Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 501: Fibroblasts in tissue culture form which type of interferon?

A. Alpha
B. Beta (Correct Answer)
C. Gamma
D. All of the above

Explanation: Interferons (IFNs) are a group of signaling proteins (cytokines) released by host cells in response to pathogens, particularly viruses. They are classified into three main types based on their structure and the cells that produce them. **Correct Option: B. Beta** Interferon-beta (IFN-β) is primarily produced by **fibroblasts** and epithelial cells. Along with IFN-alpha, it belongs to the Type I Interferon family. Its production is induced by viral infections and double-stranded RNA (dsRNA). In laboratory settings, fibroblasts in tissue culture are the classic source used to study or produce IFN-beta. **Incorrect Options:** * **A. Alpha:** IFN-alpha is primarily produced by **leukocytes** (specifically B cells, T cells, and macrophages) and plasmacytoid dendritic cells. * **C. Gamma:** IFN-gamma is a Type II Interferon produced by **T-lymphocytes (Th1 cells)** and **Natural Killer (NK) cells**. It is induced by antigens or mitogenic stimulation rather than direct viral infection. * **D. All of the above:** This is incorrect because interferon production is cell-specific. **High-Yield NEET-PG Pearls:** 1. **Type I IFNs (Alpha & Beta):** Acid-stable; primary role is antiviral. They induce an "antiviral state" in neighboring cells by activating RNAse L and Protein Kinase R (PKR). 2. **Type II IFN (Gamma):** Acid-labile; primary role is immunomodulatory (activates macrophages and increases MHC expression). 3. **Clinical Use:** IFN-beta is a mainstay in the treatment of **Multiple Sclerosis (MS)** to reduce the frequency of relapses. 4. **Inducer:** The most potent inducer of Type I Interferons is **double-stranded RNA (dsRNA)**.

Question 502: Which of the following statements concerning immunoglobulins is wrong?

A. IgM does not cross the placenta
B. IgE is increased in parasitic infections
C. IgM is increased in the primary immune response
D. Fetal infection can be diagnosed by the detection of IgG (Correct Answer)

Explanation: **Explanation** The correct answer is **D** because the statement is factually incorrect. In clinical practice, fetal infection is diagnosed by detecting **IgM**, not IgG. **1. Why Option D is the "Wrong" Statement (The Correct Answer):** IgG is the only immunoglobulin class that can cross the placenta (via neonatal Fc receptors). Therefore, if IgG is detected in a newborn’s blood, it usually represents **passive immunity** transferred from the mother rather than an active infection in the fetus. Conversely, **IgM cannot cross the placenta** due to its large pentameric size. If IgM is detected in fetal or cord blood, it indicates that the fetus has produced its own antibodies in response to an intrauterine infection (e.g., TORCH infections). **2. Analysis of Other Options:** * **Option A:** Correct. IgM is a large pentamer (MW ~900,000 Da) and is too bulky to cross the placental barrier. * **Option B:** Correct. IgE mediates type I hypersensitivity and plays a crucial role in the immune response against helminthic parasites by activating mast cells and eosinophils. * **Option C:** Correct. IgM is the first antibody produced during the **primary immune response** following initial exposure to an antigen. **High-Yield Clinical Pearls for NEET-PG:** * **IgG:** Most abundant in serum; responsible for secondary immune response; crosses placenta. * **IgM:** Largest (Pentamer); first to appear; indicates acute infection; "Millennium" molecule. * **IgA:** Found in secretions (colostrum, saliva, tears); protects mucosal surfaces. * **IgE:** Lowest serum concentration; heat-labile; mediates anaphylaxis and parasitic defense. * **IgD:** Acts primarily as a B-cell surface receptor.

Question 503: Immunoglobulin variation does not depend on which of the following?

A. Light chain (Correct Answer)
B. Heavy chain
C. Amino acid sequence
D. Constant region

Explanation: ### Explanation The classification and variation of immunoglobulins are fundamentally determined by the **Heavy (H) chain**, not the light chain. **1. Why "Light chain" is the correct answer:** Immunoglobulin classes (Isotypes) such as IgG, IgA, IgM, IgD, and IgE are defined solely by the antigenic differences in their **Heavy chains** (γ, α, μ, δ, and ε respectively). While there are two types of light chains (Kappa and Lambda), they are present in all five classes of immunoglobulins. Therefore, the specific variation or "class" of an antibody does **not** depend on which light chain is present. **2. Analysis of Incorrect Options:** * **Heavy chain:** This is the primary determinant of the immunoglobulin class and its effector functions (e.g., placental transfer, complement activation). * **Amino acid sequence:** Variations in the amino acid sequence in the constant region determine the **Isotype**, while variations in the variable region determine the **Idiotype** (antigen-binding specificity). Thus, variation is highly dependent on sequence. * **Constant region:** The constant region of the heavy chain determines the biological properties and the class of the antibody. Isotypic variation is specifically located in the constant region. ### High-Yield Clinical Pearls for NEET-PG: * **Isotype:** Determined by the Heavy chain constant region (defines IgG, M, A, D, E). * **Allotype:** Determined by allelic differences at specific loci (e.g., Gm on heavy chain, Km on light chain). * **Idiotype:** Determined by the hypervariable region (antigen-binding site). * **Ratio:** In humans, the normal Kappa (κ) to Lambda (λ) ratio is **2:1**. A significant shift in this ratio (e.g., 10:1) suggests monoclonal proliferation, such as **Multiple Myeloma**. * **Valency:** IgG is bivalent, while secreted IgA is tetravalent (dimer) and IgM is decavalent (pentamer).

