Immunology Exam 5 with correct
answers and rationale updated 2026
1. Which of the following is a primary lymphoid organ?
A. Spleen
B. Lymph node
C. Thymus
D. Tonsil
Rationale: The thymus is a primary (central) lymphoid organ where T lymphocytes mature and
undergo selection. The spleen, lymph nodes, and tonsils are secondary (peripheral) lymphoid organs
where immune responses are initiated.
2. The process by which B cells switch from producing IgM to IgG is known as:
A. Somatic recombination
B. Class switch recombination
C. Affinity maturation
D. Allelic exclusion
Rationale: Class switch recombination (CSR) is a genetic process that changes the constant region of
the antibody heavy chain, allowing a B cell to produce different isotypes (e.g., IgG, IgA, IgE) with the
same antigen specificity.
3. Which immunoglobulin is the most abundant in serum and can cross the placenta?
A. IgA
B. IgM
C. IgE
D. IgG
Rationale: IgG is the most abundant antibody isotype in serum and is the only one that can cross the
placenta, providing passive immunity to the fetus and newborn.
4. The T cell receptor (TCR) recognizes antigen presented by:
A. MHC molecules
B. Free-floating antigen
C. Antibodies
D. Complement proteins
Rationale: T cells recognize peptide fragments of antigens that are bound to and presented by Major
Histocompatibility Complex (MHC) molecules on the surface of antigen-presenting cells or other
cells.
5. Which cytokine is the primary mediator of the acute-phase response and fever?
A. IL-2
B. IL-4
C. IL-1
D. IL-10
Rationale: Interleukin-1 (IL-1), along with TNF-α and IL-6, are key pro-inflammatory cytokines that
induce the acute-phase response, including fever, in the liver and hypothalamus.
,6. The classical pathway of complement activation is initiated by:
A. Binding of C3b to a pathogen surface
B. Antigen-antibody complexes
C. Mannose-binding lectin binding to mannose
D. Spontaneous hydrolysis of C3
Rationale: The classical pathway is triggered by the binding of the C1q component to the Fc region
of antibodies (IgM or IgG) that are bound to an antigen.
7. What is the function of a cytotoxic T lymphocyte (CTL)?
A. To activate B cells
B. To phagocytose bacteria
C. To kill infected or cancerous cells
D. To release histamine
Rationale: Cytotoxic T lymphocytes (CTLs), which are typically CD8+ T cells, directly kill cells that are
infected with intracellular pathogens or have become cancerous.
8. Which of the following is NOT a professional antigen-presenting cell (APC)?
A. Dendritic cell
B. Macrophage
C. Neutrophil
D. B cell
Rationale: Professional APCs express MHC class II and costimulatory molecules to activate naive T
cells. The main professional APCs are dendritic cells, macrophages, and B cells. Neutrophils are
primarily phagocytic cells of the innate immune system and are not considered professional APCs.
9. The antibody isotype found in secretions like saliva, tears, and breast milk is:
A. IgG
B. IgM
C. IgA
D. IgD
Rationale: Secretory IgA (sIgA) is a dimeric form of IgA that is transported across mucosal epithelia
and is the primary antibody in mucosal secretions.
10. Positive selection of T cells in the thymus ensures that:
A. T cells do not react with self-antigens
B. T cells can recognize self-MHC molecules
C. T cells express both CD4 and CD8
D. T cells produce antibodies
Rationale: Positive selection tests the ability of a T cell's TCR to bind to self-MHC molecules. T cells
that fail this test undergo apoptosis. This process ensures that mature T cells are MHC-restricted.
11. Which cells are the primary producers of antibodies?
A. T cells
B. Plasma cells
C. Macrophages
D. Dendritic cells
Rationale: Plasma cells are terminally differentiated B cells that are specialized for the high-rate
synthesis and secretion of antibodies.
