PCB 3233 | PCB3233 Exam 1: Immunology -
University of Central Florida Updated and Latest
Questions and Correct Answers with Rationale
1. Which of the following serves as a primary chemical barrier in the innate immune system
by degrading peptidoglycan in bacterial cell walls?
A. Sebum
B. Defensins
C. Mucus
D. Lysozyme
Correct Answer: D
Expert Explanation: Lysozyme is an enzyme found in secretions like tears and saliva that
specifically targets the cell walls of Gram-positive bacteria. By breaking the bonds in
peptidoglycan, it causes the bacteria to undergo osmotic lysis and die. Sebum provides a
protective oily layer and lowers pH but does not degrade peptidoglycan directly. Mucus
primarily acts as a physical trap for pathogens rather than a catalytic enzyme. This
chemical defense is critical for protecting mucosal surfaces from immediate bacterial
colonization.
2. A patient presents with a bacterial infection, and lab results show a high concentration of
Lipopolysaccharide (LPS). Which Pattern Recognition Receptor (PRR) is responsible for
detecting this PAMP?
A. TLR3
B. TLR4
C. TLR5
D. TLR9
Correct Answer: B
Expert Explanation: TLR4 is the specific Toll-like receptor that recognizes the
Lipopolysaccharide found on the outer membrane of Gram-negative bacteria. When TLR4
binds LPS, it initiates a signaling pathway that activates transcription factors like NF-kB for
cytokine production. TLR3 is incorrect because it recognizes double-stranded RNA
typically associated with viral infections. TLR5 recognizes flagellin, while TLR9 detects
unmethylated CpG DNA sequences in pathogens. Understanding receptor specificity is
essential for diagnosing how the innate immune system identifies distinct microbial
threats.
,3. Which cell type is typically the first to arrive at the site of an acute infection and utilizes
Neutrophil Extracellular Traps (NETs)?
A. Macrophage
B. Neutrophil
C. Dendritic Cell
D. Natural Killer Cell
Correct Answer: B
Expert Explanation: Neutrophils are the most abundant leukocytes and are the first
responders to inflammatory signals from the site of infection. They utilize a variety of
killing mechanisms including phagocytosis, degranulation, and the release of NETs to trap
bacteria. Macrophages arrive later and are better suited for sustained defense and tissue
repair. Dendritic cells are primarily involved in antigen presentation rather than being the
primary first responders for killing. The rapid recruitment of neutrophils is a hallmark of
the acute inflammatory response in the host.
4. Where do T cells undergo their final maturation and selection process to ensure they do
not react against self-antigens?
A. Thymus
B. Spleen
C. Bone Marrow
D. Lymph Nodes
Correct Answer: A
Expert Explanation: The thymus is a primary lymphoid organ where T cell progenitors
migrate from the bone marrow to mature. During this process, T cells undergo positive and
negative selection to ensure functional and self-tolerant receptors. Bone marrow is where
B cells mature, while T cells only originate there before moving to the thymus. The spleen
and lymph nodes are secondary lymphoid organs where mature immune cells wait to
encounter antigens. Dysfunction in the thymus can lead to severe immunodeficiency or
autoimmune diseases due to improper T cell development.
5. An individual is bitten by a snake and receives an injection of pre-formed antibodies. What
type of immunity does this represent?
A. Artificial Passive Immunity
B. Natural Active Immunity
C. Artificial Active Immunity
D. Natural Passive Immunity
, Correct Answer: A
Expert Explanation: Artificial passive immunity involves the transfer of antibodies from
an external source to provide immediate protection against a pathogen or toxin. This is
common in antivenom treatments where the body does not produce its own memory
response. Natural active immunity occurs when an individual recovers from a natural
infection and develops their own antibodies. Artificial active immunity refers to
vaccination, which stimulates the host’s immune system to create a long-term memory.
Passive immunity is temporary because the injected antibodies eventually degrade and no
memory B cells are created.
6. Which component of the complement system acts as a potent opsonin, coating pathogens
to enhance their recognition by phagocytes?
A. C3b
B. C5a
C. C3a
D. C9
Correct Answer: A
Expert Explanation: C3b is the primary opsonin produced during the complement
activation cascade that binds covalently to the surface of pathogens. This coating allows
phagocytes with C3b receptors to easily identify and engulf the microbial target. C3a and
C5a act as anaphylatoxins that promote inflammation and attract other immune cells to the
site. C9 is involved in the formation of the Membrane Attack Complex, which causes direct
lysis of the cell. Efficient opsonization by C3b is one of the most important functions of the
complement system in clearing infections.
7. A patient has a deficiency in their ability to produce Reactive Oxygen Species (ROS) during
the respiratory burst. Which immune process is most likely impaired?
A. Viral entry into host cells
B. Antibody class switching
C. Intracellular killing of bacteria
D. Migration of T cells to the skin
Correct Answer: C
Expert Explanation: The respiratory burst is a metabolic process in phagocytes that
generates toxic oxygen radicals to kill ingested microorganisms. If this process is impaired,
as seen in Chronic Granulomatous Disease, phagocytes can ingest but cannot effectively
destroy certain bacteria. This lead to recurrent infections because the intracellular killing
mechanism is the final step of successful phagocytosis. Viral entry and T cell migration are
distinct processes that do not rely on the oxidative burst mechanism. This scenario
University of Central Florida Updated and Latest
Questions and Correct Answers with Rationale
1. Which of the following serves as a primary chemical barrier in the innate immune system
by degrading peptidoglycan in bacterial cell walls?
