Immunology Practice Questions

Q: A 27-year-old female presents with fever (103°F), severe fatigue, weight loss, and joint pain. She reports recently stopping aspirin and corticosteroids. Physical examination reveals a malar rash, photosensitivity, and a heart murmur. She has a history of progressive arthritis and glomerulonephritis. Laboratory tests show anemia, leukopenia, and thrombocytopenia. What is the most definitive diagnostic finding for this patient's condition?

Anti-dsDNA antibodies. ### Explanation **Diagnosis: Systemic Lupus Erythematosus (SLE)** The patient presents with a classic multisystem involvement of SLE, characterized by the **"DOPAMINE RASH"** mnemonic (Discoid rash, Oral ulcers, Photosensitivity, Arthritis, Malar rash, Immune markers, Neurologic, ESR/Hematologic, Renal, ANA, Serositis). The presence of a malar rash, glomerulonephritis, and cytopenias (anemia, leukopenia, thrombocytopenia) strongly points to SLE. **Why Anti-dsDNA is the Correct Answer:** While **Antinuclear Antibody (ANA)** is the most sensitive screening test for SLE, **Anti-dsDNA antibodies** are highly specific (approx. 95-100%) and are considered a definitive diagnostic marker. Crucially for NEET-PG, Anti-dsDNA levels correlate with **disease activity**, particularly the development of **lupus nephritis**. **Analysis of Incorrect Options:** * **A. Anticentromere antibodies:** Highly specific for **Limited Cutaneous Systemic Sclerosis (CREST syndrome)**. * **C. Antimitochondrial antibodies (AMA):** The hallmark diagnostic marker for **Primary Biliary Cholangitis (PBC)**. * **D. Antineutrophil antibodies (ANCA):** Associated with small-vessel vasculitides (e.g., c-ANCA for Granulomatosis with Polyangiitis; p-ANCA for Microscopic Polyangiitis). **High-Yield Clinical Pearls for NEET-PG:** * **Most Sensitive Test for SLE:** ANA (Indirect Immunofluorescence is the gold standard). * **Most Specific Test for SLE:** Anti-Smith (Anti-Sm) antibodies (though they do not correlate with disease activity). * **Drug-Induced Lupus:** Associated with **Anti-Histone antibodies** (Common drugs: Hydralazine, Procainamide, Isoniazid). * **Neonatal Lupus/Congenital Heart Block:** Associated with **Anti-Ro (SS-A)** and **Anti-La (SS-B)** antibodies. * **Complement Levels:** In active SLE (especially renal involvement), **C3 and C4 levels are decreased** due to consumption.

Q: Which cytokine enhances the activity of NK cells?

Interleukin-2 (IL-2). ### Explanation **Correct Answer: C. Interleukin-2 (IL-2)** Natural Killer (NK) cells are large granular lymphocytes that play a critical role in the innate immune response against virally infected and tumor cells. Their activity is primarily regulated by cytokines. **Interleukin-2 (IL-2)**, produced by Th1 cells, acts as a potent stimulator of NK cell proliferation and activation. When exposed to high concentrations of IL-2, NK cells differentiate into **Lymphokine-Activated Killer (LAK) cells**, which exhibit enhanced cytolytic activity against a broader range of tumor targets. Other key cytokines that enhance NK cell activity include **IL-12, IL-15, and Type I Interferons (IFN-α and IFN-β).** **Why the other options are incorrect:** * **IL-1:** Primarily a pro-inflammatory cytokine produced by macrophages; it mediates the acute phase response and induces fever but does not directly activate NK cells. * **TNF:** A major mediator of systemic inflammation and apoptosis; while it can be secreted *by* activated NK cells, it is not the primary driver of their activation. * **TGF-β:** This is a potent **immunosuppressive** cytokine. It inhibits the proliferation and effector functions of NK cells and T cells, acting as a "brake" on the immune system. **High-Yield Clinical Pearls for NEET-PG:** * **LAK Cells:** IL-2-stimulated NK cells used in experimental immunotherapy for renal cell carcinoma and melanoma. * **NK Cell Markers:** CD16 (FcγRIII) and CD56 are the characteristic surface markers. * **Mechanism of Killing:** NK cells use **Perforins** (create pores) and **Granzymes** (induce apoptosis). * **MHC-I Rule:** NK cells kill cells that lack **MHC Class I** expression ("Missing Self" hypothesis).

Q: Deficiency of complement proteins C5 to C8 leads to increased infection by which of the following?

