PCB 3233 | PCB3233 Exam 3: Immunology -
University of Central Florida Updated and Latest
Questions and Correct Answers with Rationale
1. Which region of an antibody molecule is responsible for determining the effector function,
such as opsonization or placental transfer?
A. The Fab region
B. The hypervariable loops
C. The Fc region
D. The light chain V domain
Correct Answer: C
Expert Explanation: The Fc region of an antibody consists of the constant domains of the
heavy chains. This region interacts with specific cell surface receptors and complement
proteins to mediate biological effects. While the Fab region binds antigens, the Fc region
determines how the immune system responds. For instance, neonatal Fc receptors
facilitate the transfer of IgG across the placenta. Therefore, the effector capability is strictly
a property of the constant heavy chain domains.
2. A patient’s serum is found to have high levels of IgM but very low levels of IgG and IgA.
This clinical finding most likely suggests a defect in which process?
A. Somatic hypermutation
B. Isotype class switching
C. V(D)J recombination
D. Affinity maturation
Correct Answer: B
Expert Explanation: Isotype class switching is the process where a B cell changes the
constant region of the heavy chain. This allows the cell to produce IgG, IgA, or IgE instead of
just IgM. If this process is defective, often due to CD40L or AID mutations, the patient
remains stuck producing IgM. This condition is clinically known as Hyper-IgM syndrome.
Understanding this mechanism explains why IgM remains the default antibody class in
early immune responses.
3. In the classical pathway of complement activation, which molecule serves as the primary
recognition unit for antigen-antibody complexes?
A. C3b
B. Factor B
,C. C1q
D. Mannose-binding lectin
Correct Answer: C
Expert Explanation: The classical pathway is typically initiated when C1q binds to the Fc
portions of IgM or IgG. C1q is part of the C1 complex, which also contains the serine
proteases C1r and C1s. Once bound, C1r and C1s are activated to cleave subsequent
complement components C4 and C2. This recognition is specific to antibodies that have
undergone a conformational change upon binding antigen. Thus, C1q serves as the critical
bridge between the adaptive and innate immune systems.
4. Which cytokine is primarily responsible for inducing the differentiation of naive T cells into
Th1 cells, promoting cell-mediated immunity?
A. IL-12
B. IL-10
C. IL-4
D. TGF-beta
Correct Answer: A
Expert Explanation: IL-12 is produced by macrophages and dendritic cells in response to
intracellular pathogens. This cytokine signaling pathway activates the transcription factor
STAT4 in naive CD4 T cells. Th1 cells then produce IFN-gamma to further activate
macrophages and enhance killing of intracellular microbes. Without IL-12, the immune
system would struggle to clear infections such as tuberculosis. It serves as a pivotal signal
in directing the adaptive immune response toward a cellular rather than humoral focus.
5. During an allergic reaction, IgE antibodies bind to which cell type to trigger the release of
histamine?
A. Neutrophils
B. Cytotoxic T cells
C. Erythrocytes
D. Mast cells
Correct Answer: D
Expert Explanation: IgE is the primary immunoglobulin involved in Type I
hypersensitivity reactions and defense against parasites. Mast cells and basophils express
high-affinity Fc epsilon receptors that bind the Fc portion of IgE. When an allergen cross-
links these bound IgE molecules, the cell undergoes degranulation. This release of pre-
formed mediators like histamine causes the classic symptoms of allergy. This mechanism
explains the rapid onset of allergic symptoms following secondary exposure to an allergen.
, 6. The Membrane Attack Complex (MAC), which creates pores in the cell membranes of
pathogens, is composed of which complement components?
A. C5b, C6, C7, C8, C9
B. C3b, Bb, C3b
C. C1, C2, C3, C4
D. C4b, C2a, C3b
Correct Answer: A
Expert Explanation: The MAC is the final effector stage of all three complement activation
pathways. It begins with the cleavage of C5 by C5 convertase into C5a and C5b. C5b then
recruits C6, C7, and C8 to the target membrane surface. Multiple C9 molecules then
polymerize to form a transmembrane channel or pore. These pores lead to osmotic lysis
and the eventual death of the target bacterial cell.
7. Which immunoglobulin class is primarily found in secretions like breast milk, saliva, and
tears, often existing as a dimer?
A. IgG
B. IgA
C. IgM
D. IgD
Correct Answer: B
Expert Explanation: IgA is the predominant antibody class at mucosal surfaces, providing
a first line of defense against pathogens. In its secretory form, it consists of two monomers
linked by a J chain and a secretory component. The secretory component protects the
antibody from degradation by proteolytic enzymes in the gut. This structure allows IgA to
neutralize toxins and prevent viral attachment to epithelial cells. Consequently, IgA plays a
crucial role in passive immunity provided to infants through breastfeeding.
8. What is the primary function of the inhibitory molecule CTLA-4 in immune regulation?
A. It activates B cells to produce more IgG
B. It stimulates the production of pro-inflammatory cytokines
C. It competes with CD28 for binding to B7 on APCs
D. It promotes the differentiation of Th17 cells
Correct Answer: C
Expert Explanation: CTLA-4 is an essential negative regulator of T cell activation
expressed shortly after stimulation. It binds to B7-1 (CD80) and B7-2 (CD86) with much
University of Central Florida Updated and Latest
Questions and Correct Answers with Rationale
1. Which region of an antibody molecule is responsible for determining the effector function,
such as opsonization or placental transfer?
