PCB 3233 | PCB3233 Exam 2: Immunology -
University of Central Florida Updated and Latest
Questions and Correct Answers with Rationale
1. Which molecule is responsible for presenting endogenous antigens to CD8+ cytotoxic T
cells?
A. CD28
B. MHC Class II
C. B cell receptor
D. MHC Class I
Correct Answer: D
Expert Explanation: MHC Class I molecules are expressed on all nucleated cells and
present peptides derived from the cytosol. These peptides are typically viral or self-
proteins that indicate the internal state of the cell. CD8+ T cells recognize these complexes
to identify and kill infected or abnormal host cells. This interaction is restricted by the
specificity of the T cell receptor for both the peptide and the MHC molecule. Thus, MHC
Class I is essential for the cell-mediated immune response against intracellular pathogens.
2. The process by which B cells with the highest affinity for an antigen are selected to survive
is known as:
A. Affinity maturation
B. Clonal deletion
C. Isotype switching
D. Anergy
Correct Answer: A
Expert Explanation: Affinity maturation occurs within the germinal centers of secondary
lymphoid organs after B cell activation. It involves somatic hypermutation of the
immunoglobulin genes followed by selection by follicular dendritic cells. B cells that bind
antigen more effectively receive survival signals, while those with lower affinity undergo
apoptosis. This process ensures that the antibody response becomes increasingly effective
over the course of an infection. Consequently, the secondary immune response produces
antibodies with much higher binding strength than the primary response.
3. Which of the following serves as the ‘Signal 2’ during the activation of a naive T cell?
A. TCR binding to MHC-peptide complex
B. IL-2 binding to the IL-2 receptor
,C. CD28 binding to B7 (CD80/86)
D. CD40 binding to CD40L
Correct Answer: C
Expert Explanation: Activation of naive T cells requires two distinct signals to prevent
accidental activation against self-antigens. Signal 1 is the recognition of the specific MHC-
peptide complex by the T cell receptor. Signal 2 is a co-stimulatory signal, most commonly
the interaction between CD28 on the T cell and B7 on the APC. Without this second signal,
the T cell may become anergic or unresponsive even if it recognizes its antigen. This dual-
checkpoint system is a vital component of peripheral immune tolerance.
4. What is the primary function of the Invariant Chain (Ii) in MHC Class II processing?
A. To transport MHC Class II to the cell surface
B. To degrade extracellular proteins into peptides
C. To prevent premature peptide binding in the ER
D. To load peptides onto MHC Class I molecules
Correct Answer: C
Expert Explanation: The Invariant Chain binds to the peptide-binding groove of MHC
Class II molecules immediately after their synthesis in the endoplasmic reticulum. This
blocking action prevents MHC Class II from binding endogenous peptides intended for MHC
Class I. The complex is then transported to endosomal compartments where the Invariant
Chain is enzymatically cleaved. A small fragment called CLIP remains until it is exchanged
for an exogenous peptide by HLA-DM. Therefore, the Invariant Chain ensures that MHC
Class II only presents antigens acquired from the extracellular environment.
5. Which cytokine is most critical for the autocrine proliferation of activated T cells?
A. IL-4
B. IL-2
C. IFN-gamma
D. TNF-alpha
Correct Answer: B
Expert Explanation: Interleukin-2 (IL-2) is a potent growth factor produced by T cells
shortly after they receive activation signals. It acts in an autocrine fashion by binding to
high-affinity IL-2 receptors expressed on the same cell’s surface. This binding triggers rapid
entry into the cell cycle and clonal expansion of the antigen-specific T cell population.
Drugs that inhibit IL-2 production are frequently used as immunosuppressants to prevent
organ transplant rejection. In summary, IL-2 is the central driver for increasing the
magnitude of the T cell response.
, 6. Somatic hypermutation occurs primarily in which anatomical location?
A. Germinal centers of lymph nodes
B. Thymus
C. Bone marrow
D. Bloodstream
Correct Answer: A
Expert Explanation: Somatic hypermutation is a late-stage mechanism of the adaptive
immune response that alters B cell receptors. This process takes place within specialized
structures called germinal centers located in secondary lymphoid tissues like lymph nodes
or the spleen. During this phase, the V-region DNA of immunoglobulin genes undergoes
rapid point mutations. These mutations are induced by the enzyme Activation-Induced
Cytidine Deaminase (AID) during B cell proliferation. This localized event is the mechanical
basis for the refinement of antibody specificity during an ongoing infection.
7. Which cell type is responsible for ‘cross-presentation’ of exogenous antigens on MHC Class
I?
A. Dendritic cells
B. Neutrophils
C. B cells
D. Red blood cells
Correct Answer: A
Expert Explanation: Cross-presentation is a unique ability of certain professional antigen-
presenting cells, primarily dendritic cells. It allows them to take up extracellular antigens
and process them into the MHC Class I pathway instead of the usual Class II pathway. This
mechanism is vital for initiating CD8+ T cell responses against viruses that do not infect
dendritic cells directly. Without cross-presentation, the immune system would struggle to
detect tumors or viruses sequestered in non-APC tissues. Thus, dendritic cells serve as the
essential bridge between exogenous threats and cytotoxic T cell activation.
8. What defines the ‘specificity’ of the adaptive immune system?
A. The unique receptors generated by gene rearrangement
B. The use of germline-encoded receptors
C. The ability to recognize PAMPs via TLRs
D. The rapid production of histamine during inflammation
Correct Answer: A
University of Central Florida Updated and Latest
Questions and Correct Answers with Rationale
1. Which molecule is responsible for presenting endogenous antigens to CD8+ cytotoxic T
cells?
