Immunology Practice Questions

Q: Which of the following immunoglobulins shows homocytotropism?

IgE. ### Explanation **Correct Answer: C. IgE** **Concept of Homocytotropism:** Homocytotropism refers to the unique property of an antibody to bind specifically to the surface of cells (primarily mast cells and basophils) of the same species. This binding occurs via the **Fc region** of the antibody to high-affinity **FcεRI receptors** on the target cells. **IgE** is the classic homocytotropic antibody. When an allergen cross-links these cell-bound IgE molecules, it triggers degranulation and the release of pharmacological mediators like histamine, leading to Type I Hypersensitivity reactions. **Analysis of Incorrect Options:** * **IgG (Option A):** While IgG is the most abundant antibody and can cross the placenta, it is generally not homocytotropic. Some subclasses (like IgG4) may show weak binding, but IgE is the definitive answer for this property. * **IgA (Option B):** Known as the "secretory antibody," its primary role is mucosal immunity. It exists as a dimer in secretions and does not bind to mast cells to mediate anaphylaxis. * **IgD (Option D):** Primarily acts as a B-cell surface receptor. Its precise systemic function is less defined, and it does not exhibit homocytotropic properties. **High-Yield NEET-PG Pearls:** * **Prausnitz-Küstner (PK) Reaction:** A classic historical test used to demonstrate homocytotropism by injecting serum from an allergic individual into the skin of a non-allergic person. * **Heat Lability:** IgE is the most heat-labile immunoglobulin (inactivated at 56°C for 30–60 minutes). * **Reaginic Antibody:** IgE is also known as the reagin antibody. * **Parasitic Infections:** IgE levels are characteristically elevated in helminthic infections and Type I hypersensitivity (Atopy).

Q: Which of the following streptococcal antigens cross-reacts with synovial fluid?

Capsular hyaluronic acid. ### Explanation The correct answer is **C. Capsular hyaluronic acid**. **Mechanism of Cross-reactivity:** *Streptococcus pyogenes* (Group A Streptococcus) possesses a capsule composed of **hyaluronic acid**. This polysaccharide is chemically identical to the hyaluronic acid found in human connective tissues, particularly in **synovial fluid** and joint cartilage. Because the body recognizes hyaluronic acid as "self," the capsule is non-immunogenic (it does not elicit an immune response), allowing the bacteria to evade phagocytosis. However, in the context of post-streptococcal sequelae like Acute Rheumatic Fever, this molecular mimicry is a key factor in tissue targeting. **Analysis of Incorrect Options:** * **A. Carbohydrate (Group A):** This is the C-substance used for Lancefield grouping. It cross-reacts specifically with the **cardiac valves** (specifically bovine heart valves and human glycoprotein). * **B. Cell wall protein (M-protein):** This is the major virulence factor. It cross-reacts with **cardiac myosin** and sarcolemmal membrane proteins, leading to rheumatic carditis. * **D. Peptidoglycan:** While it provides structural integrity and has some pyrogenic properties, it is not the primary antigen associated with synovial cross-reactivity. **NEET-PG High-Yield Pearls:** * **Molecular Mimicry Table:** * **Capsule:** Synovial fluid/Joints. * **M-Protein:** Myocardium (Myosin). * **Group A Carbohydrate:** Cardiac valves. * **Cytoplasmic Membrane:** Subthalamic/Caudate nuclei (linked to Sydenham’s Cholera). * **Protoplast membrane** is also a significant cross-reactive antigen with the sarcolemma of the heart. * **Key Concept:** The hyaluronic acid capsule is the reason why *S. pyogenes* is often described as "a wolf in sheep's clothing."

Q: What is the significance of mutation of CCR5 protein?

High resistance to HIV infection. ### Explanation **Core Concept:** HIV-1 primarily enters host CD4+ T-cells by binding to the CD4 receptor and a specific co-receptor. The **CCR5 protein** is a chemokine receptor that serves as the essential co-receptor for **M-tropic (Macrophage-tropic)** strains of HIV, which are responsible for the initial infection and transmission. A specific mutation known as the **CCR5-Δ32 (delta 32) mutation** results in a truncated, non-functional protein that is not expressed on the cell surface. Individuals who are **homozygous** for this mutation lack the "doorway" for the virus to enter, conferring **high resistance to HIV infection**. Heterozygotes typically show a slower progression to AIDS. **Analysis of Options:** * **Option A (Correct):** Without a functional CCR5 co-receptor, the virus cannot fuse with the host cell membrane, preventing infection. * **Option B:** A mutation (specifically Δ32) leads to resistance, not susceptibility, because the virus loses its binding site. * **Option C:** Resistance to ART is usually due to mutations in the viral genome (e.g., Pol gene mutations affecting Reverse Transcriptase or Protease), not host receptor mutations. * **Option D:** Virulence is a viral property; host receptor mutations affect the host's "permissiveness" to infection rather than the inherent pathogenicity of the virus. **High-Yield Clinical Pearls for NEET-PG:** * **CXCR4:** This is the co-receptor for **T-tropic** strains, which usually appear later in the course of the disease (associated with rapid CD4 decline). * **Maraviroc:** A drug that acts as a **CCR5 antagonist**, mimicking the effect of the mutation to prevent viral entry. * **Berlin Patient:** The first person cured of HIV received a bone marrow transplant from a donor homozygous for the CCR5-Δ32 mutation. * **GP120:** The viral envelope protein that initially binds to CD4 and then undergoes a conformational change to bind to CCR5/CXCR4.

