Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 411: Which of the following is an example of a Type II hypersensitivity reaction?

A. Post-streptococcal glomerulonephritis (PSGN)
B. Systemic lupus erythematosus (SLE)
C. Serum sickness
D. Goodpasture syndrome (Correct Answer)

Explanation: **Explanation:** **Type II Hypersensitivity** (Cytotoxic) involves **IgG or IgM** antibodies directed against specific antigens located on **cell surfaces or in the extracellular matrix**. This leads to cell destruction or tissue damage via the complement system, opsonization, or antibody-dependent cellular cytotoxicity (ADCC). **Why Goodpasture Syndrome is Correct:** In Goodpasture syndrome, autoantibodies are formed against the **alpha-3 chain of Type IV collagen** found in the glomerular and alveolar basement membranes. This is a classic "tissue-specific" reaction where antibodies bind directly to the target organ, resulting in a characteristic **linear immunofluorescence** pattern on biopsy. **Why the Other Options are Incorrect:** * **A, B, and C (PSGN, SLE, and Serum Sickness):** These are all examples of **Type III Hypersensitivity**. Unlike Type II, Type III involves the formation of **soluble antigen-antibody (immune) complexes** that circulate in the blood and deposit in various tissues (like joints, kidneys, or vessels), causing systemic inflammation and a "lumpy-bumpy" (granular) immunofluorescence pattern. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Type II:** "The **2** Gs" – **G**oodpasture and **G**rave’s disease (though Grave's is specifically Type II-non-cytotoxic/stimulatory). * **Key Distinction:** Type II is **Antibody-mediated** (fixed antigen); Type III is **Immune-complex mediated** (circulating antigen). * **Other Type II Examples:** Rheumatic fever, Myasthenia Gravis, Erythroblastosis fetalis, and Autoimmune Hemolytic Anemia (AIHA). * **Exam Tip:** If the question mentions "Linear IgG deposits" on a renal biopsy, always think Type II (Goodpasture). If it mentions "Granular deposits," think Type III (PSGN/SLE).

Question 412: A patient develops wheeze and hemolysis 48 hours after receiving penicillin, with no prior history of drug allergy. Antibody for penicillin is positive. What type of hypersensitivity reaction is this?

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

Explanation: **Explanation:** The correct answer is **Type II Hypersensitivity (Cytotoxic)**. In this scenario, penicillin acts as a **hapten**, binding to the surface of red blood cells (RBCs). This modifies the cell membrane, making it appear foreign to the immune system. Specific IgG or IgM antibodies then bind to these penicillin-coated RBCs, leading to complement activation or phagocytosis, resulting in **drug-induced hemolytic anemia**. **Why other options are incorrect:** * **Type I (Immediate):** Mediated by IgE and mast cell degranulation. It typically occurs within minutes to hours of exposure (anaphylaxis, urticaria). While the patient has a wheeze, the presence of hemolysis and the 48-hour timeline point toward a cytotoxic mechanism rather than classical IgE-mediated anaphylaxis. * **Type III (Immune Complex):** Involves the deposition of antigen-antibody complexes in tissues (e.g., Serum Sickness). It does not typically present with direct hemolysis of RBCs. * **Type IV (Delayed):** T-cell mediated and usually presents as contact dermatitis or Stevens-Johnson Syndrome (SJS) 48–72 hours later. It does not involve antibodies or acute hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Penicillin is unique:** It can cause all four types of hypersensitivity reactions. * **Type II Mechanism:** Remember the mnemonic **"Cy-2-toxic"**—antibodies (IgG/IgM) target antigens on specific cell surfaces. * **Coombs Test:** Drug-induced Type II hemolysis will typically show a **Positive Direct Antiglobulin Test (DAT/Direct Coombs)**. * **Other Type II Examples:** Goodpasture syndrome, Myasthenia Gravis, Rheumatic fever, and Erythroblastosis fetalis.

Question 413: What is true about monoclonal antibodies?

