MCB 3020C Exam 4 V3 | MCB 3020C General Microbiology | Actual
Q&A with Rationale (MCB3020C Exam 4) | University of Central
Florida
1. Which component of the innate immune system acts as a chemical barrier by breaking
down the peptidoglycan cell walls of bacteria?
A. Lysozyme
B. Transferrin
C. Sebum
D. Mucin
Answer: A
Explanation: Lysozyme is an enzyme found in secretions such as tears, saliva, and sweat. It
specifically targets and hydrolyzes the bonds in peptidoglycan, which is a critical structural
component of bacterial cell walls. This chemical defense is particularly effective against
Gram-positive bacteria that lack an outer membrane.
2. In the process of phagocytosis, what is the specific term for the vesicle formed when a
phagocyte engulfs a microbe?
A. Phagosome
B. Lysosome
C. Peroxisome
D. Residual body
Answer: A
Explanation: A phagosome is a membrane-bound vacuole formed around a particle
absorbed by phagocytosis. Once formed, it eventually fuses with a lysosome to create a
phagolysosome where digestion occurs. This separation of the microbe into a specialized
compartment protects the rest of the cell from toxic digestive enzymes.
3. Which of the following complement proteins acts as a powerful opsonin to enhance the
phagocytosis of pathogens?
A. C3a
B. C3b
C. C5a
D. C9
,Answer: B
Explanation: C3b is a cleavage product of the C3 protein that binds covalently to the
surface of microbial cells. This binding marks the pathogen for recognition by phagocytes
that possess C3b receptors, a process known as opsonization. Without C3b, many
encapsulated bacteria would be much harder for the immune system to ingest.
4. Which type of hypersensitivity reaction is mediated by IgE antibodies binding to mast cells
and basophils, leading to the release of histamine?
A. Type IV (Delayed Cell-Mediated)
B. Type II (Cytotoxic)
C. Type III (Immune Complex)
D. Type I (Anaphylactic)
Answer: D
Explanation: Type I hypersensitivity involves an immediate allergic reaction following
exposure to an allergen. When an allergen cross-links IgE molecules already bound to mast
cells, it triggers degranulation and the release of inflammatory mediators like histamine.
This pathway is responsible for conditions ranging from hay fever to life-threatening
systemic anaphylaxis.
5. MHC Class II molecules are primarily found on the surface of which cell type?
A. Nucleated somatic cells
B. Antigen-presenting cells (APCs)
C. Red blood cells
D. Cytotoxic T cells
Answer: B
Explanation: MHC Class II molecules are specialized proteins used to present exogenous
antigens to Helper T cells (CD4+). They are expressed almost exclusively by professional
antigen-presenting cells such as macrophages, B cells, and dendritic cells. This interaction
is crucial for the initiation of the adaptive immune response.
6. Which antibody isotype is the first to be produced during a primary immune response and
exists as a pentamer in circulation?
A. IgG
B. IgM
C. IgA
D. IgE
, Answer: B
Explanation: IgM is the initial class of antibody secreted by plasma cells upon first
exposure to an antigen. Its pentameric structure provides ten binding sites, making it
highly effective at agglutinating antigens and activating the complement system. Despite its
high valence, its large size limits its ability to leave the bloodstream and enter tissues.
7. A child receives a vaccine containing a weakened form of a virus. What type of immunity
does this provide?
A. Naturally acquired active immunity
B. Artificially acquired active immunity
C. Naturally acquired passive immunity
D. Artificially acquired passive immunity
Answer: B
Explanation: Artificially acquired active immunity occurs when a vaccine containing
antigens is introduced into the body to stimulate a long-term immune response. The body
produces its own antibodies and memory cells without the person having to suffer through
the full-blown disease. This is distinct from passive immunity, where pre-formed
antibodies are transferred from another source.
8. Which toxin produced by Staphylococcus aureus is responsible for the symptoms of Toxic
Shock Syndrome?
A. Alpha-toxin
B. Exfoliatin
C. TSST-1 (Superantigen)
D. Enterotoxin A
Answer: C
Explanation: TSST-1 acts as a superantigen, which non-specifically cross-links MHC II
molecules on APCs with T-cell receptors. This bypasses normal antigen processing and
leads to the activation of up to 20 percent of all T cells. The resulting massive release of
cytokines causes high fever, low blood pressure, and multi-organ failure.
9. Which diagnostic test uses a secondary antibody conjugated to an enzyme to detect the
presence of specific antigens or antibodies in a patient sample?