Question 504: All of the following are Integrin molecules except?

A. LFA-1
B. MAC-1
C. VLA-4
D. CD 31 (Correct Answer)

Explanation: **Explanation:** The question tests the classification of **Cell Adhesion Molecules (CAMs)**, which are critical for leukocyte migration and the inflammatory response. **Why CD 31 is the correct answer:** **CD 31**, also known as **PECAM-1** (Platelet Endothelial Cell Adhesion Molecule-1), belongs to the **Immunoglobulin (Ig) Superfamily**, not the Integrin family. Its primary role is in **diapedesis** (transendothelial migration), where it helps leukocytes squeeze through endothelial cell junctions. **Analysis of Incorrect Options (Integrin Family):** Integrins are heterodimeric surface proteins (alpha and beta chains) that mediate firm adhesion of leukocytes to the endothelium. * **LFA-1 (Lymphocyte Function-associated Antigen-1):** Also known as **CD11a/CD18**. It is a $\beta_2$-integrin that binds to ICAM-1. * **MAC-1 (Macrophage-1 antigen):** Also known as **CD11b/CD18**. It is a $\beta_2$-integrin that binds to ICAM-1 and acts as a receptor for iC3b. * **VLA-4 (Very Late Antigen-4):** Also known as **$\alpha_4\beta_1$ integrin**. It binds to VCAM-1 on endothelial cells. **High-Yield Clinical Pearls for NEET-PG:** 1. **Leukocyte Adhesion Deficiency (LAD) Type 1:** Caused by a deficiency in the **$\beta_2$ chain (CD18)** of integrins (LFA-1/MAC-1). Clinical features include delayed separation of the umbilical cord, recurrent bacterial infections without pus formation, and extreme neutrophilia. 2. **Sequence of Leukocyte Migration:** * **Rolling:** Mediated by **Selectins** (E, P, and L-selectin). * **Firm Adhesion:** Mediated by **Integrins** (LFA-1, VLA-4) binding to Ig-Superfamily ligands (ICAM-1, VCAM-1). * **Diapedesis:** Mediated by **PECAM-1 (CD 31)**. 3. **Sialyl-Lewis X:** The carbohydrate ligand on leukocytes that binds to P and E-selectins; its deficiency causes **LAD Type 2**.

Question 505: Mercaptoethanol splits an immunoglobulin molecule into which of the following?

A. Two H and two L chains (Correct Answer)
B. Two Fab fragments and one Fc fragment
C. One Fab and two Fc
D. Two H and one L chain

Explanation: **Explanation:** The structure of an Immunoglobulin (Ig) molecule is held together by two types of bonds: **disulfide bonds** (covalent) and non-covalent interactions. **Why Option A is correct:** Mercaptoethanol is a **reducing agent**. It specifically targets and breaks the **interchain disulfide bonds** that link the heavy (H) chains to each other and the light (L) chains to the heavy chains. When these bonds are cleaved, the quaternary structure of the antibody collapses, resulting in the separation of the molecule into its constituent polypeptide chains: **two heavy (H) chains and two light (L) chains**. **Why other options are incorrect:** * **Options B and C:** These describe the results of **proteolytic cleavage**, not reduction. * **Papain digestion** cleaves the molecule above the hinge region, resulting in **two Fab fragments and one Fc fragment**. * **Pepsin digestion** cleaves below the hinge region, resulting in **one F(ab')₂ fragment** (the Fc portion is degraded). * **Option D:** This is structurally incorrect; a standard monomeric IgG molecule always consists of a symmetrical pair of two H and two L chains. **High-Yield Clinical Pearls for NEET-PG:** * **Papain = 3 fragments** (2 Fab + 1 Fc). Think: "Pa-pain" has 3 syllables. * **Pepsin = 1 fragment** (F(ab')₂). The Fc part is digested into peptides. * **L-chains** are connected to H-chains by disulfide bonds. * **H-chains** are connected to each other by disulfide bonds at the hinge region. * **Bence-Jones proteins** in Multiple Myeloma are actually free monoclonal Light Chains.

