Immunology Practice Questions

Q: B cells are induced to produce IgE by which of the following cytokines?

IL-4. **Explanation:** The correct answer is **IL-4**. This question tests the concept of **Isotype Switching** (Class Switching), where B cells change the production of antibodies from IgM/IgD to other classes (IgG, IgA, or IgE) based on cytokine signals from T-helper cells. **Why IL-4 is correct:** IL-4 is primarily secreted by **Th2 cells**. It acts on B cells to induce the heavy-chain class switching from IgM to **IgE**. This is a critical step in Type I Hypersensitivity reactions and the immune response against helminthic infections. IL-13 often works synergistically with IL-4 to promote this IgE production. **Why the other options are incorrect:** * **IL-2:** Known as the "T-cell growth factor," it primarily stimulates the proliferation and differentiation of T-cells (CD4+ and CD8+). * **IL-1:** Produced by macrophages, it is a pro-inflammatory cytokine that acts as an endogenous pyrogen (induces fever) and activates T-cells. * **IL-6:** A pleiotropic cytokine that stimulates the synthesis of acute-phase reactants in the liver and promotes the final differentiation of B cells into plasma cells. **High-Yield Clinical Pearls for NEET-PG:** * **Th2 Pathway:** IL-4 (IgE switch), IL-5 (Eosinophil activation), and IL-13 (Mucus secretion). * **Th1 Pathway:** IFN-gamma (activates macrophages and induces IgG switching). * **TGF-beta & IL-5:** These are the primary cytokines responsible for switching B cells to produce **IgA** (important for mucosal immunity). * **Atopy:** Patients with atopic dermatitis or asthma often have an overactive Th2 response leading to elevated IL-4 and IgE levels.

Q: Which of the following statements is true regarding Severe Combined Immunodeficiency (SCID)?

Adenosine deaminase deficiency. **Explanation:** **Severe Combined Immunodeficiency (SCID)** is a group of rare, fatal genetic disorders characterized by the profound absence of T-cell and B-cell function. 1. **Why Option A is Correct:** **Adenosine Deaminase (ADA) deficiency** is the second most common cause of SCID (autosomal recessive). ADA is an enzyme essential for purine metabolism. Its deficiency leads to the accumulation of toxic metabolites (deoxyadenosine and dATP) within lymphocytes. These metabolites are lymphotoxic, leading to the destruction of both T and B cells, resulting in "combined" immunodeficiency. 2. **Why the other options are incorrect:** * **Option B:** While SCID involves a functional loss of lymphocytes, the hallmark is the **absence or near-absence** of T-cells. "Decreased circulating lymphocytes" is too non-specific and can occur in many conditions (e.g., HIV, stress, or steroids). * **Option C:** **NADPH oxidase deficiency** is the cause of **Chronic Granulomatous Disease (CGD)**. It affects phagocyte function (neutrophils), not the adaptive immune system (T/B cells). * **Option D:** **C1 esterase inhibitor deficiency** leads to **Hereditary Angioedema**, characterized by recurrent episodes of edema, not immunodeficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Cause:** X-linked SCID (due to a mutation in the **IL-2 receptor gamma chain**). * **Clinical Presentation:** Recurrent severe infections (fungal, viral, bacterial), failure to thrive, and chronic diarrhea in infancy. * **Radiology:** Absence of **thymic shadow** on chest X-ray. * **Treatment:** Hematopoietic stem cell transplant (HSCT) is the treatment of choice. ADA deficiency was the first disease treated with **gene therapy**.

Q: All IgG subclasses are responsible for coagglutination by binding to S. Aureus protein A, except?

IgG3. **Explanation:** The core concept behind this question is the interaction between **Staphylococcal Protein A (SpA)** and the **Fc region** of human Immunoglobulins. Protein A is a surface component of *Staphylococcus aureus* that acts as a virulence factor by binding to the Fc portion of IgG molecules, effectively orienting the antibody "upside down." This prevents opsonization and phagocytosis by immune cells. **Why IgG3 is the correct answer:** While Protein A has a high affinity for most IgG subclasses, it **does not bind to IgG3**. This is due to a structural difference in the CH2 and CH3 domains of the IgG3 heavy chain. Specifically, the presence of an **arginine** residue at position 435 in IgG3 (instead of histidine found in IgG1, 2, and 4) creates steric hindrance that prevents binding to Protein A. Therefore, IgG3 cannot participate in the coagglutination reaction used in diagnostic laboratory tests. **Analysis of incorrect options:** * **IgG1, IgG2, and IgG4:** These subclasses all possess the specific amino acid sequence in their Fc region required for high-affinity binding to Protein A. Consequently, they all facilitate coagglutination. **High-Yield NEET-PG Pearls:** * **Coagglutination Test:** Utilizes *S. aureus* (Cowan 1 strain) rich in Protein A to detect specific antigens. * **Placental Transfer:** All IgG subclasses cross the placenta, but **IgG1** is transferred most efficiently. * **Complement Activation:** **IgG3** is the most potent activator of the classical complement pathway, followed by IgG1 and IgG2. **IgG4** does not activate complement. * **Abundance:** IgG1 is the most abundant subclass in serum; IgG4 is the least.

