DMLI 2026 STUDY GUIDE Diplomate Medical
Laboratory Immunology Certification Practice
Questions with Answer Explanations Immunology •
Serology • Immunopathology • Laboratory Testing •
Quality Control • Preparation
SECTION 1: BASIC IMMUNOLOGIC PRINCIPLES & MECHANISMS
(Questions 1–18)
Question 1. Which immunoglobulin isotype is the FIRST to be produced in a
primary immune response?
A) IgG
B) IgM
C) IgA
D) IgE
E) IgD
Rationale: IgM is the first antibody isotype produced during a primary immune
response. It is a pentameric molecule with high avidity for antigens, making it
effective at agglutination and complement activation. IgG is the most abundant
isotype in serum and dominates secondary responses. IgA is found in mucosal
secretions, IgE mediates allergic reactions, and IgD is found on the surface of
naive B cells.
Question 2. Which of the following BEST describes the difference between
affinity and avidity?
A) Affinity is the strength of a single antigen-antibody bond; avidity is the overall
strength of multiple bonds
B) Affinity is the strength of a single antigen-antibody bond; avidity is the
cumulative strength of multiple antigen-antibody interactions
C) Affinity is the strength of multiple bonds; avidity is the strength of a single
bond
,D) Affinity and avidity are the same thing
E) Affinity refers to antibody class; avidity refers to antibody subclass
Rationale: Affinity is the strength of binding between a single antigenic
determinant (epitope) and a single antibody combining site. Avidity is the
cumulative strength of multiple antigen-antibody interactions, taking into account
the valency of both the antibody and the antigen. IgM, despite having lower
affinity than IgG, has high avidity due to its pentameric structure.
Question 3. Which complement pathway is activated by antigen-antibody
complexes?
A) Classical pathway
B) Alternative pathway
C) Lectin pathway
D) Coagulation pathway
E) Fibrinolytic pathway
Rationale: The classical complement pathway is activated by antigen-antibody
complexes, specifically by the binding of C1q to the Fc region of IgM or IgG
antibodies bound to antigen. The alternative pathway is activated by microbial
surfaces in the absence of antibody. The lectin pathway is activated by mannose-
binding lectin (MBL) binding to carbohydrates on microbial surfaces.
Question 4. Which of the following is TRUE about the CH50 assay?
A) It measures the alternative complement pathway
B) It measures the classical complement pathway and is used to screen for
complement deficiencies
C) It measures the lectin complement pathway
D) It is a specific test for C3 deficiency
E) It is used to diagnose rheumatoid arthritis
Rationale: The CH50 assay measures total hemolytic complement activity of the
classical pathway. It determines the dilution of patient serum needed to lyse 50%
of antibody-coated erythrocytes. A low or absent CH50 indicates a deficiency in a
classical pathway component (C1, C2, C4, C3, C5–C9) or consumption of
,complement. The AH50 assay measures the alternative pathway. The CH50 is a
screening test, not specific for a single component.
Question 5. A patient has recurrent infections with encapsulated bacteria and
absent CH50 but normal AH50. Which complement component is MOST likely
deficient?
A) C2
B) C3
C) C5
D) C9
E) Factor B
Rationale: If a classical pathway component (C1, C2, or C4) is deficient, the
CH50 is low or absent while the AH50 is normal. C3 deficiency would result in
both CH50 and AH50 being low or absent because C3 is shared by both pathways.
C5 and C9 deficiencies also affect both pathways. Factor B is an alternative
pathway component, so its deficiency would affect AH50 but not CH50.
Question 6. Which MHC class presents exogenous antigens to CD4+ T cells?
A) MHC class I
B) MHC class II
C) MHC class III
D) CD1
E) CD8
Rationale: MHC class II molecules present exogenous antigens (taken up by
phagocytosis or endocytosis) to CD4+ helper T cells. MHC class I molecules
present endogenous antigens (derived from intracellular pathogens or tumor
proteins) to CD8+ cytotoxic T cells. MHC class III encodes complement
components and other immune proteins, not antigen presentation.
Question 7. What is the function of CD40 ligand (CD40L) on activated T cells?
A) It binds to CD40 on B cells to promote class switching and differentiation
B) It binds to CD3 on T cells to promote proliferation
, C) It binds to MHC class I on antigen-presenting cells
D) It binds to complement receptors
E) It binds to Fc receptors
Rationale: CD40L (CD154) on activated CD4+ T cells binds to CD40 on B cells,
providing a critical co-stimulatory signal that promotes B cell proliferation,
immunoglobulin class switching, and differentiation into plasma cells and memory
B cells. Defects in CD40L cause X-linked hyper-IgM syndrome.
Question 8. Which cytokine is the hallmark of a Th2 immune response?
A) IFN-γ
B) IL-4
C) IL-12
D) TNF-α
E) IL-2
Rationale: IL-4 is the hallmark cytokine of Th2 responses, promoting B cell class
switching to IgE and IgG1, and driving differentiation of naive CD4+ T cells
toward the Th2 phenotype. IFN-γ and IL-12 are associated with Th1 responses.
TNF-α is a proinflammatory cytokine. IL-2 promotes T cell proliferation.
Question 9. Which of the following is TRUE about T-regulatory cells (Tregs)?
A) They express CD8 and promote cytotoxic responses
B) They express CD4, CD25, and FoxP3, and suppress immune responses
C) They express CD3 and promote B cell activation
D) They are the primary source of IL-2
E) They are found only in the thymus
Rationale: T-regulatory cells (Tregs) are CD4+CD25+FoxP3+ cells that suppress
immune responses and maintain peripheral tolerance. They are critical for
preventing autoimmunity and limiting immune pathology. Defects in Tregs cause
immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX)
syndrome.
