PCB 3233 Immunology - Complement Pathway Exam 1 2026 – UCF
1. Which of the following is the primary initiator of the Classical Pathway?
A. Spontaneous hydrolysis of C3
B. Antigen-antibody complexes (IgM or IgG)
C. Mannose-binding lectin
D. Lipopolysaccharides on bacterial surfaces
Answer: B
Rationale: The classical pathway is typically initiated by C1q binding to the Fc region of
IgM or IgG that has bound to an antigen.
2. Which complement component is common to all three activation pathways?
A. C1q
B. C3
C. Factor B
D. C4
Answer: B
Rationale: C3 is the central component where all three pathways converge to form a C3
convertase.
3. What is the C3 convertase of the Alternative Pathway?
A. C4b2a
B. C3bBb
C. C1qrs
D. C4b2a3b
Answer: B
,Rationale: In the alternative pathway, C3b binds with Factor B (cleaved to Bb) to form the
C3bBb complex.
4. Which of the following acts as an opsonin to enhance phagocytosis?
A. C3a
B. C5a
C. C9
D. C3b
Answer: D
Rationale: C3b (and its breakdown product iC3b) coats pathogens, allowing phagocytes
with CR1 receptors to recognize and ingest them.
5. Which components make up the Membrane Attack Complex (MAC)?
A. C5b, C6, C7, C8, C9
B. C1, C2, C3, C4
C. C3a, C4a, C5a
D. Factor B, Factor D, Properdin
Answer: A
Rationale: The MAC is formed by the sequential assembly of C5b, C6, C7, C8, and multiple
C9 molecules to form a pore in the membrane.
6. Which anaphylatoxin is considered the most potent in inducing
inflammation?
A. C3a
B. C5a
C. C4a
D. C3b
Answer: B
Rationale: C5a is the most powerful anaphylatoxin, attracting neutrophils and increasing
vascular permeability.
, 7. The ‘tick-over’ mechanism is associated with which pathway?
A. Classical Pathway
B. Lectin Pathway
C. Terminal Pathway
D. Alternative Pathway
Answer: D
Rationale: The alternative pathway is constantly initiated at low levels through the
spontaneous hydrolysis of C3 into C3(H2O), known as tick-over.
8. What is the role of Factor D in the alternative pathway?
A. Cleaves C3 into C3a and C3b
B. Inhibits the formation of MAC
C. Stabilizes the C3 convertase
D. Cleaves Factor B into Ba and Bb
Answer: D
Rationale: Factor D is a serine protease that cleaves Factor B only when it is bound to C3b.
9. Which molecule initiates the Lectin Pathway?
A. C1q
B. C-reactive protein
C. Mannose-Binding Lectin (MBL)
D. Properdin
Answer: C
Rationale: The lectin pathway is initiated when MBL or ficolins bind to carbohydrate
patterns (like mannose) on microbial surfaces.
1. Which of the following is the primary initiator of the Classical Pathway?
A. Spontaneous hydrolysis of C3
B. Antigen-antibody complexes (IgM or IgG)
C. Mannose-binding lectin
D. Lipopolysaccharides on bacterial surfaces
Answer: B
Rationale: The classical pathway is typically initiated by C1q binding to the Fc region of
IgM or IgG that has bound to an antigen.
2. Which complement component is common to all three activation pathways?
A. C1q
B. C3
C. Factor B
D. C4
Answer: B
Rationale: C3 is the central component where all three pathways converge to form a C3
convertase.
3. What is the C3 convertase of the Alternative Pathway?
A. C4b2a
B. C3bBb
C. C1qrs
D. C4b2a3b
Answer: B
,Rationale: In the alternative pathway, C3b binds with Factor B (cleaved to Bb) to form the
C3bBb complex.
4. Which of the following acts as an opsonin to enhance phagocytosis?
A. C3a
B. C5a
C. C9
D. C3b
Answer: D
Rationale: C3b (and its breakdown product iC3b) coats pathogens, allowing phagocytes
with CR1 receptors to recognize and ingest them.
5. Which components make up the Membrane Attack Complex (MAC)?
A. C5b, C6, C7, C8, C9
B. C1, C2, C3, C4
C. C3a, C4a, C5a
D. Factor B, Factor D, Properdin
Answer: A
Rationale: The MAC is formed by the sequential assembly of C5b, C6, C7, C8, and multiple
C9 molecules to form a pore in the membrane.
6. Which anaphylatoxin is considered the most potent in inducing
inflammation?
A. C3a
B. C5a
C. C4a
D. C3b
Answer: B
Rationale: C5a is the most powerful anaphylatoxin, attracting neutrophils and increasing
vascular permeability.
, 7. The ‘tick-over’ mechanism is associated with which pathway?
A. Classical Pathway
B. Lectin Pathway
C. Terminal Pathway
D. Alternative Pathway
Answer: D
Rationale: The alternative pathway is constantly initiated at low levels through the
spontaneous hydrolysis of C3 into C3(H2O), known as tick-over.
8. What is the role of Factor D in the alternative pathway?
A. Cleaves C3 into C3a and C3b
B. Inhibits the formation of MAC
C. Stabilizes the C3 convertase
D. Cleaves Factor B into Ba and Bb
Answer: D
Rationale: Factor D is a serine protease that cleaves Factor B only when it is bound to C3b.
9. Which molecule initiates the Lectin Pathway?
A. C1q
B. C-reactive protein
C. Mannose-Binding Lectin (MBL)
D. Properdin
Answer: C
Rationale: The lectin pathway is initiated when MBL or ficolins bind to carbohydrate
patterns (like mannose) on microbial surfaces.
