2026 DMLI Exam Prep: Medical Laboratory
Immunology Practice Questions, Answer
Rationales, Immunology, Serology, Autoimmune
Testing, Immunopathology & Laboratory
Quality Study Guide
SECTION 1: BASIC IMMUNOLOGIC PRINCIPLES AND MECHANISMS
(Questions 1–25)
1. A 28-year-old female presents with recurrent sinopulmonary infections.
Laboratory testing reveals absent serum IgA, normal IgG and IgM, and normal T-
cell function. Which of the following is the most likely diagnosis?
A) Common variable immunodeficiency
B) Selective IgA deficiency
C) X-linked agammaglobulinemia
D) Hyper-IgM syndrome
Rationale: Selective IgA deficiency is the most common primary
immunodeficiency, defined by serum IgA <7 mg/dL with normal IgG and IgM
levels and normal T-cell function. Many patients are asymptomatic, but some
experience recurrent mucosal infections. Common variable immunodeficiency
(CVID) presents with low IgG plus low IgA and/or IgM. X-linked
agammaglobulinemia (Bruton's) presents with absent B cells and very low all
immunoglobulin isotypes. Hyper-IgM syndrome presents with low IgG, IgA, and
IgE but elevated or normal IgM. The Basic Immunologic Principles domain
comprises 15% of the DMLI exam.
2. A 3-month-old male presents with failure to thrive, chronic diarrhea, and oral
thrush. Laboratory testing reveals absent thymic shadow on chest X-ray,
lymphopenia, and very low T-cell counts with normal B-cell counts. Which of the
following is the most likely diagnosis?
,A) Severe combined immunodeficiency (SCID)
B) DiGeorge syndrome
C) Wiskott-Aldrich syndrome
D) Chronic granulomatous disease
Rationale: SCID presents in early infancy with failure to thrive, chronic
infections, and lymphopenia. The absence of a thymic shadow and very low T-cell
counts with normal B cells suggest a T-cell-negative SCID variant (e.g., IL-2
receptor gamma chain deficiency, JAK3 deficiency). DiGeorge syndrome (22q11.2
deletion) presents with thymic hypoplasia, cardiac defects, and hypocalcemia.
Wiskott-Aldrich syndrome presents with eczema, thrombocytopenia, and
immunodeficiency (X-linked). Chronic granulomatous disease is a phagocyte
defect with recurrent catalase-positive infections.
3. Which complement pathway is initiated by C1q binding to antigen-antibody
complexes containing IgG or IgM?
A) Alternative pathway
B) Classical pathway
C) Lectin pathway
D) Terminal pathway
Rationale: The classical pathway is initiated when C1q binds to the Fc region of
antigen-bound IgG (subclasses 1, 3) or IgM, activating C1r and C1s, which cleave
C4 and C2 to form the C3 convertase C4b2a. The alternative pathway is initiated
by spontaneous hydrolysis of C3 and is amplified on microbial surfaces. The lectin
pathway is initiated by mannose-binding lectin (MBL) binding to carbohydrates on
microbial surfaces. The terminal pathway is the common final sequence (C5b-9
membrane attack complex) shared by all three activation pathways.
4. A patient with recurrent angioedema has decreased C4 between attacks and very
low C1 esterase inhibitor (C1-INH) functional activity. Which condition is most
likely?
A) Acquired angioedema type II
B) Hereditary angioedema type I
,C) Hereditary angioedema type II
D) ACE inhibitor-induced angioedema
Rationale: Hereditary angioedema (HAE) type I is the most common form
(~85%), characterized by a quantitative deficiency of C1-INH leading to low
antigenic and functional levels, persistently low C4 (even between attacks), and
recurrent non-pruritic angioedema. HAE type II has normal C1-INH antigenic
levels but low functional activity. Acquired angioedema is associated with
lymphoproliferative disorders or autoantibodies to C1-INH. ACE inhibitor-induced
angioedema is mediated by bradykinin accumulation but C1-INH levels are
normal.
5. Which immunoglobulin is the first to appear in a primary immune response?
A) IgG
B) IgM
C) IgA
D) IgE
Rationale: IgM is the first immunoglobulin isotype produced in a primary immune
response. It is a pentamer with high avidity for antigens and is an efficient activator
of the classical complement pathway. IgG appears later in the primary response
and dominates the secondary response. IgA is found in mucosal secretions and
serum. IgE mediates allergic reactions and defense against parasites. The Basic
Immunologic Principles domain covers antibody isotypes and subclasses.