A. Produced by hybridoma technology
B. Used for blood grouping
C. Requires in small quantity
D. All of the above (Correct Answer)

Explanation: **Explanation:** Monoclonal antibodies (mAbs) are highly specific antibodies derived from a single clone of B-cells, ensuring they all recognize the same epitope on an antigen. * **Option A (Hybridoma Technology):** This is the gold standard for mAb production, developed by Köhler and Milstein. It involves fusing a specific antibody-producing B-lymphocyte with a cancerous myeloma cell. The resulting "hybridoma" cell possesses the longevity of the cancer cell and the antibody-producing capability of the B-cell. * **Option B (Blood Grouping):** In modern clinical laboratories, mAbs are the primary reagents used for ABO and Rh blood typing. They offer high specificity and eliminate the batch-to-batch variability seen with polyclonal antisera, leading to more accurate results. * **Option C (Small Quantity):** Because mAbs are highly potent and possess extreme specificity for their target antigen, they are required in significantly smaller quantities compared to polyclonal antibodies to achieve the desired diagnostic or therapeutic effect. Since all three statements are characteristic features of monoclonal antibodies, **Option D (All of the above)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Köhler and Milstein:** Awarded the Nobel Prize (1984) for Hybridoma technology. * **Selection Medium:** **HAT Medium** (Hypoxanthine, Aminopterin, Thymidine) is used to select hybridoma cells; only fused cells survive. * **Therapeutic Examples:** * *Infliximab:* Anti-TNF-α (Rheumatoid Arthritis). * *Rituximab:* Anti-CD20 (Non-Hodgkin Lymphoma). * *Trastuzumab:* Anti-HER2 (Breast Cancer). * **Nomenclature:** Drugs ending in **-omab** are murine (mouse), **-ximab** are chimeric, **-zumab** are humanized, and **-umab** are fully human.

Question 414: T-helper cells (TH2) initiate which type of immunity?

A. Cell-mediated immunity
B. Humoral immunity (Correct Answer)
C. Immunity by activation of cytotoxic T-cells
D. None of the above

Explanation: ### Explanation The differentiation of CD4+ T-helper (Th) cells into specific subsets determines the nature of the immune response. **Why Humoral Immunity is Correct:** **Th2 cells** are the primary drivers of **Humoral Immunity** (antibody-mediated). Upon activation, Th2 cells secrete cytokines such as **IL-4, IL-5, IL-10, and IL-13**. * **IL-4** induces B-cell proliferation and isotype switching to **IgE**. * **IL-5** activates eosinophils and promotes switching to **IgA**. This pathway is essential for defense against extracellular parasites (helminths) and is central to allergic reactions. **Analysis of Incorrect Options:** * **Option A (Cell-mediated immunity):** This is primarily mediated by **Th1 cells**. Th1 cells secrete **IFN-γ and IL-2**, which activate macrophages and promote intracellular pathogen clearance. * **Option C (Activation of cytotoxic T-cells):** This is also a feature of the Th1 response. IL-2 produced by Th1 cells provides the necessary signal for the proliferation of CD8+ Cytotoxic T-lymphocytes (CTLs), which kill virally infected or tumor cells. **NEET-PG High-Yield Pearls:** * **The "Rule of 4":** IL-4 induces Th2; IL-12 (from macrophages) induces Th1. * **Cross-regulation:** IFN-γ (Th1) inhibits Th2 proliferation, while IL-10 (Th2) inhibits Th1 cytokine production. * **Clinical Correlation:** * **Tuberculoid Leprosy:** Strong **Th1** response (contained infection). * **Lepromatous Leprosy:** Predominant **Th2** response (disseminated infection due to ineffective humoral response against intracellular *M. leprae*). * **Mnemonic:** Th**1** is for **1**nside (intracellular/cell-mediated); Th**2** is for **2** (B-cells/antibodies/humoral).

Question 415: Which of the following is an example of Type IV hypersensitivity?

A. Farmer's lung
B. Contact hypersensitivity (Correct Answer)
C. Immediate hypersensitivity
D. Myasthenia gravis

Explanation: **Explanation:** **Type IV Hypersensitivity**, also known as **Delayed-Type Hypersensitivity (DTH)**, is unique because it is cell-mediated (T-cells) rather than antibody-mediated. It typically manifests 48–72 hours after exposure. **Why Option B is correct:** **Contact hypersensitivity** (e.g., Nickel allergy, Poison Ivy) occurs when small molecules called haptens penetrate the skin and bind to self-proteins. These are captured by Langerhans cells (APCs) and presented to **CD4+ Th1 cells**. Upon re-exposure, these sensitized T-cells release cytokines (IFN-γ), activating macrophages and causing local tissue damage and dermatitis. **Why the other options are incorrect:** * **A. Farmer’s Lung:** This is an example of **Type III Hypersensitivity** (Immune-complex mediated). It involves the inhalation of actinomycete antigens which react with IgG antibodies, leading to complex deposition in the alveoli. * **C. Immediate Hypersensitivity:** This is **Type I Hypersensitivity**, mediated by **IgE** antibodies and mast cell degranulation (e.g., Anaphylaxis, Atopy). * **D. Myasthenia Gravis:** This is **Type II Hypersensitivity** (Antibody-dependent). Autoantibodies block or destroy Acetylcholine receptors at the neuromuscular junction. **NEET-PG High-Yield Pearls:** * **Mnemonic for Types:** **ACID** (Type I: **A**naphylactic; Type II: **C**ytotoxic; Type III: **I**mmune-complex; Type IV: **D**elayed). * **Key Type IV Examples:** Mantoux Test (Tuberculin), Lepromin test, Sarcoidosis, and Granuloma formation. * **Cells involved:** T-lymphocytes and Macrophages (No antibodies/complement involved).