A. The Fab region
B. The hypervariable loops
C. The Fc region
D. The light chain V domain
Correct Answer: C
Expert Explanation: The Fc region of an antibody consists of the constant domains of the
heavy chains. This region interacts with specific cell surface receptors and complement
proteins to mediate biological effects. While the Fab region binds antigens, the Fc region
determines how the immune system responds. For instance, neonatal Fc receptors
facilitate the transfer of IgG across the placenta. Therefore, the effector capability is strictly
a property of the constant heavy chain domains.
2. A patient’s serum is found to have high levels of IgM but very low levels of IgG and IgA.
This clinical finding most likely suggests a defect in which process?
A. Somatic hypermutation
B. Isotype class switching
C. V(D)J recombination
D. Affinity maturation
Correct Answer: B
Expert Explanation: Isotype class switching is the process where a B cell changes the
constant region of the heavy chain. This allows the cell to produce IgG, IgA, or IgE instead of
just IgM. If this process is defective, often due to CD40L or AID mutations, the patient
remains stuck producing IgM. This condition is clinically known as Hyper-IgM syndrome.
Understanding this mechanism explains why IgM remains the default antibody class in
early immune responses.
3. In the classical pathway of complement activation, which molecule serves as the primary
recognition unit for antigen-antibody complexes?
A. C3b
B. Factor B
,C. C1q
D. Mannose-binding lectin
Correct Answer: C
Expert Explanation: The classical pathway is typically initiated when C1q binds to the Fc
portions of IgM or IgG. C1q is part of the C1 complex, which also contains the serine
proteases C1r and C1s. Once bound, C1r and C1s are activated to cleave subsequent
complement components C4 and C2. This recognition is specific to antibodies that have
undergone a conformational change upon binding antigen. Thus, C1q serves as the critical
bridge between the adaptive and innate immune systems.
4. Which cytokine is primarily responsible for inducing the differentiation of naive T cells into
Th1 cells, promoting cell-mediated immunity?
A. IL-12
B. IL-10
C. IL-4
D. TGF-beta
Correct Answer: A
Expert Explanation: IL-12 is produced by macrophages and dendritic cells in response to
intracellular pathogens. This cytokine signaling pathway activates the transcription factor
STAT4 in naive CD4 T cells. Th1 cells then produce IFN-gamma to further activate
macrophages and enhance killing of intracellular microbes. Without IL-12, the immune
system would struggle to clear infections such as tuberculosis. It serves as a pivotal signal
in directing the adaptive immune response toward a cellular rather than humoral focus.
5. During an allergic reaction, IgE antibodies bind to which cell type to trigger the release of
histamine?
A. Neutrophils
B. Cytotoxic T cells
C. Erythrocytes
D. Mast cells
Correct Answer: D
Expert Explanation: IgE is the primary immunoglobulin involved in Type I
hypersensitivity reactions and defense against parasites. Mast cells and basophils express
high-affinity Fc epsilon receptors that bind the Fc portion of IgE. When an allergen cross-
links these bound IgE molecules, the cell undergoes degranulation. This release of pre-
formed mediators like histamine causes the classic symptoms of allergy. This mechanism
explains the rapid onset of allergic symptoms following secondary exposure to an allergen.
, 6. The Membrane Attack Complex (MAC), which creates pores in the cell membranes of
pathogens, is composed of which complement components?
A. C5b, C6, C7, C8, C9
B. C3b, Bb, C3b
C. C1, C2, C3, C4
D. C4b, C2a, C3b
Correct Answer: A
Expert Explanation: The MAC is the final effector stage of all three complement activation
pathways. It begins with the cleavage of C5 by C5 convertase into C5a and C5b. C5b then
recruits C6, C7, and C8 to the target membrane surface. Multiple C9 molecules then
polymerize to form a transmembrane channel or pore. These pores lead to osmotic lysis
and the eventual death of the target bacterial cell.
7. Which immunoglobulin class is primarily found in secretions like breast milk, saliva, and
tears, often existing as a dimer?
A. IgG
B. IgA
C. IgM
D. IgD
Correct Answer: B
Expert Explanation: IgA is the predominant antibody class at mucosal surfaces, providing
a first line of defense against pathogens. In its secretory form, it consists of two monomers
linked by a J chain and a secretory component. The secretory component protects the
antibody from degradation by proteolytic enzymes in the gut. This structure allows IgA to
neutralize toxins and prevent viral attachment to epithelial cells. Consequently, IgA plays a
crucial role in passive immunity provided to infants through breastfeeding.
8. What is the primary function of the inhibitory molecule CTLA-4 in immune regulation?
A. It activates B cells to produce more IgG
B. It stimulates the production of pro-inflammatory cytokines
C. It competes with CD28 for binding to B7 on APCs
D. It promotes the differentiation of Th17 cells
Correct Answer: C
Expert Explanation: CTLA-4 is an essential negative regulator of T cell activation
expressed shortly after stimulation. It binds to B7-1 (CD80) and B7-2 (CD86) with much