Neisseria. **Explanation:** The complement system is a vital component of innate immunity. Complement proteins **C5, C6, C7, C8, and C9** assemble to form the **Membrane Attack Complex (MAC)**. The primary function of the MAC is to create pores in the lipid bilayer of bacterial cell walls, leading to osmotic lysis and death of the pathogen. **Why Neisseria is the correct answer:** *Neisseria* species (*N. meningitidis* and *N. gonorrhoeae*) are Gram-negative bacteria with thin cell walls, making them uniquely susceptible to direct lysis by the MAC. While other bacteria can be cleared via opsonization (C3b), *Neisseria* relies heavily on the terminal complement pathway for elimination. Patients with deficiencies in C5–C9 have a **7,000 to 10,000-fold increased risk** of systemic Neisserial infections, particularly recurrent meningococcal meningitis. **Why other options are incorrect:** * **A & D (Streptococcus and Staphylococcus):** These are Gram-positive organisms with thick peptidoglycan layers that protect them from MAC-mediated lysis. They are primarily cleared through **opsonization** (C3b) and subsequent phagocytosis. Deficiencies in early complement components (C1, C2, C4) or C3 would predispose patients to these infections. * **C (Pseudomonas):** While Gram-negative, *Pseudomonas* is typically cleared through robust neutrophilic action and opsonization rather than a specific reliance on the terminal MAC assembly. **NEET-PG High-Yield Pearls:** * **C1, C2, C4 deficiency:** Associated with Immune Complex diseases like **Systemic Lupus Erythematosus (SLE)**. * **C3 deficiency:** The most severe; leads to recurrent infections with **encapsulated bacteria** (e.g., *S. pneumoniae, H. influenzae*). * **C1 Esterase Inhibitor deficiency:** Causes **Hereditary Angioedema** (characterized by low C4 levels). * **DAF (CD55) / MIRL (CD59) deficiency:** Leads to **Paroxysmal Nocturnal Hemoglobinuria (PNH)**.

Q: Which subtype of IgG activates the alternate complement system?

IgG4. ### Explanation The correct answer is **IgG4**. **1. Why IgG4 is Correct:** In the complement system, the **Classical Pathway** is typically activated by antigen-antibody complexes involving **IgG1, IgG2, IgG3, and IgM**. However, **IgG4** is unique among the IgG subclasses because it cannot activate the classical pathway (due to its inability to bind C1q). Instead, IgG4 is the only IgG subtype capable of activating the **Alternative Complement Pathway**, albeit weakly. This is a high-yield distinction for competitive exams. **2. Why Other Options are Incorrect:** * **IgG1 & IgG3:** These are the most potent activators of the **Classical Pathway**. IgG3 is the most effective, followed closely by IgG1. * **IgG2:** This subtype is a weak activator of the Classical Pathway, primarily responding to polysaccharide antigens (e.g., encapsulated bacteria like *S. pneumoniae*). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Potency Order (Classical Pathway):** IgM > IgG3 > IgG1 > IgG2. (Mnemonic: **M**any **3** **1** **2**). * **Placental Transfer:** All IgG subclasses cross the placenta, but **IgG2** crosses the least efficiently. * **IgG4 Characteristics:** It is known for "Fab-arm exchange," making it functionally monovalent and often anti-inflammatory. It is also associated with **IgG4-related diseases (IgG4-RD)**, characterized by tissue fibrosis and tumefactive lesions. * **Alternative Pathway Activators:** Apart from IgG4, this pathway is triggered by IgA, Endotoxins (LPS), Cobra Venom Factor, and Nephritic Factor.

Q: Which cell surface markers are expressed by both TH1 cells and macrophages?