Q: Which of the following is the most important antigen-presenting cell (APC) for initiating T-cell response against protein antigens?

Dendritic cell. **Explanation:** The correct answer is **B. Dendritic cell**. **Why Dendritic Cells are the most important APCs:** Dendritic cells (DCs) are considered the "professional" and most potent antigen-presenting cells for initiating **primary T-cell responses**. Their superiority lies in their unique ability to capture antigens in peripheral tissues, migrate to regional lymph nodes, and express high levels of **MHC Class II** and **co-stimulatory molecules** (like B7-1 and B7-2). Unlike other APCs, mature dendritic cells are the only cells capable of activating **naive T-lymphocytes**, thereby bridging innate and adaptive immunity. **Analysis of Incorrect Options:** * **A. NK cell:** Natural Killer cells are part of the innate immune system involved in killing virally infected or tumor cells. They are **not** antigen-presenting cells. * **C. Macrophage:** While macrophages are professional APCs, their primary role is to present antigens to **already differentiated effector T-cells** to enhance their own microbicidal activity (cell-mediated immunity), rather than initiating a primary response. * **D. B-lymphocyte:** B-cells act as APCs by presenting antigens to **Helper T-cells (CD4+)** to receive signals for antibody production (humoral immunity). They are not the primary initiators of the systemic T-cell response. **High-Yield NEET-PG Pearls:** * **Langerhans cells** are specialized dendritic cells found in the epidermis. * **Follicular Dendritic Cells (FDCs)** are found in germinal centers; they trap antigens for B-cells but, interestingly, do **not** express MHC Class II. * The most potent stimulator of naive T-cells is the **Mature Dendritic Cell**. * **Cross-presentation:** A unique feature of certain DCs where they present exogenous antigens via MHC Class I to CD8+ T-cells.

Q: Which of the following statements is NOT true about the secondary immunoglobulin response?

Takes 5 days to appear. ### Explanation The **secondary immune response** (anamnestic response) occurs when the immune system encounters an antigen for the second or subsequent time. This response is characterized by its speed, intensity, and specificity due to the presence of **memory B-cells**. **Why Option B is the correct answer (The False Statement):** The secondary response is significantly faster than the primary response. While the primary response has a long lag phase of 5–10 days, the secondary response has a very short lag phase, typically appearing within **1–3 days**. Therefore, stating it takes 5 days to appear is incorrect in the context of a secondary response. **Analysis of Incorrect Options:** * **Option A (Predominance of IgG):** This is true. While the primary response is dominated by IgM, the secondary response involves "class switching," leading to a predominance of **IgG** (and sometimes IgA or IgE) with higher affinity for the antigen. * **Option C (Depends on immunologic memory):** This is true. The rapid mobilization is entirely dependent on memory B and T cells generated during the initial exposure. * **Option D (May be repeated):** This is true. The secondary response can be triggered multiple times upon re-exposure to the same pathogen, which is the fundamental principle behind **booster doses** in vaccination. ### NEET-PG High-Yield Pearls * **Lag Phase:** Primary (5–10 days) vs. Secondary (1–3 days). * **Antibody Titer:** Much higher and persists longer in the secondary response. * **Affinity Maturation:** Antibodies produced in the secondary response have a higher binding affinity for the antigen compared to the primary response. * **Antigens:** Primary response is elicited by both T-dependent and T-independent antigens; the secondary response is typically elicited by **T-dependent antigens**.

Q: Which immunoglobulin is produced first by the fetus in response to infection?

