Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 491: Papain cleaves the immunoglobulin molecule into:

A. 1 Fc and 1 Fab
B. 1 Fc and 2 Fab (Correct Answer)
C. 2 Fc and 1 Fab
D. 2 Fc and 2 Fab

Explanation: ### Explanation **1. Understanding the Concept (The Correct Answer)** Immunoglobulins (antibodies) are Y-shaped molecules composed of two heavy chains and two light chains held together by disulfide bonds. The enzyme **Papain** (derived from papaya) acts on the **hinge region** of the immunoglobulin molecule, specifically cleaving it *above* the inter-chain disulfide bonds. This enzymatic digestion results in three distinct fragments: * **Two Fab fragments (Fragment Antigen Binding):** Each Fab fragment consists of one light chain and a portion of one heavy chain. They are monovalent, meaning each can bind to one antigen epitope. * **One Fc fragment (Fragment Crystallizable):** This consists of the remaining portions of the two heavy chains. It is responsible for biological effector functions, such as complement fixation and binding to cell surface receptors. **2. Why Other Options are Incorrect** * **Option A & C:** These are incorrect because the stoichiometry of the Y-shaped antibody molecule dictates that there are two identical "arms" (Fab) and one "stem" (Fc). Cleavage at the hinge region will always yield two binding fragments. * **Option D:** This is incorrect because the heavy chains in the Fc region are held together by disulfide bonds; papain does not separate these two chains into individual units. **3. High-Yield Clinical Pearls for NEET-PG** * **Pepsin Digestion:** Unlike Papain, the enzyme **Pepsin** cleaves *below* the hinge region. This results in **one F(ab')₂ fragment** (bivalent, as the two Fab units remain linked) and several small peptides (the Fc portion is degraded). * **Mercaptoethanol:** This is a reducing agent that breaks disulfide bonds, separating the molecule into **two individual heavy chains** and **two individual light chains**. * **Memory Aid:** **Pa**pain = **Pa**rted (splits the Fabs apart into 2 pieces); **Pe**psin = **Pe**rmanent (keeps the Fabs together as 1 piece).

Question 492: Which immunoglobulin mediates the primary immune response?

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

Explanation: **Explanation:** The correct answer is **IgM**. This is because IgM is the first antibody isotype produced by B cells during the **primary immune response** following initial exposure to an antigen. **Why IgM is correct:** * **Structure:** It exists as a pentamer in secretions, providing 10 antigen-binding sites (high avidity), which allows it to compensate for the low affinity typical of early antibodies. * **Kinetics:** It appears within days of infection. Its presence in serum typically indicates an **acute or recent infection**. * **Function:** It is highly efficient at activating the classical complement pathway and causing agglutination. **Why other options are incorrect:** * **IgE:** Primarily involved in **Type I hypersensitivity** reactions (allergies) and provides immunity against helminthic parasitic infections. * **IgA:** The predominant antibody in **mucosal secretions** (tears, saliva, colostrum, GI tract). It provides local immunity but is not the primary systemic responder. * **IgD:** Found mainly on the surface of naive B lymphocytes, where it acts as an antigen receptor; its secreted form has no clearly defined systemic immune role. **NEET-PG High-Yield Pearls:** * **IgG** is the most abundant antibody in serum and mediates the **secondary (anamnestic) immune response**. It is also the only antibody that **crosses the placenta**. * **J-Chain:** Present in both IgM (pentamer) and secretory IgA (dimer). * **Isotype Switching:** The process where B cells change from producing IgM to IgG, IgA, or IgE, triggered by cytokines from T-helper cells. * **Intrauterine Infection:** Detection of IgM in a newborn’s serum is diagnostic of a congenital infection (e.g., TORCH), as maternal IgM cannot cross the placenta.

Question 493: An infant with a history of delayed separation of the umbilical cord now presents with recurrent pneumonia. The total white blood cell count is 20,000/ml. Which of the following genetic defects is most likely present?