6. A laboratory is evaluating a patient with suspected complement deficiency.
CH50 is undetectable, but AP50 is normal. Which complement component is most
likely deficient?
A) C3
B) C2
C) Factor B
D) C5
Rationale: CH50 measures the classical pathway (C1, C4, C2, C3, C5-9). An
undetectable CH50 with a normal AP50 indicates a deficiency in a classical
pathway-specific component (C1, C4, or C2). C3 deficiency would affect both
, CH50 and AP50 (since C3 is common to all pathways). Factor B is specific to the
alternative pathway and would cause low AP50. C5 deficiency would affect both
pathways.
7. Which cytokine signature defines a Th17 helper T-cell response?
A) IFN-gamma and IL-2
B) IL-4, IL-5, and IL-13
C) IL-17 and IL-22, driven by IL-23
D) TGF-beta and IL-10
Rationale: Th17 cells produce IL-17A/F and IL-22 and require IL-23 for stable
maintenance. They drive neutrophil recruitment, mucosal defense against
extracellular bacteria and fungi, and contribute to autoimmune diseases such as
psoriasis and ankylosing spondylitis. Th1 cells produce IFN-gamma and IL-2. Th2
cells produce IL-4, IL-5, and IL-13. Treg cells produce TGF-beta and IL-10.
8. MHC class I antigen presentation primarily processes peptides derived from
which source?
A) Extracellular bacteria phagocytosed into endosomes
B) Endogenous cytosolic proteins, including viral proteins
C) Exogenous proteins after lysosomal degradation
D) Lipid antigens loaded onto CD1 molecules
Rationale: MHC class I presents peptides (8–10 amino acids) derived from
cytosolic proteins (including viral and tumor antigens) generated by the
proteasome. These peptides are translocated into the ER by TAP1/TAP2 and
loaded onto class I for CD8+ T-cell recognition. MHC class II presents exogenous
peptides (13–25 amino acids) derived from lysosomal degradation to CD4+ T
cells. Lipid antigens are presented by CD1 molecules.
9. In MHC class II antigen processing, which molecule facilitates the exchange of
CLIP for an antigenic peptide in the MIIC compartment?
A) TAP1/TAP2
B) Tapasin
Immunology Practice Questions, Answer
Rationales, Immunology, Serology, Autoimmune
Testing, Immunopathology & Laboratory
Quality Study Guide
SECTION 1: BASIC IMMUNOLOGIC PRINCIPLES AND MECHANISMS
(Questions 1–25)
1. A 28-year-old female presents with recurrent sinopulmonary infections.
Laboratory testing reveals absent serum IgA, normal IgG and IgM, and normal T-
cell function. Which of the following is the most likely diagnosis?
A) Common variable immunodeficiency
B) Selective IgA deficiency
C) X-linked agammaglobulinemia
D) Hyper-IgM syndrome
Rationale: Selective IgA deficiency is the most common primary
immunodeficiency, defined by serum IgA <7 mg/dL with normal IgG and IgM
levels and normal T-cell function. Many patients are asymptomatic, but some
experience recurrent mucosal infections. Common variable immunodeficiency
(CVID) presents with low IgG plus low IgA and/or IgM. X-linked
agammaglobulinemia (Bruton's) presents with absent B cells and very low all
immunoglobulin isotypes. Hyper-IgM syndrome presents with low IgG, IgA, and
IgE but elevated or normal IgM. The Basic Immunologic Principles domain
comprises 15% of the DMLI exam.
2. A 3-month-old male presents with failure to thrive, chronic diarrhea, and oral
thrush. Laboratory testing reveals absent thymic shadow on chest X-ray,
lymphopenia, and very low T-cell counts with normal B-cell counts. Which of the
following is the most likely diagnosis?
,A) Severe combined immunodeficiency (SCID)
B) DiGeorge syndrome
C) Wiskott-Aldrich syndrome
D) Chronic granulomatous disease
Rationale: SCID presents in early infancy with failure to thrive, chronic
infections, and lymphopenia. The absence of a thymic shadow and very low T-cell
counts with normal B cells suggest a T-cell-negative SCID variant (e.g., IL-2
receptor gamma chain deficiency, JAK3 deficiency). DiGeorge syndrome (22q11.2
deletion) presents with thymic hypoplasia, cardiac defects, and hypocalcemia.