A. Agglutination
B. Western Blot
C. ELISA
Q&A with Rationale (MCB3020C Exam 4) | University of Central
Florida
1. Which component of the innate immune system acts as a chemical barrier by breaking
down the peptidoglycan cell walls of bacteria?
A. Lysozyme
B. Transferrin
C. Sebum
D. Mucin
Answer: A
Explanation: Lysozyme is an enzyme found in secretions such as tears, saliva, and sweat. It
specifically targets and hydrolyzes the bonds in peptidoglycan, which is a critical structural
component of bacterial cell walls. This chemical defense is particularly effective against
Gram-positive bacteria that lack an outer membrane.
2. In the process of phagocytosis, what is the specific term for the vesicle formed when a
phagocyte engulfs a microbe?
A. Phagosome
B. Lysosome
C. Peroxisome
D. Residual body
Answer: A
Explanation: A phagosome is a membrane-bound vacuole formed around a particle
absorbed by phagocytosis. Once formed, it eventually fuses with a lysosome to create a
phagolysosome where digestion occurs. This separation of the microbe into a specialized
compartment protects the rest of the cell from toxic digestive enzymes.
3. Which of the following complement proteins acts as a powerful opsonin to enhance the
phagocytosis of pathogens?
A. C3a
B. C3b
C. C5a
D. C9
,Answer: B
Explanation: C3b is a cleavage product of the C3 protein that binds covalently to the
surface of microbial cells. This binding marks the pathogen for recognition by phagocytes
that possess C3b receptors, a process known as opsonization. Without C3b, many
encapsulated bacteria would be much harder for the immune system to ingest.
4. Which type of hypersensitivity reaction is mediated by IgE antibodies binding to mast cells
and basophils, leading to the release of histamine?
A. Type IV (Delayed Cell-Mediated)
B. Type II (Cytotoxic)
C. Type III (Immune Complex)
D. Type I (Anaphylactic)
Answer: D
Explanation: Type I hypersensitivity involves an immediate allergic reaction following
exposure to an allergen. When an allergen cross-links IgE molecules already bound to mast
cells, it triggers degranulation and the release of inflammatory mediators like histamine.
This pathway is responsible for conditions ranging from hay fever to life-threatening
systemic anaphylaxis.
5. MHC Class II molecules are primarily found on the surface of which cell type?
A. Nucleated somatic cells
B. Antigen-presenting cells (APCs)
C. Red blood cells
D. Cytotoxic T cells
Answer: B
Explanation: MHC Class II molecules are specialized proteins used to present exogenous
antigens to Helper T cells (CD4+). They are expressed almost exclusively by professional
antigen-presenting cells such as macrophages, B cells, and dendritic cells. This interaction
is crucial for the initiation of the adaptive immune response.
6. Which antibody isotype is the first to be produced during a primary immune response and
exists as a pentamer in circulation?
A. IgG
B. IgM
C. IgA
D. IgE
, Answer: B
Explanation: IgM is the initial class of antibody secreted by plasma cells upon first
exposure to an antigen. Its pentameric structure provides ten binding sites, making it
highly effective at agglutinating antigens and activating the complement system. Despite its
high valence, its large size limits its ability to leave the bloodstream and enter tissues.
7. A child receives a vaccine containing a weakened form of a virus. What type of immunity
does this provide?
A. Naturally acquired active immunity
B. Artificially acquired active immunity
C. Naturally acquired passive immunity
D. Artificially acquired passive immunity
Answer: B
Explanation: Artificially acquired active immunity occurs when a vaccine containing
antigens is introduced into the body to stimulate a long-term immune response. The body
produces its own antibodies and memory cells without the person having to suffer through
the full-blown disease. This is distinct from passive immunity, where pre-formed
antibodies are transferred from another source.
8. Which toxin produced by Staphylococcus aureus is responsible for the symptoms of Toxic
Shock Syndrome?
A. Alpha-toxin
B. Exfoliatin
C. TSST-1 (Superantigen)
D. Enterotoxin A
Answer: C
Explanation: TSST-1 acts as a superantigen, which non-specifically cross-links MHC II
molecules on APCs with T-cell receptors. This bypasses normal antigen processing and
leads to the activation of up to 20 percent of all T cells. The resulting massive release of
cytokines causes high fever, low blood pressure, and multi-organ failure.
9. Which diagnostic test uses a secondary antibody conjugated to an enzyme to detect the
presence of specific antigens or antibodies in a patient sample?
A. Agglutination
B. Western Blot
C. ELISA
