Immunology — MCQs
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All of the following are true regarding Natural Killer (NK) cells, except:
A 35-year-old male stockbroker presents with excessive nervousness, irritability, and a subjective complaint of the office being too hot. Physical examination reveals a goiter and exophthalmia. Laboratory analysis of his blood reveals high antibody titers against the thyroid-stimulating hormone (TSH) receptor. Which of the following is the most likely diagnosis?
What is the location of the antigen-binding site on an antibody?
Which of the following is a type III hypersensitivity reaction?
What is the most sensitive test for antigen detection?
Beta microglobulin is derived from which cell type?
Steatorrhea associated with Giardia infection is seen in which immunoglobulin deficiency?
Which HLA allele is associated with Graves' disease?
What is the primary function of Toll-like receptors (TLRs)?
What does a superantigen cause?
Immunology Indian Medical PG Practice Questions and MCQs
Question 251: All of the following are true regarding Natural Killer (NK) cells, except:
- A. CD16 and CD56 positive
- B. Secrete complement-like substances
- C. Play an important role in eliminating viral-infected cells
- D. None of the above (Correct Answer)
Explanation: **Explanation:** Natural Killer (NK) cells are large granular lymphocytes that form a crucial part of the innate immune system. They provide a rapid response to virally infected cells and tumor cells without prior sensitization. **Why "None of the above" is the correct answer:** All the statements provided (A, B, and C) are factually correct descriptions of NK cell characteristics. Therefore, none of them are "false." * **Option A (CD16 and CD56 positive):** This is the classic phenotypic marker for NK cells. **CD56** (NCAM) is the primary marker used for identification, while **CD16** (FcγRIII) allows NK cells to bind to the Fc portion of IgG, enabling **Antibody-Dependent Cellular Cytotoxicity (ADCC)**. * **Option B (Secrete complement-like substances):** NK cells kill target cells by releasing granules containing **Perforins** and **Granzymes**. Perforins are structurally and functionally similar to the Membrane Attack Complex (MAC) of the complement system; they create pores in the target cell membrane to facilitate apoptosis. * **Option C (Eliminating viral-infected cells):** NK cells are the first line of defense against intracellular pathogens. They identify "stressed" cells that have downregulated **MHC Class I** molecules (a common viral evasion tactic), a mechanism known as the **"Missing Self" hypothesis.** **High-Yield Clinical Pearls for NEET-PG:** * **Origin:** Derived from Common Lymphoid Progenitors (CLP) but do not require the thymus for maturation. * **Cytokine Production:** They are a major source of **IFN-gamma**, which activates macrophages. * **Inhibitory Receptors:** They possess **KIR (Killer Immunoglobulin-like Receptors)** which recognize MHC-I on healthy cells and inhibit the killing action. * **IL-2 and IL-15:** These cytokines are potent stimulators of NK cell proliferation and activation.
Question 252: A 35-year-old male stockbroker presents with excessive nervousness, irritability, and a subjective complaint of the office being too hot. Physical examination reveals a goiter and exophthalmia. Laboratory analysis of his blood reveals high antibody titers against the thyroid-stimulating hormone (TSH) receptor. Which of the following is the most likely diagnosis?
- A. Goodpasture syndrome
- B. Graves' disease (Correct Answer)
- C. Hashimoto disease
- D. Juvenile-onset diabetes mellitus
Explanation: **Explanation:** The clinical presentation of nervousness, heat intolerance, goiter, and exophthalmia (proptosis) is classic for **Graves' disease**, the most common cause of hyperthyroidism. **1. Why Graves' Disease is Correct:** Graves' disease is a **Type II Hypersensitivity** reaction. It is characterized by the production of **Thyroid Stimulating Immunoglobulins (TSI)**. These autoantibodies bind to and activate the **TSH receptors** on thyroid follicular cells, mimicking the action of TSH. This leads to autonomous overproduction of thyroid hormones (T3 and T4) and hypertrophy of the gland (goiter). The exophthalmia is due to antibody-mediated inflammation and glycosaminoglycan deposition in the retro-orbital tissues. **2. Why Other Options are Incorrect:** * **Goodpasture Syndrome:** Also a Type II hypersensitivity, but involves antibodies against the **alpha-3 chain of Type IV collagen** in glomerular and alveolar basement membranes, leading to hemoptysis and hematuria. * **Hashimoto Disease:** The most common cause of hypothyroidism. It involves **Type IV (cell-mediated)** and Type II hypersensitivity, with antibodies against **Thyroid Peroxidase (TPO)** and Thyroglobulin, leading to follicular destruction rather than stimulation. * **Juvenile-onset Diabetes (Type 1 DM):** Primarily a **Type IV hypersensitivity** where T-cells destroy insulin-producing beta cells in the pancreatic islets. **NEET-PG High-Yield Pearls:** * **Mechanism:** Graves' is a unique Type II hypersensitivity because the antibody is **stimulatory** rather than cytotoxic. * **HLA Association:** Strongly associated with **HLA-DR3** and HLA-B8. * **Triad:** Hyperthyroidism, Exophthalmos, and Pretibial Myxedema. * **Lab Profile:** Low TSH, High Free T4/T3, and positive Anti-TSH receptor antibodies.