Question 506: Toll-like receptor 5 (TLR-5) binds to which of the following?

A. Bacterial peptidoglycan
B. Double-stranded RNA of viruses
C. Flagella of bacteria (Correct Answer)
D. Bacterial DNA

Explanation: **Explanation:** Toll-like receptors (TLRs) are a class of **Pattern Recognition Receptors (PRRs)** that play a crucial role in the innate immune system by recognizing conserved microbial structures known as **Pathogen-Associated Molecular Patterns (PAMPs)**. **Why Option C is Correct:** **TLR-5** is specifically localized on the cell surface and is responsible for recognizing **Flagellin**, the principal protein component of bacterial flagella. This recognition triggers a signaling cascade (via the MyD88 pathway) leading to the activation of NF-κB and the production of pro-inflammatory cytokines to combat motile bacteria. **Analysis of Incorrect Options:** * **A. Bacterial peptidoglycan:** This is primarily recognized by **TLR-2** (often in association with TLR-1 or TLR-6). Peptidoglycan is a major component of the Gram-positive bacterial cell wall. * **B. Double-stranded RNA (dsRNA):** This is recognized by **TLR-3**, which is located intracellularly within endosomes. It is a hallmark of viral replication. * **D. Bacterial DNA:** Unmethylated CpG DNA motifs common in bacteria and some viruses are recognized by **TLR-9**, also located in endosomal compartments. **High-Yield Clinical Pearls for NEET-PG:** * **Cell Surface TLRs:** 1, 2, 4, 5, 6 (Recognize lipids, proteins, and glycans). * **Endosomal (Intracellular) TLRs:** 3, 7, 8, 9 (Recognize nucleic acids). * **TLR-4:** Recognizes **Lipopolysaccharide (LPS)** of Gram-negative bacteria (requires MD2 and CD14). * **TLR-7/8:** Recognizes single-stranded RNA (ssRNA) of viruses. * **Memory Aid:** "High **5** for the **Flag**" (TLR-5 = Flagellin).

Question 507: Atopy is primarily mediated by which immunoglobulin class?

A. IgE (Correct Answer)
B. IgD
C. IgM
D. IgA

Explanation: **Explanation:** **Atopy** refers to the genetic predisposition to develop localized hypersensitivity reactions to common environmental allergens. This process is the hallmark of **Type I Hypersensitivity** (Immediate Hypersensitivity). 1. **Why IgE is Correct:** In atopic individuals, exposure to an allergen triggers Th2 cells to release cytokines (IL-4, IL-13), which induce B-cells to undergo class switching to produce **IgE**. These IgE antibodies bind to high-affinity **FcεRI receptors** on the surface of **mast cells and basophils** (sensitization). Upon re-exposure, the allergen cross-links the bound IgE, leading to degranulation and the release of vasoactive amines like histamine, causing symptoms of asthma, allergic rhinitis, or eczema. 2. **Why Other Options are Incorrect:** * **IgD:** Primarily acts as a B-cell surface receptor; its secreted function is largely unknown and not involved in allergy. * **IgM:** The first antibody produced in a primary immune response and a potent activator of the classical complement pathway. It is involved in Type II and Type III hypersensitivity, but not atopy. * **IgA:** The primary secretory immunoglobulin found in mucosal surfaces (tears, saliva, colostrum). It provides local mucosal immunity rather than mediating allergic triggers. **High-Yield Clinical Pearls for NEET-PG:** * **Prausnitz-Küstner (PK) Reaction:** A classic (though now obsolete) test used to demonstrate IgE-mediated serum transfer of allergy. * **Casoni’s Test:** An immediate hypersensitivity skin test used for Hydatid disease. * **Key Cytokine:** **IL-4** is the essential cytokine for IgE class switching. * **Atopic Triad:** Asthma, Allergic Rhinitis, and Atopic Dermatitis.

Question 508: Which protein is responsible for the clearance of C3 convertase?