, 12. An epitope is:
A. The part of an antibody that binds antigen
B. The specific portion of an antigen that is recognized by the immune system
C. A molecule that enhances the immune response
D. A type of cytokine
Rationale: An epitope, or antigenic determinant, is the specific three-dimensional region of an
antigen that is physically bound by an antibody or T cell receptor.
13. The main function of helper T cells (CD4+) is to:
A. Directly kill virus-infected cells
B. Secrete cytokines to activate and direct other immune cells
C. Produce antibodies
D. Release histamine during allergic reactions
Rationale: Helper T cells are the "generals" of the immune response. They recognize antigen on
MHC class II and secrete cytokines that help activate B cells, cytotoxic T cells, and macrophages.
14. Which of the following describes the "affinity maturation" process?
A. The change from IgM to IgG production
B. The development of T cells in the thymus
C. The increase in the binding strength of antibodies during an immune response
D. The process of a B cell becoming a plasma cell
Rationale: Affinity maturation occurs in germinal centers during a T-dependent immune response. It
involves somatic hypermutation of antibody genes followed by selection of B cells with higher-
affinity receptors.
15. A patient has a genetic defect causing a lack of MHC class II molecules. Which cell type would
be most directly affected in its function?
A. Cytotoxic T cells
B. Helper T cells
C. B cells
D. Neutrophils
Rationale: Helper T cells (CD4+) are restricted by MHC class II molecules. Without MHC class II, they
cannot be positively selected in the thymus and cannot recognize antigen on APCs, leading to a
severe immunodeficiency.
16. The inflammasome is a cytosolic multiprotein complex that activates:
A. Caspase-1
B. NF-κB
C. The proteasome
D. The classical complement pathway
Rationale: The inflammasome activates caspase-1, which in turn cleaves pro-IL-1β and pro-IL-18 into
their active forms, leading to a potent inflammatory response.
17. Which immunoglobulin is the first to be produced in a primary immune response?
A. IgG
B. IgM
C. IgA
D. IgE
Rationale: Naive B cells express IgM and IgD on their surface. Upon activation, they first differentiate
into plasma cells that secrete IgM before class switching to other isotypes occurs.
answers and rationale updated 2026
1. Which of the following is a primary lymphoid organ?
A. Spleen
B. Lymph node
C. Thymus
D. Tonsil
Rationale: The thymus is a primary (central) lymphoid organ where T lymphocytes mature and
undergo selection. The spleen, lymph nodes, and tonsils are secondary (peripheral) lymphoid organs
where immune responses are initiated.
2. The process by which B cells switch from producing IgM to IgG is known as:
A. Somatic recombination
B. Class switch recombination
C. Affinity maturation
D. Allelic exclusion
Rationale: Class switch recombination (CSR) is a genetic process that changes the constant region of
the antibody heavy chain, allowing a B cell to produce different isotypes (e.g., IgG, IgA, IgE) with the
same antigen specificity.
3. Which immunoglobulin is the most abundant in serum and can cross the placenta?
A. IgA
B. IgM
C. IgE
D. IgG
Rationale: IgG is the most abundant antibody isotype in serum and is the only one that can cross the
placenta, providing passive immunity to the fetus and newborn.
4. The T cell receptor (TCR) recognizes antigen presented by:
A. MHC molecules
B. Free-floating antigen
C. Antibodies
D. Complement proteins
Rationale: T cells recognize peptide fragments of antigens that are bound to and presented by Major
Histocompatibility Complex (MHC) molecules on the surface of antigen-presenting cells or other
cells.
5. Which cytokine is the primary mediator of the acute-phase response and fever?
A. IL-2
B. IL-4
C. IL-1
D. IL-10
Rationale: Interleukin-1 (IL-1), along with TNF-α and IL-6, are key pro-inflammatory cytokines that
induce the acute-phase response, including fever, in the liver and hypothalamus.
,6. The classical pathway of complement activation is initiated by:
A. Binding of C3b to a pathogen surface
B. Antigen-antibody complexes
C. Mannose-binding lectin binding to mannose
D. Spontaneous hydrolysis of C3
Rationale: The classical pathway is triggered by the binding of the C1q component to the Fc region
of antibodies (IgM or IgG) that are bound to an antigen.