Question 253: What is the location of the antigen-binding site on an antibody?
- A. Hinge region
- B. Constant region
- C. Variable region (Correct Answer)
- D. Hypervariable region
Explanation: ### Explanation **Correct Option: C. Variable region** The antigen-binding site (also known as the **paratope**) is located at the N-terminal ends of the antibody molecule. It is formed by the association of the **Variable regions** of both the heavy (VH) and light (VL) chains. These regions possess unique amino acid sequences that allow the antibody to specifically recognize and bind to a particular epitope on an antigen. **Analysis of Options:** * **A. Hinge region:** This is a flexible amino acid segment (rich in proline and cysteine) located between the CH1 and CH2 domains. It allows the two antigen-binding arms to move independently but does not bind antigens. * **B. Constant region:** This part of the antibody (Fc fragment) determines the biological properties of the immunoglobulin, such as complement fixation and binding to cell surface receptors. It is identical for all antibodies of the same isotype. * **D. Hypervariable region:** While these are the specific loops *within* the variable region that make direct contact with the antigen (also called Complementarity Determining Regions or CDRs), the standard anatomical location for the binding site is defined as the **Variable region**. In many exams, if both are present, "Variable region" is the broader structural answer, though CDRs are the functional units. **High-Yield Clinical Pearls for NEET-PG:** * **Papain Digestion:** Cleaves the antibody *above* the hinge region into **two Fab fragments** (antigen-binding) and **one Fc fragment**. * **Pepsin Digestion:** Cleaves *below* the hinge region, resulting in one **F(ab')2 fragment** (bivalent) and degraded Fc fragments. * **Isotype Switching:** Changes the Constant region (e.g., IgM to IgG) but the **Variable region remains the same**, ensuring antigen specificity does not change. * **Idiotype:** Determined by the unique structure of the Variable region.
Question 254: Which of the following is a type III hypersensitivity reaction?
- A. Prausnitz-kustner reaction
- B. Contact dermatitis
- C. Arthus reaction (Correct Answer)
- D. Rh incompatibility
Explanation: ### Explanation **Type III Hypersensitivity** is an **immune-complex-mediated** reaction. It occurs when soluble antigen-antibody (IgG or IgM) complexes are not adequately cleared by the reticuloendothelial system, leading to their deposition in tissues (like blood vessel walls, synovial membrane, or glomerular basement membrane). This triggers the classical complement pathway, attracting neutrophils that release lysosomal enzymes, causing tissue damage and vasculitis. **Why Option C is Correct:** The **Arthus reaction** is the classic localized example of Type III hypersensitivity. It occurs when an antigen is injected intradermally into an individual who already has high levels of circulating IgG antibodies. The resulting local immune complexes deposit in small dermal blood vessels, causing edema, hemorrhage, and necrosis. **Analysis of Incorrect Options:** * **A. Prausnitz-Kustner (PK) reaction:** This is a classic test for **Type I (Immediate) Hypersensitivity**. It involves the passive transfer of IgE antibodies via serum from an allergic individual to a non-allergic one. * **B. Contact dermatitis:** This is a **Type IV (Delayed-type) Hypersensitivity** reaction mediated by T-cells (specifically Th1 and CD8+ cells), not antibodies. * **D. Rh incompatibility:** This is a **Type II (Cytotoxic) Hypersensitivity** reaction where antibodies (IgG) target antigens on the surface of red blood cells, leading to their destruction. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hypersensitivity (ACID):** **A**naphlyactic (I), **C**ytotoxic (II), **I**mmune-Complex (III), **D**elayed (IV). * **Systemic Type III examples:** Serum sickness, Systemic Lupus Erythematosus (SLE), Post-streptococcal glomerulonephritis (PSGN), and Rheumatoid Arthritis. * **Key Mediator:** Complement components **C3a, C4a, and C5a** (anaphylatoxins) are crucial in Type III reactions for recruiting neutrophils.