A. CD 59
B. CD 55 (Correct Answer)
C. Factor D
D. Factor E

Explanation: **Explanation:** The correct answer is **CD 55**, also known as **Decay-Accelerating Factor (DAF)**. **Why CD 55 is correct:** CD 55 is a membrane-bound protein found on the surface of blood cells and endothelial cells. Its primary function is to protect host cells from accidental damage by the complement system. It acts by binding to the **C3 convertase** (C4b2a in the classical pathway and C3bBb in the alternative pathway) and accelerating its dissociation (decay). By clearing C3 convertase, it prevents the amplification of the complement cascade and the subsequent formation of the Membrane Attack Complex (MAC). **Analysis of Incorrect Options:** * **CD 59 (Protectin):** This protein acts further downstream in the cascade. It prevents the final assembly of the **Membrane Attack Complex (MAC)** by inhibiting the binding of C9 to the C5b-8 complex. * **Factor D:** This is a serine protease essential for the **activation** of the alternative pathway (it cleaves Factor B into Bb). It promotes, rather than clears, C3 convertase formation. * **Factor E:** This is a distractor; there is no "Factor E" in the human complement system. **Clinical Pearls for NEET-PG:** * **Paroxysmal Nocturnal Hemoglobinuria (PNH):** This condition is caused by a deficiency of **PIGA**, a protein required to anchor CD 55 and CD 59 to the cell membrane via GPI anchors. Without these regulators, RBCs are susceptible to complement-mediated lysis, leading to hemolytic anemia. * **C3 Convertases:** Remember C4b2a (Classical/Lectin) and C3bBb (Alternative). * **Factor I and Factor H:** These are other important regulators; Factor I cleaves C3b and C4b, while Factor H is the main soluble regulator of the alternative pathway.

Question 509: Analysis of protein antigen is by which method?

A. Southern blot
B. Northern blot
C. Western blot (Correct Answer)
D. Eastern blot

Explanation: **Explanation:** The correct answer is **Western blot**. This technique is the gold standard for the detection and analysis of specific **protein antigens** in a given sample. ### Why Western Blot is Correct: Western blotting involves the separation of proteins based on their molecular weight via gel electrophoresis (usually SDS-PAGE), followed by their transfer to a membrane (nitrocellulose or PVDF). Specific proteins are then identified using labeled **antibodies** that bind to the target antigen. This makes it the primary method for analyzing protein expression and confirming infections. ### Analysis of Incorrect Options: * **A. Southern Blot:** Used for the detection of specific **DNA** sequences. (Mnemonic: **S**outhern = **D**NA). * **B. Northern Blot:** Used for the detection of specific **RNA** (mRNA) sequences to study gene expression. (Mnemonic: **N**orthern = **R**NA). * **C. Eastern Blot:** A specialized technique used to analyze **post-translational modifications** of proteins, such as lipids, phosphates, or glycoconjugates. It is not the standard method for general protein antigen analysis. ### High-Yield Clinical Pearls for NEET-PG: * **Mnemonic (SNOW DROP):** * **S**outhern — **D**NA * **N**orthern — **R**NA * **O** — **O** (Nothing) * **W**estern — **P**rotein * **Clinical Application:** Western blot was historically the definitive **confirmatory test for HIV** (detecting antibodies against p24, gp41, and gp120/160), though it has largely been replaced by 4th generation ELISA and NAT in modern algorithms. * **Southwestern Blot:** A hybrid technique used to identify proteins that bind to DNA (e.g., transcription factors).

Question 510: Human immunoglobulins are divided based on what criteria?

A. Functional differences
B. Structural differences (Correct Answer)
C. Complement fixation
D. None of the above

Explanation: ### Explanation **1. Why "Structural differences" is correct:** Human immunoglobulins (IgG, IgA, IgM, IgE, and IgD) are classified into five distinct classes (isotypes) based on **structural differences in their heavy chains**. Specifically, these differences reside in the amino acid sequences of the **constant (Fc) region** of the heavy chains, designated by Greek letters: * **Gamma (γ):** IgG * **Alpha (α):** IgA * **Mu (μ):** IgM * **Epsilon (ε):** IgE * **Delta (δ):** IgD Additionally, immunoglobulins are further divided into subclasses (e.g., IgG1–IgG4) based on minor structural variations in these heavy chains. **2. Why other options are incorrect:** * **Functional differences:** While each class has unique functions (e.g., IgE in allergy, IgA in mucosal immunity), these functions are a *consequence* of their structural design, not the primary criteria for their fundamental classification. * **Complement fixation:** This is a specific functional property. Only IgM and certain subclasses of IgG (IgG3 > IgG1 > IgG2) can activate the classical complement pathway. It is not used to define the five primary classes. **3. High-Yield Clinical Pearls for NEET-PG:** * **Most abundant Ig in serum:** IgG (80%). * **Largest Ig (Pentamer):** IgM (highest molecular weight; first to appear in primary response). * **Mucosal Immunity:** IgA (exists as a dimer with a J-chain and secretory component). * **Placental Transfer:** Only IgG can cross the placenta (specifically IgG1, IgG3, and IgG4). * **Heat Labile Ig:** IgE (reaginic antibody involved in Type I Hypersensitivity). * **Light Chains:** Regardless of the heavy chain, all Igs have either **Kappa (κ)** or **Lambda (λ)** light chains, usually in a 2:1 ratio.

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Immunology — MCQs

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