IgM. **Explanation:** The correct answer is **IgM**. **Why IgM is correct:** IgM is the first immunoglobulin class produced by the fetus during intrauterine life, starting around the 20th week of gestation. Unlike IgG, IgM is a large pentameric molecule that **cannot cross the placenta**. Therefore, if IgM antibodies are detected in the cord blood or neonatal serum, it is a definitive diagnostic indicator of an **intrauterine (congenital) infection** (e.g., TORCH infections), as these antibodies must have been synthesized by the fetus’s own immune system in response to a pathogen. **Why other options are incorrect:** * **IgG:** While IgG is the most abundant immunoglobulin in a neonate, it is almost entirely derived from the mother via **passive placental transfer**. The fetus only begins significant endogenous production of IgG after birth. * **IgA:** Secretory IgA is primarily involved in mucosal immunity and is provided to the infant through colostrum and breast milk. Fetal production of IgA is negligible. * **IgD:** This is primarily found on the surface of B-lymphocytes as a receptor and is not produced in significant quantities by the fetus in response to infection. **High-Yield Clinical Pearls for NEET-PG:** * **IgG:** The only antibody that crosses the placenta (provides passive immunity). * **IgM:** The first antibody produced in a primary immune response (both in adults and fetuses). * **IgA:** The most abundant antibody in body secretions (tears, saliva, breast milk). * **IgE:** Mediates Type I hypersensitivity and provides defense against helminthic infections. * **Memory Trick:** **M** is for **M**omentary/Immediate (first response) and **G** is for **G**estation (crosses placenta).

Q: Which CD marker is characteristic of NK cells?

CD16. **Explanation:** **Natural Killer (NK) cells** are large granular lymphocytes that play a critical role in the innate immune system. The correct answer is **CD16** (also known as FcγRIII). This marker is a low-affinity receptor for the Fc portion of IgG antibodies. Its presence allows NK cells to bind to antibody-coated target cells and destroy them via **Antibody-Dependent Cellular Cytotoxicity (ADCC)**. Along with **CD56** (NCAM), CD16 is considered a definitive phenotypic marker for identifying NK cells in clinical practice. **Analysis of Incorrect Options:** * **CD60:** This is a carbohydrate antigen (sialylated ganglioside) primarily found on a subset of T cells and melanocytes; it is not a primary marker for NK cells. * **CD32 (FcγRII):** This is an inhibitory Fc receptor found on B cells, macrophages, and neutrophils. It regulates immune responses but is not the characteristic marker for NK cells. * **CD25:** This is the alpha chain of the **IL-2 receptor**. While it can be expressed on activated NK cells, it is classically the hallmark of **Regulatory T cells (Tregs)** and activated T/B cells. **High-Yield Clinical Pearls for NEET-PG:** * **NK Cell Markers:** Always look for **CD16 and CD56**. Notably, NK cells are **CD3 negative** (distinguishing them from NKT cells). * **Function:** NK cells provide the first line of defense against viral infections and tumor cells by recognizing the **absence of MHC Class I** (the "Missing Self" hypothesis). * **Cytokines:** NK cell activity is significantly enhanced by **IL-2 and IL-12**. * **Clinical Correlation:** Deficiencies in NK cell function or markers like CD16 are associated with recurrent viral infections (especially Herpesviruses).

Q: Which cells are primarily responsible for innate immunity against cancer cells?

NK cells. **Explanation:** The correct answer is **NK (Natural Killer) cells**. **1. Why NK Cells are Correct:** NK cells are large granular lymphocytes that form a critical component of the **innate immune system**. Unlike T-cells, they do not require prior sensitization or MHC-restricted antigen presentation to function. They specialize in identifying and killing "stressed" cells, specifically **virally-infected cells** and **tumor cells** [1]. They operate via the "missing-self" hypothesis: when cancer cells downregulate MHC Class I molecules to evade T-cells, NK cells detect this absence and trigger apoptosis using perforins and granzymes [1]. **2. Why Other Options are Incorrect:** * **Basophils (A):** These are granulocytes primarily involved in type I hypersensitivity reactions (allergy) and defense against ectoparasites [2]. They release histamine and heparin. * **Eosinophils (B):** These are mainly responsible for combating multicellular parasites (helminths) and are involved in allergic inflammation (e.g., asthma) [2]. * **Neutrophils (D):** These are the "first responders" of the innate system, primarily focused on phagocytosis and killing of **extracellular bacteria** and fungi through oxidative burst and NETs (Neutrophil Extracellular Traps) [2]. **3. NEET-PG High-Yield Pearls:** * **Markers:** NK cells are identified by the presence of **CD56** and **CD16**, and the absence of CD3. * **Cytokine Activation:** Their activity is significantly enhanced by **IL-2** and **IL-12** [1]. * **LAK Cells:** NK cells treated with high-dose IL-2 become Lymphokine-Activated Killer (LAK) cells, used in experimental cancer immunotherapy [1]. * **Antibody-Dependent Cellular Cytotoxicity (ADCC):** NK cells use their CD16 receptor to bind to the Fc portion of IgG, allowing them to kill antibody-coated target cells.