Question 255: What is the most sensitive test for antigen detection?
- A. Radioimmunoassay (RIA) (Correct Answer)
- B. Enzyme-linked immunosorbent assay (ELISA)
- C. Immunofluorescence
- D. Passive hemagglutination
Explanation: **Explanation:** The correct answer is **Radioimmunoassay (RIA)**. **Why RIA is the most sensitive:** Radioimmunoassay (RIA) is a competitive binding assay that utilizes radioactive isotopes (typically Iodine-125) as labels to detect antigens or antibodies. Its extreme sensitivity (capable of detecting concentrations as low as nanograms or picograms) stems from the high precision with which radioactive decay can be measured. In a competitive RIA, a fixed amount of labeled antigen competes with an unlabeled antigen (the patient sample) for a limited number of antibody binding sites. The sensitivity of RIA remains the gold standard benchmark, though it is often replaced in clinical practice by ELISA due to the hazards of radiation. **Analysis of Incorrect Options:** * **B. ELISA:** While highly sensitive and the most commonly used screening test (e.g., for HIV), it is generally considered slightly less sensitive than RIA. However, it is preferred in labs because it uses enzymes instead of radioactive isotopes, making it safer and giving it a longer shelf life. * **C. Immunofluorescence:** This technique uses fluorescent dyes (like fluorescein) to visualize antigens in tissues or cells. While excellent for localization, its sensitivity for quantifying minute amounts of soluble antigen is lower than RIA or ELISA. * **D. Passive Hemagglutination:** This is an agglutination-based test where soluble antigens are coated onto RBCs. It is a semi-quantitative method and is significantly less sensitive than primary binding assays like RIA. **High-Yield Clinical Pearls for NEET-PG:** * **Sensitivity Hierarchy:** RIA > ELISA > Immunofluorescence > Agglutination > Precipitation. * **RIA Principle:** Based on **competitive binding**. * **Application:** RIA is frequently used for measuring hormones (e.g., T3, T4), drugs (digitoxin), and hepatitis B surface antigen (HBsAg). * **Pro-tip:** If the question asks for the "most common" or "preferred" screening test, the answer is usually ELISA. If it asks for the "most sensitive," the answer is RIA.
Question 256: Beta microglobulin is derived from which cell type?
- A. B-cells (Correct Answer)
- B. T-cells
- C. Both
- D. None of the above
Explanation: **Explanation:** **Beta-2 Microglobulin (β2M)** is a low-molecular-weight protein that serves as the light chain component of the **MHC Class I molecule**. While MHC Class I molecules are expressed on the surface of almost all nucleated cells, the primary source of free circulating β2M in the serum is **lymphocytes**, specifically **B-cells**. 1. **Why B-cells (Option A) is correct:** B-lymphocytes have a high rate of membrane turnover. During the synthesis and degradation of MHC Class I complexes, β2M is shed into the serum. In clinical practice, serum β2M levels are used as a reliable marker of B-cell activity and tumor burden. 2. **Why T-cells (Option B) is incorrect:** While T-cells do express MHC Class I (and thus contain β2M), they are not the primary source of the circulating protein compared to the high turnover rate seen in B-lineage cells. 3. **Why Both (Option C) is incorrect:** In the context of competitive exams like NEET-PG, when asked for the "derivation" or primary source, B-cells are the specific answer due to their clinical significance in lymphoproliferative disorders. **Clinical Pearls for NEET-PG:** * **Tumor Marker:** β2M is a crucial prognostic marker for **Multiple Myeloma** and **B-cell Lymphomas**. Higher levels correlate with a higher tumor burden and poorer prognosis. * **Renal Link:** β2M is filtered by the glomerulus but reabsorbed by proximal tubules. Elevated levels can indicate renal failure or tubular damage. * **Dialysis-related Amyloidosis:** In long-term hemodialysis patients, β2M can accumulate and deposit in joints and bones, leading to "Dialysis-associated amyloidosis." * **Structure:** It is encoded by a gene on **Chromosome 15** (unlike the MHC heavy chain, which is on Chromosome 6).
