PCB 3233 Exam 2 V2 | PCB 3233
Immunology | Actual Q&A with Rationale
(PCB3233 Exam 2) | University of Central
Florida
1. In the MHC Class I processing pathway, which protein is responsible for transporting
proteasome-derived peptides into the endoplasmic reticulum?
A. HLA-DM
B. TAP (Transporter associated with Antigen Processing)
C. Invariant chain
D. Calnexin
Answer: B
Rationale: TAP proteins form a heterodimer in the ER membrane that pumps peptides
from the cytosol to the ER lumen. This step is critical because MHC Class I molecules are
loaded within the ER before being sent to the surface. Without functional TAP, MHC Class I
molecules cannot acquire peptides and remain unstable.
2. What is the primary function of the invariant chain (Ii) in MHC Class II molecules?
A. To facilitate peptide loading in the ER
B. To transport peptides into the endosome
C. To block the peptide-binding groove and prevent premature loading
,D. To degrade extracellular proteins into smaller fragments
Answer: C
Rationale: The invariant chain binds to the MHC Class II groove in the ER to prevent
endogenous peptides from binding. It also contains sorting signals that direct the MHC-Ii
complex to the late endosomal compartments. This ensures that MHC II only presents
exogenous antigens acquired through the endocytic pathway.
3. During T cell development in the thymus, what does ‘Positive Selection’ determine?
A. The ability of the TCR to recognize self-MHC molecules
B. The rearrangement of the TCR alpha chain
C. The migration of thymocytes to the medulla
D. The elimination of self-reactive T cells
Answer: A
Rationale: Positive selection occurs in the thymic cortex where double-positive
thymocytes interact with cortical epithelial cells. Thymocytes that have a TCR capable of
binding self-MHC receive survival signals. Cells that fail to recognize self-MHC undergo
apoptosis by neglect.
4. Which of the following molecules is responsible for the removal of the CLIP peptide from
the MHC Class II binding groove?
A. TAP-1
, B. Cathepsin S
C. B7-1
D. HLA-DM
Answer: D
Rationale: HLA-DM is a non-classical MHC molecule that acts as a peptide editor in the
endosome. It catalyzes the release of CLIP (Class II-associated invariant chain peptide)
from the MHC II molecule. This allows the MHC II groove to bind higher-affinity exogenous
peptides.
5. Which transcription factor is uniquely expressed by Regulatory T cells (Tregs)?
A. T-bet
B. FoxP3
C. ROR-gamma-t
D. GATA-3
Answer: B
Rationale: FoxP3 is the master regulator for the development and inhibitory function of
regulatory T cells. Mutations in the FoxP3 gene lead to severe autoimmune disorders like
IPEX syndrome. This factor distinguishes Tregs from other effector CD4 T cell subsets like
Th1 or Th2.
Immunology | Actual Q&A with Rationale
(PCB3233 Exam 2) | University of Central
Florida
1. In the MHC Class I processing pathway, which protein is responsible for transporting
proteasome-derived peptides into the endoplasmic reticulum?
A. HLA-DM
B. TAP (Transporter associated with Antigen Processing)
C. Invariant chain
D. Calnexin
Answer: B
Rationale: TAP proteins form a heterodimer in the ER membrane that pumps peptides
from the cytosol to the ER lumen. This step is critical because MHC Class I molecules are
loaded within the ER before being sent to the surface. Without functional TAP, MHC Class I
molecules cannot acquire peptides and remain unstable.
2. What is the primary function of the invariant chain (Ii) in MHC Class II molecules?
A. To facilitate peptide loading in the ER
B. To transport peptides into the endosome
C. To block the peptide-binding groove and prevent premature loading
,D. To degrade extracellular proteins into smaller fragments
Answer: C
Rationale: The invariant chain binds to the MHC Class II groove in the ER to prevent
endogenous peptides from binding. It also contains sorting signals that direct the MHC-Ii
complex to the late endosomal compartments. This ensures that MHC II only presents
exogenous antigens acquired through the endocytic pathway.
3. During T cell development in the thymus, what does ‘Positive Selection’ determine?
A. The ability of the TCR to recognize self-MHC molecules
B. The rearrangement of the TCR alpha chain
C. The migration of thymocytes to the medulla
D. The elimination of self-reactive T cells
Answer: A
Rationale: Positive selection occurs in the thymic cortex where double-positive
thymocytes interact with cortical epithelial cells. Thymocytes that have a TCR capable of
binding self-MHC receive survival signals. Cells that fail to recognize self-MHC undergo
apoptosis by neglect.
4. Which of the following molecules is responsible for the removal of the CLIP peptide from
the MHC Class II binding groove?
A. TAP-1
, B. Cathepsin S
C. B7-1
D. HLA-DM
Answer: D
Rationale: HLA-DM is a non-classical MHC molecule that acts as a peptide editor in the
endosome. It catalyzes the release of CLIP (Class II-associated invariant chain peptide)
from the MHC II molecule. This allows the MHC II groove to bind higher-affinity exogenous
peptides.
5. Which transcription factor is uniquely expressed by Regulatory T cells (Tregs)?
A. T-bet
B. FoxP3
C. ROR-gamma-t
D. GATA-3
Answer: B
Rationale: FoxP3 is the master regulator for the development and inhibitory function of
regulatory T cells. Mutations in the FoxP3 gene lead to severe autoimmune disorders like
IPEX syndrome. This factor distinguishes Tregs from other effector CD4 T cell subsets like
Th1 or Th2.