Q: Which toll-like receptors are involved in the action of bacterial endotoxins?

IV. **Explanation:** The correct answer is **Option C (TLR-4)**. **Why TLR-4 is correct:** Toll-like receptors (TLRs) are a class of Pattern Recognition Receptors (PRRs) that recognize conserved microbial structures known as Pathogen-Associated Molecular Patterns (PAMPs). **TLR-4** is specifically responsible for recognizing **Lipopolysaccharide (LPS)**, which is the primary component of the **endotoxin** found in the outer membrane of Gram-negative bacteria. When LPS binds to TLR-4 (with the help of MD2 and CD14), it triggers a signaling cascade (via NF-κB) that leads to the release of pro-inflammatory cytokines (TNF-α, IL-1, IL-6), potentially resulting in septic shock. **Why other options are incorrect:** * **TLR-1:** Usually forms a heterodimer with TLR-2 to recognize triacylated lipopeptides (found in bacteria and mycobacteria). * **TLR-2:** Primarily recognizes **Peptidoglycan**, lipoteichoic acid (Gram-positive bacteria), and fungal zymosan. * **TLR-3:** Located on endosomal membranes; it recognizes **double-stranded RNA (dsRNA)**, which is characteristic of certain viruses. **High-Yield Clinical Pearls for NEET-PG:** * **TLR-5:** Recognizes **Flagellin** (bacterial flagella). * **TLR-7 & 8:** Recognize single-stranded RNA (ssRNA). * **TLR-9:** Recognizes unmethylated **CpG DNA** (bacterial/viral DNA). * **Location:** TLRs 1, 2, 4, 5, and 6 are found on the **cell surface**, while TLRs 3, 7, 8, and 9 are located in **endosomes** (intracellular). * **Genetic Link:** Mutations in the TLR-4 gene are associated with increased susceptibility to Gram-negative septicemia.

Question 1

B cells are induced to produce IgE by which of the following cytokines?

Accepted Answer

IL-4

Answer

IL-2

Answer

IL-1

Answer

IL-6

Question 2

Which of the following statements is true regarding Severe Combined Immunodeficiency (SCID)?

Accepted Answer

Adenosine deaminase deficiency

Answer

Decreased circulating lymphocytes

Answer

NADPH oxidase deficiency

Answer

C1 esterase deficiency

Question 3

All IgG subclasses are responsible for coagglutination by binding to S. Aureus protein A, except?

Accepted Answer

IgG3

Answer

IgG1

Answer

IgG2

Answer

IgG4

Question 4

Which of the following is the most important antigen-presenting cell (APC) for initiating T-cell response against protein antigens?

Accepted Answer

Dendritic cell

Answer

NK cell

Answer

Macrophage

Answer

B-lymphocyte

Question 5

Which of the following statements is NOT true about the secondary immunoglobulin response?

Accepted Answer

Takes 5 days to appear

Answer

Predominance of IgG

Answer

Depends on immunologic memory

Answer

May be repeated within physiological limits

Question 6

Which immunoglobulin is produced first by the fetus in response to infection?

Accepted Answer

IgM

Answer

IgG

Answer

IgA

Answer

IgD

Question 7

Cytotoxic T cells induced by infection with virus A will kill target cells that are infected by virus A and identical at class I MHC loci to the cytotoxic T cells?

Accepted Answer

Infected by virus A and identical at class I MHC loci to the cytotoxic T cells

Answer

From the same host infected with any virus

Answer

Infected by virus A and identical at class II MHC loci to the cytotoxic T cells

Answer

Infected with any virus and identical at class I MHC loci to the cytotoxic cells

Question 8

Which CD marker is characteristic of NK cells?

Accepted Answer

CD16

Answer

CD60

Answer

CD32

Answer

CD25

Question 9

Which cells are primarily responsible for innate immunity against cancer cells?

Accepted Answer

NK cells

Answer

Basophil

Answer

Eosinophils

Answer

Neutrophil

Question 10

Which toll-like receptors are involved in the action of bacterial endotoxins?

Accepted Answer

IV

Answer

I

Answer

II

Answer

III

Immunology — MCQs

Immunology — MCQs

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