A. Low levels of NADPH oxidase and a negative DHR test
B. Mutation of the Bruton tyrosine kinase gene
C. Excessive IgM with reduced IgG and IgA
D. Reduced phagocyte surface expression of Sialyl-Lewis x (Correct Answer)

Explanation: **Explanation:** The clinical presentation of **delayed separation of the umbilical cord** (typically >3 weeks), recurrent bacterial infections without pus formation, and marked **leukocytosis** (neutrophilia) is pathognomonic for **Leukocyte Adhesion Deficiency (LAD)**. **Why the correct answer is right:** LAD occurs due to a failure of leukocytes to migrate from the bloodstream into tissues. * **LAD Type 1:** Deficiency of **CD18** (common β2-chain of integrins), preventing firm adhesion. * **LAD Type 2:** Deficiency of **Sialyl-Lewis X** (ligand for E- and P-selectins), preventing the initial "rolling" phase of leukocyte extravasation. Since the neutrophils cannot exit the blood vessels to reach the site of infection, they accumulate in the blood (leukocytosis), and the umbilical cord fails to detach due to a lack of neutrophilic enzymatic activity. **Analysis of Incorrect Options:** * **Option A:** Describes **Chronic Granulomatous Disease (CGD)**. While it causes recurrent infections, it does not cause delayed umbilical cord separation and is characterized by granuloma formation. * **Option B:** Refers to **X-linked Agammaglobulinemia (Bruton’s)**. This presents after 6 months of age (once maternal IgG wanes) with absent B-cells and low immunoglobulins, but not delayed cord separation. * **Option C:** Describes **Hyper-IgM Syndrome**, a defect in CD40L or AID. It presents with pyogenic infections but does not affect leukocyte adhesion or cord separation. **NEET-PG High-Yield Pearls:** * **Classic Triad of LAD:** Delayed umbilical cord separation + Recurrent skin/mucosal infections + **Absent pus formation** (cold infections). * **LAD-1:** Defect in **CD18** (Integrin LFA-1/Mac-1). * **LAD-2:** Defect in **Sialyl-Lewis X** (Selectin ligand); often associated with growth retardation and Bombay blood group. * **LAD-3:** Defect in **Kindlin-3** (Integrin activation); presents with LAD-1 features plus a bleeding diathesis (platelet dysfunction).

Question 494: Hemolytic disease of the newborn is which type of hypersensitivity reaction?

A. Type I
B. Type II (Correct Answer)
C. Type III
D. Type IV

Explanation: **Explanation:** **Hemolytic Disease of the Newborn (HDN)**, also known as Erythroblastosis Fetalis, is a classic example of **Type II (Cytotoxic) Hypersensitivity**. **Why Type II is correct:** Type II hypersensitivity is mediated by **IgG or IgM antibodies** directed against antigens on specific cell surfaces or tissues. In HDN (specifically Rh incompatibility), maternal anti-D IgG antibodies cross the placenta and bind to Rh antigens on the fetal red blood cell (RBC) membrane. This leads to RBC destruction via two mechanisms: 1. **Opsonization** and subsequent phagocytosis by splenic macrophages. 2. **Complement-mediated lysis.** Since the damage is localized to a specific cell type (RBCs) via antibody binding, it fits the Type II definition. **Why other options are incorrect:** * **Type I (Immediate):** Mediated by **IgE** and mast cell degranulation (e.g., Anaphylaxis, Asthma). * **Type III (Immune-complex):** Involves soluble antigen-antibody complexes depositing in tissues/vessels (e.g., SLE, Post-streptococcal glomerulonephritis). * **Type IV (Delayed):** Cell-mediated reaction involving **T-lymphocytes**, not antibodies (e.g., Mantoux test, Contact dermatitis). **High-Yield Clinical Pearls for NEET-PG:** * **Coombs Test:** The **Direct Coombs Test** is used to detect antibodies already bound to the baby’s RBCs (positive in HDN). The **Indirect Coombs Test** checks for anti-Rh antibodies in the mother's serum. * **Prophylaxis:** Administering **Anti-D (RhoGAM)** to an Rh-negative mother at 28 weeks and within 72 hours of delivery prevents sensitization. * **Other Type II Examples:** Myasthenia Gravis, Goodpasture Syndrome, Rheumatic Fever, and Pemphigus Vulgaris.