Question 257: Steatorrhea associated with Giardia infection is seen in which immunoglobulin deficiency?
- A. IgE
- B. IgA (Correct Answer)
- C. IgG
- D. IgM
Explanation: **Explanation:** **1. Why IgA is the correct answer:** Secretory IgA (sIgA) is the primary immunoglobulin responsible for mucosal immunity. It acts as the first line of defense in the gastrointestinal tract by preventing the attachment of pathogens to the intestinal epithelium. In patients with **Selective IgA Deficiency** or **Common Variable Immunodeficiency (CVID)**, the lack of mucosal IgA allows *Giardia lamblia* trophozoites to adhere extensively to the duodenal and jejunal mucosa. This leads to villous atrophy and malabsorption, manifesting clinically as chronic diarrhea and **steatorrhea** (fatty stools). **2. Why other options are incorrect:** * **IgE:** Primarily involved in Type I hypersensitivity reactions and defense against helminths (multicellular parasites). It does not play a significant role in preventing protozoal adhesion. * **IgG:** The most abundant systemic antibody, crucial for opsonization and neutralizing toxins in the blood and tissues, but it is not the dominant antibody on mucosal surfaces. * **IgM:** The first antibody produced during a primary immune response. While it can be secreted onto mucosa in small amounts (especially as a compensatory mechanism in IgA deficiency), it is not the primary diagnostic association for Giardia-induced steatorrhea. **3. NEET-PG High-Yield Pearls:** * **Selective IgA Deficiency** is the most common primary immunodeficiency. * **Clinical Association:** Patients are often asymptomatic but have an increased risk of respiratory infections, GI infections (*Giardia*), and **Anaphylaxis during blood transfusions** (due to anti-IgA antibodies). * **Giardia Diagnosis:** Stool microscopy (cysts/trophozoites) is standard, but the **Entero-test (String test)** or duodenal aspiration may be used if stool samples are negative. * **Drug of Choice:** Metronidazole or Tinidazole.
Question 258: Which HLA allele is associated with Graves' disease?
- A. DR4
- B. B27
- C. B8 (Correct Answer)
- D. DQ8
Explanation: **Explanation:** The association between the Human Leukocyte Antigen (HLA) system and autoimmune diseases is a high-yield topic for NEET-PG. Graves' disease, an autoimmune hyperthyroidism caused by stimulating antibodies against the TSH receptor, is strongly linked to **HLA-B8** and **HLA-DR3**. These alleles are often inherited together as part of a conserved extended haplotype, predisposing individuals to various organ-specific autoimmune conditions. **Analysis of Options:** * **HLA-B8 (Correct):** This Class I MHC allele is classically associated with Graves' disease, Myasthenia gravis, and Celiac disease. * **HLA-DR4:** This is primarily associated with **Rheumatoid Arthritis**, Type 1 Diabetes Mellitus, and Pemphigus vulgaris. (Mnemonic: *DR4 for 4-walled joints*). * **HLA-B27:** This is the most famous association in PG exams, linked to **Seronegative Spondyloarthropathies** (PAIR: Psoriatic arthritis, Ankylosing spondylitis, Inflammatory bowel disease-associated arthritis, and Reactive arthritis). * **HLA-DQ8:** This allele, along with DQ2, is specifically associated with **Celiac disease**. **High-Yield Clinical Pearls for NEET-PG:** * **HLA-DR3 Association:** If HLA-B8 is not in the options, look for **DR3**, as they are frequently linked in Graves' disease and Systemic Lupus Erythematosus (SLE). * **Narcolepsy:** Associated with **HLA-DR2/DQB1*0602** (Strongest known HLA association). * **Multiple Sclerosis:** Associated with **HLA-DR2**. * **Behcet’s Disease:** Associated with **HLA-B51**.
Question 259: What is the primary function of Toll-like receptors (TLRs)?
- A. Activation of oncogenes by viruses and playing a role in cancer.
- B. Regulation of glucose homeostasis.
- C. Facilitating the entry of retroviruses into immune cells.