Question 10. Which immunoglobulin is transported across the placenta?
Laboratory Immunology Certification Practice
Questions with Answer Explanations Immunology •
Serology • Immunopathology • Laboratory Testing •
Quality Control • Preparation
SECTION 1: BASIC IMMUNOLOGIC PRINCIPLES & MECHANISMS
(Questions 1–18)
Question 1. Which immunoglobulin isotype is the FIRST to be produced in a
primary immune response?
A) IgG
B) IgM
C) IgA
D) IgE
E) IgD
Rationale: IgM is the first antibody isotype produced during a primary immune
response. It is a pentameric molecule with high avidity for antigens, making it
effective at agglutination and complement activation. IgG is the most abundant
isotype in serum and dominates secondary responses. IgA is found in mucosal
secretions, IgE mediates allergic reactions, and IgD is found on the surface of
naive B cells.
Question 2. Which of the following BEST describes the difference between
affinity and avidity?
A) Affinity is the strength of a single antigen-antibody bond; avidity is the overall
strength of multiple bonds
B) Affinity is the strength of a single antigen-antibody bond; avidity is the
cumulative strength of multiple antigen-antibody interactions
C) Affinity is the strength of multiple bonds; avidity is the strength of a single
bond
,D) Affinity and avidity are the same thing
E) Affinity refers to antibody class; avidity refers to antibody subclass
Rationale: Affinity is the strength of binding between a single antigenic
determinant (epitope) and a single antibody combining site. Avidity is the
cumulative strength of multiple antigen-antibody interactions, taking into account
the valency of both the antibody and the antigen. IgM, despite having lower
affinity than IgG, has high avidity due to its pentameric structure.
Question 3. Which complement pathway is activated by antigen-antibody
complexes?
A) Classical pathway
B) Alternative pathway
C) Lectin pathway
D) Coagulation pathway
E) Fibrinolytic pathway
Rationale: The classical complement pathway is activated by antigen-antibody
complexes, specifically by the binding of C1q to the Fc region of IgM or IgG
antibodies bound to antigen. The alternative pathway is activated by microbial
surfaces in the absence of antibody. The lectin pathway is activated by mannose-
binding lectin (MBL) binding to carbohydrates on microbial surfaces.
Question 4. Which of the following is TRUE about the CH50 assay?
A) It measures the alternative complement pathway
B) It measures the classical complement pathway and is used to screen for
complement deficiencies
C) It measures the lectin complement pathway
D) It is a specific test for C3 deficiency
E) It is used to diagnose rheumatoid arthritis
Rationale: The CH50 assay measures total hemolytic complement activity of the
classical pathway. It determines the dilution of patient serum needed to lyse 50%
of antibody-coated erythrocytes. A low or absent CH50 indicates a deficiency in a
classical pathway component (C1, C2, C4, C3, C5–C9) or consumption of
,complement. The AH50 assay measures the alternative pathway. The CH50 is a
screening test, not specific for a single component.
Question 5. A patient has recurrent infections with encapsulated bacteria and
absent CH50 but normal AH50. Which complement component is MOST likely
deficient?
A) C2
B) C3
C) C5
D) C9
E) Factor B
Rationale: If a classical pathway component (C1, C2, or C4) is deficient, the
CH50 is low or absent while the AH50 is normal. C3 deficiency would result in
both CH50 and AH50 being low or absent because C3 is shared by both pathways.
C5 and C9 deficiencies also affect both pathways. Factor B is an alternative
pathway component, so its deficiency would affect AH50 but not CH50.
Question 6. Which MHC class presents exogenous antigens to CD4+ T cells?
A) MHC class I
B) MHC class II
C) MHC class III
D) CD1
E) CD8
Rationale: MHC class II molecules present exogenous antigens (taken up by
phagocytosis or endocytosis) to CD4+ helper T cells. MHC class I molecules
present endogenous antigens (derived from intracellular pathogens or tumor
proteins) to CD8+ cytotoxic T cells. MHC class III encodes complement
components and other immune proteins, not antigen presentation.
Question 7. What is the function of CD40 ligand (CD40L) on activated T cells?
A) It binds to CD40 on B cells to promote class switching and differentiation
B) It binds to CD3 on T cells to promote proliferation
, C) It binds to MHC class I on antigen-presenting cells
D) It binds to complement receptors
E) It binds to Fc receptors
Rationale: CD40L (CD154) on activated CD4+ T cells binds to CD40 on B cells,
providing a critical co-stimulatory signal that promotes B cell proliferation,
immunoglobulin class switching, and differentiation into plasma cells and memory
B cells. Defects in CD40L cause X-linked hyper-IgM syndrome.
Question 8. Which cytokine is the hallmark of a Th2 immune response?
A) IFN-γ
B) IL-4
C) IL-12
D) TNF-α
E) IL-2
Rationale: IL-4 is the hallmark cytokine of Th2 responses, promoting B cell class
switching to IgE and IgG1, and driving differentiation of naive CD4+ T cells
toward the Th2 phenotype. IFN-γ and IL-12 are associated with Th1 responses.
TNF-α is a proinflammatory cytokine. IL-2 promotes T cell proliferation.
Question 9. Which of the following is TRUE about T-regulatory cells (Tregs)?
A) They express CD8 and promote cytotoxic responses
B) They express CD4, CD25, and FoxP3, and suppress immune responses
C) They express CD3 and promote B cell activation
D) They are the primary source of IL-2
E) They are found only in the thymus
Rationale: T-regulatory cells (Tregs) are CD4+CD25+FoxP3+ cells that suppress
immune responses and maintain peripheral tolerance. They are critical for
preventing autoimmunity and limiting immune pathology. Defects in Tregs cause
immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX)
syndrome.
Question 10. Which immunoglobulin is transported across the placenta?