Question 495: What is true about Restriction Fragment Length Polymorphism (RFLP)?

A. Detects mutation (Correct Answer)
B. Recognizes trinucleotide repeat
C. Detects deletion
D. Blunt ends are produced

Explanation: **Explanation:** **Restriction Fragment Length Polymorphism (RFLP)** is a molecular biology technique used to detect variations in DNA sequences. It relies on the principle that a specific **mutation** (single nucleotide polymorphism) can either create or abolish a recognition site for a **restriction endonuclease** enzyme. 1. **Why Option A is Correct:** When a mutation occurs at a restriction site, the enzyme fails to cut the DNA at that specific point, or it may cut at a new site. This results in DNA fragments of varying lengths (polymorphism) when separated by gel electrophoresis. Therefore, RFLP is primarily used to **detect mutations** that alter restriction sites. 2. **Why Other Options are Incorrect:** * **Option B:** Trinucleotide repeats (e.g., in Fragile X or Huntington’s) are typically detected using PCR or Southern Blotting to measure expansion size, rather than RFLP. * **Option C:** While large deletions can change fragment size, RFLP is specifically designed to identify variations at specific restriction enzyme recognition sequences. Deletions are more accurately detected by Comparative Genomic Hybridization (CGH) or multiplex PCR. * **Option D:** Restriction enzymes can produce either "sticky ends" (overhangs) or "blunt ends." Producing blunt ends is not a defining characteristic or a requirement for RFLP analysis. **Clinical Pearls for NEET-PG:** * **Applications:** RFLP is used in forensic science (DNA fingerprinting), paternity testing, and identifying genetic carriers for diseases like **Sickle Cell Anemia** (where a mutation destroys the *MstII* enzyme site). * **Key Requirement:** You must have a specific DNA probe that complementary binds to the region of interest. * **Evolution:** RFLP has largely been replaced by faster, PCR-based SNP genotyping, but remains a fundamental concept in genetic linkage analysis.

Question 496: Interleukin-1 primarily acts on which of the following cell types?

A. T-lymphocytes (Correct Answer)
B. B-lymphocytes
C. Neutrophils
D. Macrophages

Explanation: **Explanation:** **Interleukin-1 (IL-1)**, primarily produced by activated macrophages and monocytes, is a key pro-inflammatory cytokine that serves as a critical link between innate and adaptive immunity. **Why T-lymphocytes is the correct answer:** The primary immunological role of IL-1 is the **activation of T-lymphocytes**. When an Antigen-Presenting Cell (APC) processes a pathogen, it releases IL-1, which acts as a "second signal" (co-stimulation). This induces T-cells to transition from the G0 to the G1 phase of the cell cycle, triggering the production of **Interleukin-2 (IL-2)** and the expression of IL-2 receptors. This sequence is essential for T-cell proliferation and the subsequent orchestration of the adaptive immune response. **Analysis of Incorrect Options:** * **B-lymphocytes:** While IL-1 can promote B-cell proliferation and antibody synthesis, this is a secondary effect. B-cell activation is more directly governed by IL-4, IL-5, and IL-6. * **Neutrophils:** IL-1 does induce the release of neutrophils from bone marrow and increases their adhesion to endothelium, but it is not their primary activator. **IL-8** is the primary chemotactic factor for neutrophils. * **Macrophages:** Macrophages are the **producers** of IL-1 rather than its primary target. They are primarily activated by Interferon-gamma (IFN-γ). **High-Yield Clinical Pearls for NEET-PG:** * **Endogenous Pyrogen:** IL-1 acts on the anterior hypothalamus to increase prostaglandin E2 (PGE2) production, resulting in **fever**. * **Acute Phase Response:** Along with IL-6 and TNF-α, IL-1 stimulates the liver to produce acute-phase proteins (e.g., CRP). * **Osteoclast Activation:** IL-1 is also known as "Osteoclast Activating Factor," leading to bone resorption.