Q: Which statement is true about the relative concentrations of immunoglobulins in serum?

IgE has the minimum concentration.. In human serum, the concentration of immunoglobulins follows a specific hierarchy based on their physiological roles and half-lives. The correct order from highest to lowest concentration is **IgG > IgA > IgM > IgD > IgE**. ### Why the Correct Answer is Right **IgE** is the least abundant immunoglobulin in the serum of healthy individuals, with a concentration of approximately **0.00005 mg/mL** (or <0.001% of total serum Ig). Its low concentration is due to its short half-life (approx. 2 days) and the fact that most IgE is sequestered on the surface of mast cells and basophils via high-affinity FcεRI receptors, rather than circulating freely. ### Why the Other Options are Wrong * **IgG (Option B):** This is the **most abundant** immunoglobulin (75–80% of total serum Ig), providing long-term immunity and the only class that crosses the placenta. * **IgA (Option C):** This is the **second most abundant** serum immunoglobulin (10–15%). It is the primary antibody found in secretions (colostrum, saliva, tears). * **IgM (Option D):** This is the **third most abundant** (5–10%). It is a pentamer and the first antibody produced during a primary immune response. ### NEET-PG High-Yield Pearls * **Mnemonic for Concentration:** **GAMDE** (IgG > IgA > IgM > IgD > IgE). * **IgG:** Longest half-life (23 days); responsible for secondary immune response. * **IgM:** Highest molecular weight (Millionaire antibody); best for complement fixation. * **IgA:** Most produced antibody *per day* in the body (due to mucosal surface area), but serum levels are second to IgG. * **IgE:** Levels significantly increase in **Type I Hypersensitivity** (allergies) and **helminthic (parasitic) infections**.

Q: Which of the immunoglobulins is primarily found in Peyer's patches?

IgA. **Explanation:** The correct answer is **IgA**. **Underlying Concept:** Peyer’s patches are organized lymphoid tissues located in the lamina propria of the small intestine, forming a critical part of the **Mucosa-Associated Lymphoid Tissue (MALT)**. Their primary function is to monitor intestinal bacteria and prevent the growth of pathogenic microorganisms. **IgA** (specifically secretory IgA) is the predominant antibody class produced at mucosal surfaces. Within Peyer’s patches, B-cells undergo class-switching to become IgA-producing plasma cells, which then secrete dimeric IgA to neutralize pathogens before they can penetrate the intestinal epithelium. **Analysis of Incorrect Options:** * **IgM:** While IgM is the first antibody produced in a primary immune response and can be secreted across mucosa, it is not the primary resident of Peyer’s patches. * **IgG:** This is the most abundant immunoglobulin in the **serum** and provides systemic immunity. It reaches mucosal surfaces primarily via passive diffusion rather than local production in MALT. * **IgD:** This is primarily found on the surface of B-lymphocytes as an antigen receptor; it has no significant role in mucosal defense. **Clinical Pearls for NEET-PG:** * **Secretory Component:** IgA acquires a "secretory component" (derived from the poly-Ig receptor) which protects it from digestion by intestinal enzymes. * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients often present with recurrent sinopulmonary and GI infections (e.g., *Giardia lamblia*). * **M-Cells:** Specialized cells in the epithelium overlying Peyer’s patches that transport antigens from the gut lumen to the underlying lymphoid tissue.

Q: Amniocentesis conducted during genetic counseling of a pregnant woman reveals a fetal adenosine deaminase deficiency. This autosomal recessive immunodeficiency is usually associated with which of the following?

Humoral Immunity - Deficient; Cellular Immunity - Deficient. ### Explanation **Adenosine Deaminase (ADA) deficiency** is the second most common cause of **Severe Combined Immunodeficiency (SCID)**, accounting for approximately 15% of cases. It is inherited as an autosomal recessive trait. #### Why Option D is Correct: The enzyme adenosine deaminase is essential for the purine salvage pathway. Its deficiency leads to the accumulation of toxic metabolites, specifically **deoxyadenosine** and **dATP**, within lymphocytes. These metabolites are lymphotoxic; they inhibit DNA synthesis and induce apoptosis. Because this metabolic defect affects the common lymphoid progenitor cell line, it results in the failure of both **T-cell and B-cell maturation**. Consequently, patients exhibit a profound lack of both **Cellular Immunity** (T-cells) and **Humoral Immunity** (B-cells/Antibodies). #### Why Other Options are Incorrect: * **Option A:** This describes an immunocompetent state. ADA deficiency is a primary immunodeficiency. * **Option B:** This pattern (Deficient Cellular, Normal Humoral) is characteristic of **DiGeorge Syndrome** (thymic hypoplasia), where T-cells are low but B-cells are initially preserved. * **Option C:** This pattern (Deficient Humoral, Normal Cellular) is characteristic of **X-linked Agammaglobulinemia (Bruton’s)**, where B-cell maturation is blocked but T-cell function remains intact. #### NEET-PG High-Yield Pearls: * **SCID Presentation:** Recurrent "failure to thrive," chronic diarrhea, and severe opportunistic infections (e.g., *Pneumocystis jirovecii*, *Candida*) in early infancy. * **Imaging:** Characteristically shows an **absent thymic shadow** on chest X-ray. * **Historical Significance:** ADA deficiency was the first disease treated with **Gene Therapy**. * **Treatment:** Hematopoietic stem cell transplant (HSCT) is the treatment of choice; Enzyme Replacement Therapy (PEG-ADA) is a temporizing measure.