MHC Class I. **Explanation:** The correct answer is **MHC Class I**. **1. Why MHC Class I is correct:** Major Histocompatibility Complex (MHC) Class I molecules are expressed on **all nucleated cells** in the human body. Since both TH1 cells (a subset of T-lymphocytes) and macrophages are nucleated cells, they both express MHC Class I. These molecules are essential for presenting endogenous antigens to CD8+ Cytotoxic T-cells, allowing the immune system to monitor cellular health. **2. Why the other options are incorrect:** * **Immunoglobulin (Option A):** These are B-cell receptors (BCR) or antibodies. They are specifically expressed on the surface of **B-lymphocytes** and secreted by plasma cells. * **CD3 (Option B):** This is a definitive marker for **T-lymphocytes**. While present on TH1 cells, it is absent on macrophages. * **TCR (T-Cell Receptor) (Option C):** Similar to CD3, the TCR is unique to **T-lymphocytes**. It is used for antigen recognition in association with MHC molecules but is not found on macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Distribution:** MHC I is on all nucleated cells; MHC II is only on **Professional Antigen Presenting Cells (APCs)** like Macrophages, B-cells, and Dendritic cells. * **Rule of 8:** MHC I interacts with CD8 cells (1 × 8 = 8); MHC II interacts with CD4 cells (2 × 4 = 8). * **TH1 vs. Macrophage Interaction:** TH1 cells secrete **IFN-γ**, which is the most potent activator of macrophages, enhancing their microbicidal activity. * **Non-nucleated exception:** Mature Red Blood Cells (RBCs) lack MHC Class I, which is why they cannot be infected by viruses that require MHC for entry or presentation.

Q: Polysaccharide antigens are:

T cell independent antigens. ### Explanation **1. Why Option A is Correct:** Polysaccharide antigens (such as those found in the capsules of *S. pneumoniae* or *H. influenzae*) are classified as **T-cell Independent (TI) antigens**. Unlike proteins, polysaccharides cannot be processed and presented on MHC molecules to T-cells. Instead, they contain large, repeating identical epitopes that can **cross-link multiple B-cell receptors (BCRs)** simultaneously. This cross-linking provides a signal strong enough to activate B-cells directly to produce antibodies (mainly IgM) without the "help" of T-helper cells. **2. Why the Other Options are Incorrect:** * **Option B (T-cell Dependent):** These are typically **protein antigens**. They require processing by Antigen Presenting Cells (APCs) and presentation to T-helper cells via MHC II. This interaction is necessary for isotype switching (to IgG, IgA, IgE) and the formation of memory B-cells. * **Options C & D (MHC I/II Dependent):** MHC molecules can only bind and present **peptide (protein) fragments**. Since polysaccharides are not proteins, they cannot be loaded onto MHC I or MHC II molecules. Therefore, they do not trigger a classical T-cell mediated immune response. **3. High-Yield Clinical Pearls for NEET-PG:** * **Age Factor:** TI antigens are poorly immunogenic in children under **2 years of age** because their splenic marginal zone (where B-cells respond to TI antigens) is immature. * **Vaccine Strategy:** To make polysaccharide vaccines (like the Pneumococcal vaccine) effective in infants, they are **conjugated** to a protein carrier (e.g., diphtheria toxoid). This converts the TI antigen into a T-cell dependent one, allowing for isotype switching and long-term memory. * **Memory:** Pure polysaccharide antigens do **not** produce immunological memory or secondary (anamnestic) responses.

Q: What role does the lipopolysaccharide of Gram-negative bacteria play in the immune response?

Stimulator for B lymphocytes. **Explanation:** The correct answer is **C. Stimulator for B lymphocytes.** Lipopolysaccharide (LPS), found in the outer membrane of Gram-negative bacteria, acts as a classic **T-cell independent (TI) antigen**. Specifically, it is a **polyclonal B-cell activator** (mitogen). Unlike T-dependent antigens that require helper T-cells to trigger an immune response, LPS can directly cross-link B-cell receptors or bind to **Toll-like Receptor 4 (TLR4)**. This interaction stimulates B-cells to proliferate and secrete antibodies (primarily IgM) without the need for T-cell mediation. **Analysis of Incorrect Options:** * **A. Hapten:** A hapten is a small molecule that is antigenic but not immunogenic on its own; it requires a carrier protein to elicit an immune response. LPS is a large, complex molecule that is highly immunogenic by itself. * **B. Heterophile antibody:** These are antibodies that react with antigens from phylogenetically unrelated species (e.g., Paul-Bunnell test). LPS is an *antigen*, not an antibody. * **D. Inducer of cell-mediated immunity (CMI):** CMI is primarily driven by T-lymphocytes and intracellular pathogens. As a TI-antigen, LPS predominantly triggers a humoral (antibody) response rather than a T-cell-mediated one. **High-Yield NEET-PG Pearls:** * **Endotoxin Structure:** LPS consists of three parts: Lipid A (responsible for **toxicity** and cytokine storm), Core polysaccharide, and O-antigen (responsible for **serological specificity**). * **Cytokine Release:** LPS triggers macrophages to release **IL-1, IL-6, and TNF-α**, which can lead to septic shock and DIC. * **Schwartzman Reaction:** This is a local or systemic reaction to repeated injections of LPS, characterized by hemorrhagic necrosis.