Question 497: Increased level of IgM indicates which of the following?

A. Vaccination
B. An immunized person
C. Acute infection (Correct Answer)
D. Chronic infection

Explanation: **Explanation:** The correct answer is **C. Acute infection**. **1. Why Acute Infection is Correct:** IgM (Immunoglobulin M) is the first antibody isotype produced by B-cells during a primary immune response. It is a pentameric molecule, making it highly efficient at agglutination and complement activation. Because IgM has a relatively short half-life (approximately 5–10 days) and is eventually replaced by IgG through "class switching," its presence in high titers is a definitive diagnostic marker for a **recent or acute infection**. **2. Why Other Options are Incorrect:** * **A & B (Vaccination/Immunized Person):** Long-term immunity and the secondary immune response are characterized by **IgG**. While IgM may appear briefly after the first dose of a vaccine, a person considered "immunized" or successfully vaccinated will primarily show elevated levels of IgG, which provides long-lasting protection. * **D (Chronic Infection):** Chronic or persistent infections (like Tuberculosis or HIV) are associated with a sustained **IgG** response. IgM levels typically decline after the initial phase of the disease. **3. NEET-PG High-Yield Pearls:** * **IgM:** The largest antibody (Pentamer), "Millionaire molecule," and the first to appear in phylogeny and ontogeny (fetal life). It **does not cross the placenta**; therefore, IgM in a newborn indicates an *in utero* (congenital) infection. * **IgG:** The most abundant antibody in serum, the only one that **crosses the placenta**, and the marker for secondary immune response/past exposure. * **IgA:** The primary antibody found in secretions (colostrum, saliva, tears) and provides mucosal immunity.

Question 498: The Rose-Waaler test is a type of?

A. Co-agglutination test
B. Latex agglutination test
C. Slide agglutination test
D. Haemagglutination test (Correct Answer)

Explanation: **Explanation:** The **Rose-Waaler test** is a classic diagnostic method used to detect **Rheumatoid Factor (RF)** in a patient's serum. Rheumatoid Factor is an autoantibody (usually IgM) directed against the Fc portion of the patient's own IgG. **Why the correct answer is right:** The Rose-Waaler test is a **Passive Haemagglutination test**. It utilizes **sheep erythrocytes (RBCs)** that have been sensitized (coated) with a sub-agglutinating dose of rabbit anti-sheep erythrocyte antibody (IgG). When the patient's serum containing RF is added, the RF acts as a bridge between the IgG molecules on the RBCs, leading to visible clumping or **haemagglutination**. **Analysis of incorrect options:** * **A. Co-agglutination test:** This uses *Staphylococcus aureus* (Cowan 1 strain) containing Protein A to coat antibodies; it is not the principle used in Rose-Waaler. * **B. Latex agglutination test:** While commonly used for RF (e.g., the RA factor test), this uses polystyrene latex particles instead of RBCs. It is more sensitive but less specific than the Rose-Waaler test. * **C. Slide agglutination test:** This is a general format (like the Widal test or blood grouping) rather than a specific physiological category of the Rose-Waaler mechanism. **High-Yield Clinical Pearls for NEET-PG:** * **Paul-Bunnell Test:** Another famous haemagglutination test, used for Infectious Mononucleosis (detects heterophile antibodies). * **Sensitivity vs. Specificity:** The Rose-Waaler test is highly specific for Rheumatoid Arthritis but has been largely replaced in modern labs by the more sensitive Latex Agglutination and the highly specific **Anti-CCP antibody** test. * **RF Fact:** RF is not exclusive to RA; it can be positive in SLE, Sjögren’s syndrome, and chronic infections like Leprosy or TB.

Question 499: HLA Class I genes are linked with which of the following?