7. What is the function of a cytotoxic T lymphocyte (CTL)?
A. To activate B cells
B. To phagocytose bacteria
C. To kill infected or cancerous cells
D. To release histamine
Rationale: Cytotoxic T lymphocytes (CTLs), which are typically CD8+ T cells, directly kill cells that are
infected with intracellular pathogens or have become cancerous.
8. Which of the following is NOT a professional antigen-presenting cell (APC)?
A. Dendritic cell
B. Macrophage
C. Neutrophil
D. B cell
Rationale: Professional APCs express MHC class II and costimulatory molecules to activate naive T
cells. The main professional APCs are dendritic cells, macrophages, and B cells. Neutrophils are
primarily phagocytic cells of the innate immune system and are not considered professional APCs.
9. The antibody isotype found in secretions like saliva, tears, and breast milk is:
A. IgG
B. IgM
C. IgA
D. IgD
Rationale: Secretory IgA (sIgA) is a dimeric form of IgA that is transported across mucosal epithelia
and is the primary antibody in mucosal secretions.
10. Positive selection of T cells in the thymus ensures that:
A. T cells do not react with self-antigens
B. T cells can recognize self-MHC molecules
C. T cells express both CD4 and CD8
D. T cells produce antibodies
Rationale: Positive selection tests the ability of a T cell's TCR to bind to self-MHC molecules. T cells
that fail this test undergo apoptosis. This process ensures that mature T cells are MHC-restricted.
11. Which cells are the primary producers of antibodies?
A. T cells
B. Plasma cells
C. Macrophages
D. Dendritic cells
Rationale: Plasma cells are terminally differentiated B cells that are specialized for the high-rate
synthesis and secretion of antibodies.
, 12. An epitope is:
A. The part of an antibody that binds antigen
B. The specific portion of an antigen that is recognized by the immune system
C. A molecule that enhances the immune response
D. A type of cytokine
Rationale: An epitope, or antigenic determinant, is the specific three-dimensional region of an
antigen that is physically bound by an antibody or T cell receptor.
13. The main function of helper T cells (CD4+) is to:
A. Directly kill virus-infected cells
B. Secrete cytokines to activate and direct other immune cells
C. Produce antibodies
D. Release histamine during allergic reactions
Rationale: Helper T cells are the "generals" of the immune response. They recognize antigen on
MHC class II and secrete cytokines that help activate B cells, cytotoxic T cells, and macrophages.
14. Which of the following describes the "affinity maturation" process?
A. The change from IgM to IgG production
B. The development of T cells in the thymus
C. The increase in the binding strength of antibodies during an immune response
D. The process of a B cell becoming a plasma cell
Rationale: Affinity maturation occurs in germinal centers during a T-dependent immune response. It
involves somatic hypermutation of antibody genes followed by selection of B cells with higher-
affinity receptors.
15. A patient has a genetic defect causing a lack of MHC class II molecules. Which cell type would
be most directly affected in its function?
A. Cytotoxic T cells
B. Helper T cells
C. B cells
D. Neutrophils
Rationale: Helper T cells (CD4+) are restricted by MHC class II molecules. Without MHC class II, they
cannot be positively selected in the thymus and cannot recognize antigen on APCs, leading to a
severe immunodeficiency.
16. The inflammasome is a cytosolic multiprotein complex that activates:
A. Caspase-1
B. NF-κB
C. The proteasome
D. The classical complement pathway
Rationale: The inflammasome activates caspase-1, which in turn cleaves pro-IL-1β and pro-IL-18 into
their active forms, leading to a potent inflammatory response.
17. Which immunoglobulin is the first to be produced in a primary immune response?
A. IgG
B. IgM
C. IgA
D. IgE
Rationale: Naive B cells express IgM and IgD on their surface. Upon activation, they first differentiate
into plasma cells that secrete IgM before class switching to other isotypes occurs.