- D. Recognition of microbial products to initiate acute inflammation. (Correct Answer)
Explanation: ### Explanation **1. Why Option D is Correct:** Toll-like receptors (TLRs) are a class of **Pattern Recognition Receptors (PRRs)** that play a pivotal role in the **Innate Immune System**. They are designed to recognize highly conserved molecular structures known as **Pathogen-Associated Molecular Patterns (PAMPs)** found on microbes. When a TLR binds to its specific ligand (e.g., LPS, flagellin, or viral RNA), it triggers a signaling cascade (often via the **MyD88 pathway**) that activates transcription factors like **NF-κB**. This leads to the production of pro-inflammatory cytokines and chemokines, effectively initiating **acute inflammation** and bridging the gap to the adaptive immune response. **2. Why Other Options are Incorrect:** * **Option A:** While some viruses (like HPV or EBV) can activate oncogenes, this is a pathological process of carcinogenesis, not the physiological function of TLRs. * **Option B:** Glucose homeostasis is primarily regulated by hormones like insulin and glucagon; TLRs are sensory proteins for immunity, not metabolic regulation. * **Option C:** Retroviruses (like HIV) enter cells via specific receptors and co-receptors (CD4, CCR5, CXCR4). TLRs recognize the virus to *fight* infection, rather than facilitating its entry. **3. High-Yield Clinical Pearls for NEET-PG:** * **TLR-4:** Recognizes **Lipopolysaccharide (LPS)** of Gram-negative bacteria (Crucial for Septic Shock). * **TLR-2:** Recognizes Peptidoglycan (Gram-positive bacteria). * **TLR-3:** Recognizes double-stranded RNA (dsRNA). * **TLR-5:** Recognizes Flagellin. * **TLR-7 & 8:** Recognize single-stranded RNA (ssRNA). * **TLR-9:** Recognizes unmethylated CpG DNA. * **Location:** TLRs 1, 2, 4, 5, and 6 are on the **plasma membrane**; TLRs 3, 7, 8, and 9 are located in **endosomes**.
Question 260: What does a superantigen cause?
- A. Polyclonal activation of T-cells (Correct Answer)
- B. Stimulation of B cells
- C. Enhancement of phagocytosis
- D. Activation of complement
Explanation: ### Explanation **Correct Answer: A. Polyclonal activation of T-cells** **Mechanism of Action:** Superantigens (SAgs) are unique proteins that bypass the standard rules of antigen processing. Unlike conventional antigens, which are processed into peptides and presented within the peptide-binding groove of MHC Class II molecules, superantigens bind **externally** to the **Vβ region of the T-cell receptor (TCR)** and the **alpha chain of MHC Class II** on antigen-presenting cells. By cross-linking these receptors regardless of the antigen specificity, superantigens can activate up to **20% of the body’s naive T-cells** simultaneously. This massive, **polyclonal activation** leads to a "cytokine storm" (excessive release of IL-1, IL-2, TNF-α, and IFN-γ), resulting in systemic toxicity and shock. **Why Other Options are Incorrect:** * **B. Stimulation of B cells:** Superantigens primarily target T-cells. While B-cells may be indirectly affected by the cytokine milieu, they are not the primary target of SAg binding. * **C. Enhancement of phagocytosis:** SAgs do not act as opsonins; in fact, the overwhelming inflammatory response often impairs coordinated immune functions like effective phagocytosis. * **D. Activation of complement:** Complement activation is typically triggered by the classical (antibody-antigen), lectin, or alternative pathways, not by the direct binding mechanism of superantigens. **High-Yield Clinical Pearls for NEET-PG:** * **Endogenous Superantigens:** Integrated retroviral proteins (e.g., Mouse Mammary Tumor Virus). * **Exogenous Superantigens:** * *Staphylococcus aureus:* **TSST-1** (Toxic Shock Syndrome), Enterotoxins (Food poisoning), Exfoliatin (Scalded Skin Syndrome). * *Streptococcus pyogenes:* **SpeA and SpeC** (Streptococcal Toxic Shock-like Syndrome). * **Key Distinction:** Conventional antigens activate <0.01% of T-cells; Superantigens activate up to 20-25%.
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Cells and Organs of Immune System
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Innate Immunity
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Adaptive Immunity
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Antigens and Antibodies
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Major Histocompatibility Complex
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Complement System
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Cytokines and Chemokines
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Hypersensitivity Reactions
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Autoimmunity and Autoimmune Diseases
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Immunodeficiency Disorders
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Transplantation Immunology
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Tumor Immunology
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