A. CD28
B. MHC Class II
C. B cell receptor
D. MHC Class I
Correct Answer: D
Expert Explanation: MHC Class I molecules are expressed on all nucleated cells and
present peptides derived from the cytosol. These peptides are typically viral or self-
proteins that indicate the internal state of the cell. CD8+ T cells recognize these complexes
to identify and kill infected or abnormal host cells. This interaction is restricted by the
specificity of the T cell receptor for both the peptide and the MHC molecule. Thus, MHC
Class I is essential for the cell-mediated immune response against intracellular pathogens.
2. The process by which B cells with the highest affinity for an antigen are selected to survive
is known as:
A. Affinity maturation
B. Clonal deletion
C. Isotype switching
D. Anergy
Correct Answer: A
Expert Explanation: Affinity maturation occurs within the germinal centers of secondary
lymphoid organs after B cell activation. It involves somatic hypermutation of the
immunoglobulin genes followed by selection by follicular dendritic cells. B cells that bind
antigen more effectively receive survival signals, while those with lower affinity undergo
apoptosis. This process ensures that the antibody response becomes increasingly effective
over the course of an infection. Consequently, the secondary immune response produces
antibodies with much higher binding strength than the primary response.
3. Which of the following serves as the ‘Signal 2’ during the activation of a naive T cell?
A. TCR binding to MHC-peptide complex
B. IL-2 binding to the IL-2 receptor
,C. CD28 binding to B7 (CD80/86)
D. CD40 binding to CD40L
Correct Answer: C
Expert Explanation: Activation of naive T cells requires two distinct signals to prevent
accidental activation against self-antigens. Signal 1 is the recognition of the specific MHC-
peptide complex by the T cell receptor. Signal 2 is a co-stimulatory signal, most commonly
the interaction between CD28 on the T cell and B7 on the APC. Without this second signal,
the T cell may become anergic or unresponsive even if it recognizes its antigen. This dual-
checkpoint system is a vital component of peripheral immune tolerance.
4. What is the primary function of the Invariant Chain (Ii) in MHC Class II processing?
A. To transport MHC Class II to the cell surface
B. To degrade extracellular proteins into peptides
C. To prevent premature peptide binding in the ER
D. To load peptides onto MHC Class I molecules
Correct Answer: C
Expert Explanation: The Invariant Chain binds to the peptide-binding groove of MHC
Class II molecules immediately after their synthesis in the endoplasmic reticulum. This
blocking action prevents MHC Class II from binding endogenous peptides intended for MHC
Class I. The complex is then transported to endosomal compartments where the Invariant
Chain is enzymatically cleaved. A small fragment called CLIP remains until it is exchanged
for an exogenous peptide by HLA-DM. Therefore, the Invariant Chain ensures that MHC
Class II only presents antigens acquired from the extracellular environment.
5. Which cytokine is most critical for the autocrine proliferation of activated T cells?
A. IL-4
B. IL-2
C. IFN-gamma
D. TNF-alpha
Correct Answer: B
Expert Explanation: Interleukin-2 (IL-2) is a potent growth factor produced by T cells
shortly after they receive activation signals. It acts in an autocrine fashion by binding to
high-affinity IL-2 receptors expressed on the same cell’s surface. This binding triggers rapid
entry into the cell cycle and clonal expansion of the antigen-specific T cell population.
Drugs that inhibit IL-2 production are frequently used as immunosuppressants to prevent
organ transplant rejection. In summary, IL-2 is the central driver for increasing the
magnitude of the T cell response.
, 6. Somatic hypermutation occurs primarily in which anatomical location?
A. Germinal centers of lymph nodes
B. Thymus
C. Bone marrow
D. Bloodstream
Correct Answer: A
Expert Explanation: Somatic hypermutation is a late-stage mechanism of the adaptive
immune response that alters B cell receptors. This process takes place within specialized
structures called germinal centers located in secondary lymphoid tissues like lymph nodes
or the spleen. During this phase, the V-region DNA of immunoglobulin genes undergoes
rapid point mutations. These mutations are induced by the enzyme Activation-Induced
Cytidine Deaminase (AID) during B cell proliferation. This localized event is the mechanical
basis for the refinement of antibody specificity during an ongoing infection.
7. Which cell type is responsible for ‘cross-presentation’ of exogenous antigens on MHC Class
I?
A. Dendritic cells
B. Neutrophils
C. B cells
D. Red blood cells
Correct Answer: A
Expert Explanation: Cross-presentation is a unique ability of certain professional antigen-
presenting cells, primarily dendritic cells. It allows them to take up extracellular antigens
and process them into the MHC Class I pathway instead of the usual Class II pathway. This
mechanism is vital for initiating CD8+ T cell responses against viruses that do not infect
dendritic cells directly. Without cross-presentation, the immune system would struggle to
detect tumors or viruses sequestered in non-APC tissues. Thus, dendritic cells serve as the
essential bridge between exogenous threats and cytotoxic T cell activation.
8. What defines the ‘specificity’ of the adaptive immune system?
A. The unique receptors generated by gene rearrangement
B. The use of germline-encoded receptors
C. The ability to recognize PAMPs via TLRs
D. The rapid production of histamine during inflammation
Correct Answer: A