A. Sebum
B. Defensins
C. Mucus
D. Lysozyme
Correct Answer: D
Expert Explanation: Lysozyme is an enzyme found in secretions like tears and saliva that
specifically targets the cell walls of Gram-positive bacteria. By breaking the bonds in
peptidoglycan, it causes the bacteria to undergo osmotic lysis and die. Sebum provides a
protective oily layer and lowers pH but does not degrade peptidoglycan directly. Mucus
primarily acts as a physical trap for pathogens rather than a catalytic enzyme. This
chemical defense is critical for protecting mucosal surfaces from immediate bacterial
colonization.
2. A patient presents with a bacterial infection, and lab results show a high concentration of
Lipopolysaccharide (LPS). Which Pattern Recognition Receptor (PRR) is responsible for
detecting this PAMP?
A. TLR3
B. TLR4
C. TLR5
D. TLR9
Correct Answer: B
Expert Explanation: TLR4 is the specific Toll-like receptor that recognizes the
Lipopolysaccharide found on the outer membrane of Gram-negative bacteria. When TLR4
binds LPS, it initiates a signaling pathway that activates transcription factors like NF-kB for
cytokine production. TLR3 is incorrect because it recognizes double-stranded RNA
typically associated with viral infections. TLR5 recognizes flagellin, while TLR9 detects
unmethylated CpG DNA sequences in pathogens. Understanding receptor specificity is
essential for diagnosing how the innate immune system identifies distinct microbial
threats.
,3. Which cell type is typically the first to arrive at the site of an acute infection and utilizes
Neutrophil Extracellular Traps (NETs)?
A. Macrophage
B. Neutrophil
C. Dendritic Cell
D. Natural Killer Cell
Correct Answer: B
Expert Explanation: Neutrophils are the most abundant leukocytes and are the first
responders to inflammatory signals from the site of infection. They utilize a variety of
killing mechanisms including phagocytosis, degranulation, and the release of NETs to trap
bacteria. Macrophages arrive later and are better suited for sustained defense and tissue
repair. Dendritic cells are primarily involved in antigen presentation rather than being the
primary first responders for killing. The rapid recruitment of neutrophils is a hallmark of
the acute inflammatory response in the host.
4. Where do T cells undergo their final maturation and selection process to ensure they do
not react against self-antigens?
A. Thymus
B. Spleen
C. Bone Marrow
D. Lymph Nodes
Correct Answer: A
Expert Explanation: The thymus is a primary lymphoid organ where T cell progenitors
migrate from the bone marrow to mature. During this process, T cells undergo positive and
negative selection to ensure functional and self-tolerant receptors. Bone marrow is where
B cells mature, while T cells only originate there before moving to the thymus. The spleen
and lymph nodes are secondary lymphoid organs where mature immune cells wait to
encounter antigens. Dysfunction in the thymus can lead to severe immunodeficiency or
autoimmune diseases due to improper T cell development.
5. An individual is bitten by a snake and receives an injection of pre-formed antibodies. What
type of immunity does this represent?
A. Artificial Passive Immunity
B. Natural Active Immunity
C. Artificial Active Immunity
D. Natural Passive Immunity
, Correct Answer: A
Expert Explanation: Artificial passive immunity involves the transfer of antibodies from
an external source to provide immediate protection against a pathogen or toxin. This is
common in antivenom treatments where the body does not produce its own memory
response. Natural active immunity occurs when an individual recovers from a natural
infection and develops their own antibodies. Artificial active immunity refers to
vaccination, which stimulates the host’s immune system to create a long-term memory.
Passive immunity is temporary because the injected antibodies eventually degrade and no
memory B cells are created.
6. Which component of the complement system acts as a potent opsonin, coating pathogens
to enhance their recognition by phagocytes?
A. C3b
B. C5a
C. C3a
D. C9
Correct Answer: A
Expert Explanation: C3b is the primary opsonin produced during the complement
activation cascade that binds covalently to the surface of pathogens. This coating allows
phagocytes with C3b receptors to easily identify and engulf the microbial target. C3a and
C5a act as anaphylatoxins that promote inflammation and attract other immune cells to the
site. C9 is involved in the formation of the Membrane Attack Complex, which causes direct
lysis of the cell. Efficient opsonization by C3b is one of the most important functions of the
complement system in clearing infections.
7. A patient has a deficiency in their ability to produce Reactive Oxygen Species (ROS) during
the respiratory burst. Which immune process is most likely impaired?
A. Viral entry into host cells
B. Antibody class switching
C. Intracellular killing of bacteria
D. Migration of T cells to the skin
Correct Answer: C
Expert Explanation: The respiratory burst is a metabolic process in phagocytes that
generates toxic oxygen radicals to kill ingested microorganisms. If this process is impaired,
as seen in Chronic Granulomatous Disease, phagocytes can ingest but cannot effectively
destroy certain bacteria. This lead to recurrent infections because the intracellular killing
mechanism is the final step of successful phagocytosis. Viral entry and T cell migration are
distinct processes that do not rely on the oxidative burst mechanism. This scenario