Q: Delayed hypersensitivity in skin tests is assessed by which of the following findings?

Induration. **Explanation:** **Induration** is the hallmark of a positive Type IV (Delayed-type) hypersensitivity reaction. Unlike immediate hypersensitivity (Type I), which is mediated by IgE and histamine, Type IV reactions are **cell-mediated**. 1. **Why Induration is Correct:** When an antigen (e.g., Tuberculin/PPD) is injected intradermally, sensitized **T-lymphocytes** (Th1 cells) migrate to the site. They release cytokines (IFN-γ, IL-2) that recruit and activate **macrophages**. This massive cellular infiltration and the resulting deposition of fibrin in the local tissue cause the area to become firm and raised—a process known as **induration**. This reaction typically peaks at **48–72 hours**. 2. **Why Other Options are Incorrect:** * **Erythema (A):** While redness often accompanies the reaction due to vasodilation, it is non-specific and can occur in Type I reactions or simple irritation. It is **not** used to measure the strength of a delayed hypersensitivity test. * **Bulla (B) and Necrosis (C):** These represent extreme, severe tissue damage. While they can occur in hyper-responsive individuals (e.g., a strongly positive Mantoux test), they are complications rather than the standard diagnostic criteria for assessing the reaction. **NEET-PG High-Yield Pearls:** * **Classic Examples:** Mantoux test (Tuberculosis), Lepromin test (Leprosy), Frei test (Lymphogranuloma Venereum), and Contact Dermatitis. * **Key Cells:** CD4+ T-cells (Th1) and Macrophages. * **Measurement:** In a Mantoux test, only the **transverse diameter of the induration** is measured in millimeters, not the erythema. * **Time Frame:** "Delayed" refers to the 48–72 hour window required for T-cell recruitment and cytokine production.

Question 1

A 27-year-old female presents with fever (103°F), severe fatigue, weight loss, and joint pain. She reports recently stopping aspirin and corticosteroids. Physical examination reveals a malar rash, photosensitivity, and a heart murmur. She has a history of progressive arthritis and glomerulonephritis. Laboratory tests show anemia, leukopenia, and thrombocytopenia. What is the most definitive diagnostic finding for this patient's condition?

Accepted Answer

Anti-dsDNA antibodies

Answer

Anticentromere antibodies

Answer

Antimitochondrial antibodies

Answer

Antineutrophil antibodies

Question 2

Which cytokine enhances the activity of NK cells?

Accepted Answer

Interleukin-2 (IL-2)

Answer

Interleukin-1 (IL-1)

Answer

Tumor Necrosis Factor (TNF)

Answer

Transforming Growth Factor-beta 3 (TGF-β3)

Question 3

Deficiency of complement proteins C5 to C8 leads to increased infection by which of the following?

Accepted Answer

Neisseria

Answer

Streptococcus

Answer

Pseudomonas

Answer

Staphylococcus

Question 4

Which subtype of IgG activates the alternate complement system?

Accepted Answer

IgG4

Answer

IgG1

Answer

IgG2

Answer

IgG3

Question 5

Which cell surface markers are expressed by both TH1 cells and macrophages?

Accepted Answer

MHC Class I

Answer

Immunoglobulin

Answer

CD3

Answer

TCR

Question 6

Polysaccharide antigens are:

Accepted Answer

T cell independent antigens

Answer

T cell dependent antigens

Answer

MHC I dependent antigens

Answer

MHC II dependent antigens

Question 7

The prozone phenomenon is responsible for which of the following?

Accepted Answer

False negative test

Answer

False positive test

Answer

May cause any of the above

Answer

Has no relation with the accuracy of the test

Question 8

What is a possible source of a "second signal" to a B-cell bound by a specific antigen?

Accepted Answer

Antigen-specific T-cells

Answer

Epstein-Barr virus

Answer

Endotoxin

Answer

Plasma cells

Question 9

What role does the lipopolysaccharide of Gram-negative bacteria play in the immune response?

Accepted Answer

Stimulator for B lymphocytes

Answer

Hapten

Answer

Heterophile antibody

Answer

Inducer of cell-mediated immunity

Question 10

Delayed hypersensitivity in skin tests is assessed by which of the following findings?

Accepted Answer

Induration

Answer

Erythema

Answer

Bulla

Answer

Necrosis

Immunology — MCQs

Immunology — MCQs

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