Wiskott-Aldrich syndrome presents with eczema, thrombocytopenia, and
immunodeficiency (X-linked). Chronic granulomatous disease is a phagocyte
defect with recurrent catalase-positive infections.
3. Which complement pathway is initiated by C1q binding to antigen-antibody
complexes containing IgG or IgM?
A) Alternative pathway
B) Classical pathway
C) Lectin pathway
D) Terminal pathway
Rationale: The classical pathway is initiated when C1q binds to the Fc region of
antigen-bound IgG (subclasses 1, 3) or IgM, activating C1r and C1s, which cleave
C4 and C2 to form the C3 convertase C4b2a. The alternative pathway is initiated
by spontaneous hydrolysis of C3 and is amplified on microbial surfaces. The lectin
pathway is initiated by mannose-binding lectin (MBL) binding to carbohydrates on
microbial surfaces. The terminal pathway is the common final sequence (C5b-9
membrane attack complex) shared by all three activation pathways.
4. A patient with recurrent angioedema has decreased C4 between attacks and very
low C1 esterase inhibitor (C1-INH) functional activity. Which condition is most
likely?
A) Acquired angioedema type II
B) Hereditary angioedema type I
,C) Hereditary angioedema type II
D) ACE inhibitor-induced angioedema
Rationale: Hereditary angioedema (HAE) type I is the most common form
(~85%), characterized by a quantitative deficiency of C1-INH leading to low
antigenic and functional levels, persistently low C4 (even between attacks), and
recurrent non-pruritic angioedema. HAE type II has normal C1-INH antigenic
levels but low functional activity. Acquired angioedema is associated with
lymphoproliferative disorders or autoantibodies to C1-INH. ACE inhibitor-induced
angioedema is mediated by bradykinin accumulation but C1-INH levels are
normal.
5. Which immunoglobulin is the first to appear in a primary immune response?
A) IgG
B) IgM
C) IgA
D) IgE
Rationale: IgM is the first immunoglobulin isotype produced in a primary immune
response. It is a pentamer with high avidity for antigens and is an efficient activator
of the classical complement pathway. IgG appears later in the primary response
and dominates the secondary response. IgA is found in mucosal secretions and
serum. IgE mediates allergic reactions and defense against parasites. The Basic
Immunologic Principles domain covers antibody isotypes and subclasses.
6. A laboratory is evaluating a patient with suspected complement deficiency.
CH50 is undetectable, but AP50 is normal. Which complement component is most
likely deficient?
A) C3
B) C2
C) Factor B
D) C5
Rationale: CH50 measures the classical pathway (C1, C4, C2, C3, C5-9). An
undetectable CH50 with a normal AP50 indicates a deficiency in a classical
pathway-specific component (C1, C4, or C2). C3 deficiency would affect both
, CH50 and AP50 (since C3 is common to all pathways). Factor B is specific to the
alternative pathway and would cause low AP50. C5 deficiency would affect both
pathways.
7. Which cytokine signature defines a Th17 helper T-cell response?
A) IFN-gamma and IL-2
B) IL-4, IL-5, and IL-13
C) IL-17 and IL-22, driven by IL-23
D) TGF-beta and IL-10
Rationale: Th17 cells produce IL-17A/F and IL-22 and require IL-23 for stable
maintenance. They drive neutrophil recruitment, mucosal defense against
extracellular bacteria and fungi, and contribute to autoimmune diseases such as
psoriasis and ankylosing spondylitis. Th1 cells produce IFN-gamma and IL-2. Th2
cells produce IL-4, IL-5, and IL-13. Treg cells produce TGF-beta and IL-10.
8. MHC class I antigen presentation primarily processes peptides derived from
which source?
A) Extracellular bacteria phagocytosed into endosomes
B) Endogenous cytosolic proteins, including viral proteins
C) Exogenous proteins after lysosomal degradation
D) Lipid antigens loaded onto CD1 molecules
Rationale: MHC class I presents peptides (8–10 amino acids) derived from
cytosolic proteins (including viral and tumor antigens) generated by the
proteasome. These peptides are translocated into the ER by TAP1/TAP2 and
loaded onto class I for CD8+ T-cell recognition. MHC class II presents exogenous
peptides (13–25 amino acids) derived from lysosomal degradation to CD4+ T
cells. Lipid antigens are presented by CD1 molecules.
9. In MHC class II antigen processing, which molecule facilitates the exchange of
CLIP for an antigenic peptide in the MIIC compartment?
A) TAP1/TAP2
B) Tapasin