A. Graft rejection
B. Killing of viral infected cells
C. Graft versus host disease (Correct Answer)
D. Susceptibility to autoimmune diseases

Explanation: **Explanation:** The correct answer is **C. Graft versus host disease (GVHD)**. **1. Why Option C is Correct:** Graft-versus-host disease occurs primarily in bone marrow or hematopoietic stem cell transplants. It is mediated by the **donor’s T-cells** (the graft) recognizing the **host’s HLA Class I and Class II antigens** as foreign. Since HLA Class I molecules (HLA-A, B, and C) are expressed on almost all nucleated cells in the recipient's body, they serve as the primary targets for the donor’s cytotoxic T-cells (CD8+), leading to widespread systemic damage (skin, liver, and GI tract). **2. Why Other Options are Incorrect:** * **A. Graft Rejection:** While HLA Class I is involved, graft rejection is primarily a host-versus-graft response. In the context of standard NEET-PG questions, GVHD is the classic clinical scenario specifically linked to the systemic recognition of Class I/II disparities by donor cells. * **B. Killing of Viral Infected Cells:** This is a **physiological function** of HLA Class I (MHC restriction), not a disease linkage. HLA Class I presents endogenous viral peptides to CD8+ T-cells. * **D. Susceptibility to Autoimmune Diseases:** While some Class I alleles are linked (e.g., HLA-B27 and Ankylosing Spondylitis), the majority of autoimmune disease susceptibilities are more strongly associated with **HLA Class II** genes (e.g., HLA-DR3/DR4). **Clinical Pearls for NEET-PG:** * **HLA Class I:** Encoded by HLA-A, B, and C loci. Found on all nucleated cells and platelets (not RBCs). * **HLA Class II:** Encoded by HLA-DP, DQ, and DR loci. Found only on Antigen Presenting Cells (APCs). * **Rule of 8:** HLA Class I × CD8 = 8; HLA Class II × CD4 = 8. * **GVHD Requirements:** The graft must contain immunocompetent cells, the recipient must be immunocompromised, and there must be an HLA mismatch.

Question 500: Toll-like receptors are expressed in all of the following cell types except:

A. Macrophages
B. Dendritic cells
C. B cells (Correct Answer)
D. T cells

Explanation: **Explanation:** Toll-like receptors (TLRs) are a class of **Pattern Recognition Receptors (PRRs)** that play a critical role in the innate immune system by recognizing conserved microbial structures known as Pathogen-Associated Molecular Patterns (PAMPs). **Why B cells is the correct answer:** While TLRs are widely expressed on cells of the **innate immune system** (like macrophages and dendritic cells) to initiate rapid defense, they are generally **not expressed on B cells**. B cells primarily rely on the B-cell receptor (BCR) for antigen recognition. In the context of standard medical examinations like NEET-PG, TLRs are classically associated with myeloid lineage cells rather than lymphoid cells like B cells. **Analysis of other options:** * **Macrophages (Option A):** These are the prototypical innate immune cells. They express a wide array of TLRs (e.g., TLR4 for LPS) to trigger phagocytosis and cytokine release. * **Dendritic Cells (Option B):** Known as the "bridge" between innate and adaptive immunity, DCs express various TLRs to sense pathogens and subsequently undergo maturation to present antigens. * **T cells (Option D):** While primarily part of adaptive immunity, certain subsets of T cells (especially γδ T cells and some memory T cells) do express TLRs which act as co-stimulatory signals. **High-Yield NEET-PG Pearls:** * **TLR-4:** Recognizes **Lipopolysaccharide (LPS)** of Gram-negative bacteria (associated with septic shock). * **TLR-3:** Recognizes **double-stranded RNA (dsRNA)** (viral infections). * **TLR-5:** Recognizes **Flagellin**. * **TLR-7 & 8:** Recognize single-stranded RNA (ssRNA). * **TLR-9:** Recognizes unmethylated **CpG DNA**. * **Location:** TLR 1, 2, 4, 5, and 6 are on the **cell surface**; TLR 3, 7, 8, and 9 are located in **endosomes**.

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Immunology — MCQs

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