBL to which carbohydrate
on microbial surfaces?
A. Sialic acid
B. Galactose
C. Mannose
D. Glucose
Answer: C
Rationale: Mannose-binding lectin (MBL) recognizes mannose residues common on the
surface of many pathogens.
5. Which fragment is known as a potent anaphylatoxin and chemoattractant for
neutrophils?
A. C3b
B. C4b
C. C5a
D. C9
Answer: C
Rationale: C5a is the most potent anaphylatoxin, causing mast cell degranulation and
attracting neutrophils to the site of infection.
, 6. What is the function of the Membrane Attack Complex (MAC)?
A. Opsonization
B. Cell lysis by pore formation
C. Neutralization of toxins
D. Activation of B cells
Answer: B
Rationale: The MAC (C5b-C9) inserts into the target cell membrane to create a pore,
leading to osmotic lysis.
7. Which regulatory protein displaces Bb from the C3bBb complex to inhibit the
alternative pathway?
A. S protein
B. Properdin
C. C1 inhibitor
D. Factor H
Answer: D
Rationale: Factor H competes with Factor B for binding to C3b and accelerates the decay of
the C3bBb convertase.
8. C4b2a is the C3 convertase for which pathways?
A. Classical and Lectin
B. Lectin and Alternative
C. Classical and Alternative
D. Alternative only
Answer: A
Rationale: Both the classical and lectin pathways use C4 and C2 to form the C4b2a C3
convertase.
1. Which molecule acts as the initial recognition component for the classical
complement pathway?
A. C1q
B. C3b
C. Mannose-binding lectin
D. Factor B
Answer: A
Rationale: The classical pathway is initiated when C1q binds to the Fc region of antibodies
(IgM or IgG) bound to an antigen.
2. Which of the following serves as the C3 convertase in the alternative
pathway?
A. C4b2a
B. C1qr2s2
C. C5b6789
D. C3bBb
Answer: D
Rationale: In the alternative pathway, C3b binds Factor B, which is then cleaved by Factor
D to form the C3bBb complex.
,3. Which complement component is common to all three activation pathways?
A. C1q
B. C4
C. Factor D
D. C3
Answer: D
Rationale: C3 is the central component where all three pathways (Classical, Lectin, and
Alternative) converge.
4. The Lectin pathway is initiated by the binding of MBL to which carbohydrate
on microbial surfaces?
A. Sialic acid
B. Galactose
C. Mannose
D. Glucose
Answer: C
Rationale: Mannose-binding lectin (MBL) recognizes mannose residues common on the
surface of many pathogens.
5. Which fragment is known as a potent anaphylatoxin and chemoattractant for
neutrophils?
A. C3b
B. C4b
C. C5a
D. C9
Answer: C
Rationale: C5a is the most potent anaphylatoxin, causing mast cell degranulation and
attracting neutrophils to the site of infection.
, 6. What is the function of the Membrane Attack Complex (MAC)?
A. Opsonization
B. Cell lysis by pore formation
C. Neutralization of toxins
D. Activation of B cells
Answer: B
Rationale: The MAC (C5b-C9) inserts into the target cell membrane to create a pore,
leading to osmotic lysis.
7. Which regulatory protein displaces Bb from the C3bBb complex to inhibit the
alternative pathway?
A. S protein
B. Properdin
C. C1 inhibitor
D. Factor H
Answer: D
Rationale: Factor H competes with Factor B for binding to C3b and accelerates the decay of
the C3bBb convertase.
8. C4b2a is the C3 convertase for which pathways?
A. Classical and Lectin
B. Lectin and Alternative
C. Classical and Alternative
D. Alternative only
Answer: A
Rationale: Both the classical and lectin pathways use C4 and C2 to form the C4b2a C3
convertase.