Q: Which marker is lineage-specific for B cells?

CD 19. **Explanation:** The correct answer is **CD 19**. **Why CD 19 is the correct answer:** CD 19 is considered the most reliable **lineage-specific marker** for B cells because it is expressed from the earliest stages of B-cell commitment (pro-B cell) through all stages of maturation, until it is finally downregulated during terminal differentiation into plasma cells. Because it appears before other markers and is present on almost all B-lineage cells, it is the gold standard for identifying B cells in flow cytometry. **Analysis of Incorrect Options:** * **CD 20:** While highly specific to B cells, it is not expressed on the earliest pro-B cells. It is primarily found on mature B cells and is lost during the transition to plasma cells. (Clinical Note: It is the target for **Rituximab**). * **CD 21:** Also known as Complement Receptor 2 (CR2), it is the receptor for the **Epstein-Barr Virus (EBV)**. It is expressed only on mature B cells and follicular dendritic cells, making it less "lineage-encompassing" than CD 19. * **CD 22:** This is a regulatory molecule appearing later in B-cell maturation (mature B cells) and is not present on the earliest precursors. **High-Yield NEET-PG Pearls:** * **Pan B-cell markers:** CD 19, CD 20, and CD 22. * **Plasma Cell Markers:** CD 138 (Syndecan-1) and CD 38. Note that plasma cells typically **lose** CD 19 and CD 20. * **T-cell Lineage Marker:** CD 3 is the definitive lineage-specific marker for T cells. * **NK Cell Markers:** CD 16 and CD 56. * **Hairy Cell Leukemia:** Characterized by CD 11c, CD 25, and CD 103.

Question 1

Which of the following immunoglobulins shows homocytotropism?

Accepted Answer

IgE

Answer

IgG

Answer

IgA

Answer

IgD

Question 2

A latent, measles-like viral infection and, presumably, a defect in cellular immunity is associated with which of the following diseases?

Accepted Answer

Subacute sclerosing panencephalitis (SSPE)

Answer

Progressive multifocal leukoencephalopathy (PML)

Answer

Multiple sclerosis (MS)

Answer

Creutzfeldt-Jakob disease

Question 3

Which of the following streptococcal antigens cross-reacts with synovial fluid?

Accepted Answer

Capsular hyaluronic acid

Answer

Carbohydrate (group A)

Answer

Cell wall protein

Answer

Peptidoglycan

Question 4

All of the following are TRUE about ASO titre, EXCEPT?

Accepted Answer

ASO titre is included in the major criteria of the Jones criteria.

Answer

ASO titre can be increased in school children.

Answer

ASO titre may be negative in post-streptococcal glomerulonephritis.

Answer

ASO titre may not be elevated in 20% of cases of carditis.

Question 5

What is the significance of mutation of CCR5 protein?

Accepted Answer

High resistance to HIV infection

Answer

High susceptibility to HIV infection

Answer

Causes resistance to anti-retroviral therapy

Answer

Causes increased virulence of the virus

Question 6

Which statement is true about the relative concentrations of immunoglobulins in serum?

Accepted Answer

IgE has the minimum concentration.

Answer

IgG has the minimum concentration.

Answer

IgA has the minimum concentration.

Answer

IgM has the minimum concentration.

Question 7

Passive cutaneous anaphylaxis is detected by which of the following?

Accepted Answer

Heterocytotropic antibody

Answer

Wheal and flare reaction

Answer

Atopy

Answer

Cutaneous anaphylaxis

Question 8

Which of the immunoglobulins is primarily found in Peyer's patches?

Accepted Answer

IgA

Answer

IgM

Answer

IgG

Answer

IgD

Question 9

Amniocentesis conducted during genetic counseling of a pregnant woman reveals a fetal adenosine deaminase deficiency. This autosomal recessive immunodeficiency is usually associated with which of the following?

Accepted Answer

Humoral Immunity - Deficient; Cellular Immunity - Deficient

Answer

Humoral Immunity - Normal; Cellular Immunity - Normal

Answer

Humoral Immunity - Normal; Cellular Immunity - Deficient

Answer

Humoral Immunity - Deficient; Cellular Immunity - Normal

Question 10

Which marker is lineage-specific for B cells?

Accepted Answer

CD 19

Answer

CD 20

Answer

CD 21

Answer

CD 22

Immunology — MCQs

Immunology — MCQs

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