Question 416: Which of the following statements best describes a hapten?

A. Haptens activate T cells
B. Penicillin is a hapten (Correct Answer)
C. Haptens do not react with specific antibody
D. Haptens bind the major histocompatibility complex

Explanation: **Explanation:** A **hapten** is a small molecule that is **antigenic but not immunogenic**. This means it can react specifically with antibodies once they are formed, but it cannot induce an immune response on its own because of its low molecular weight. To become immunogenic, a hapten must bind to a larger protein molecule called a **carrier**. **Why Option B is correct:** **Penicillin** is a classic clinical example of a hapten. By itself, the penicillin molecule is too small to trigger an immune response. However, in susceptible individuals, penicillin (or its degradation products like penicilloic acid) binds to endogenous serum proteins (carriers). This hapten-carrier complex is recognized as foreign, leading to the production of IgE antibodies and potentially causing Type I hypersensitivity (anaphylaxis). **Analysis of Incorrect Options:** * **Option A:** Haptens cannot activate T cells or B cells independently. They lack the complexity and size required to cross-link receptors or be processed for presentation. * **Option C:** This is incorrect. While haptens cannot *induce* an immune response, they **do react** with pre-formed specific antibodies (Antigenicity is present). * **Option D:** Haptens do not bind directly to MHC molecules. Only processed peptide fragments (from immunogens) are presented via MHC to T-cell receptors. **NEET-PG High-Yield Pearls:** * **Landsteiner’s Experiment:** Karl Landsteiner discovered haptens by showing that small chemical groups (like dinitrobenzene) only caused antibody production when conjugated to a protein. * **Clinical Relevance:** Many drugs (Hydralazine, Procainamide) and toxins (Urushiol from Poison Ivy) act as haptens, leading to drug-induced lupus or contact dermatitis. * **Formula:** Hapten + Carrier = Complete Antigen (Immunogen).

Question 417: HLA is the major histocompatibility antigen determining homograft rejection. This falls under which category of specificity?

A. Antigen specificity
B. Species specificity
C. Isospecificity (Correct Answer)
D. Auto specificity

Explanation: ### Explanation **1. Why Isospecificity is Correct:** Isospecificity (also known as **allospecificity**) refers to antigens that are present in some, but not all, members of the same species. **HLA (Human Leukocyte Antigen)** molecules are the classic example. While all humans belong to the same species, the specific alleles of HLA vary between individuals (except identical twins). When a graft is transplanted between two genetically different members of the same species (**homograft/allograft**), the recipient’s immune system recognizes the donor's HLA as "non-self," leading to rejection. **2. Analysis of Incorrect Options:** * **Antigen Specificity:** This is a general term referring to the ability of an antibody or T-cell receptor to distinguish between different chemical structures (epitopes). It does not describe the distribution of antigens within a population. * **Species Specificity:** These are antigens found in all members of a particular species but not in others (e.g., human serum albumin vs. bovine serum albumin). These are responsible for the rejection of **xenografts** (between different species). * **Auto Specificity:** This refers to "self-antigens." Normally, the body is tolerant to these, but in autoimmune diseases, the immune system attacks its own tissues (e.g., DNA in SLE). **3. High-Yield Clinical Pearls for NEET-PG:** * **HLA Class I (A, B, C):** Found on all nucleated cells; recognized by CD8+ T-cells. * **HLA Class II (DR, DQ, DP):** Found only on Antigen-Presenting Cells (APCs); recognized by CD4+ T-cells. * **MHC Mapping:** In humans, the MHC gene complex is located on the **short arm of Chromosome 6**. * **Direct vs. Indirect Allorecognition:** Direct recognition (recipient T-cells recognize donor MHC) is the primary driver of **acute cellular rejection**.

Question 418: The indirect Coombs test is used to detect which of the following?

A. Antibodies bound to red blood cells
B. Free antibodies in the serum (Correct Answer)
C. Agglutinated red blood cells
D. None of the above

Explanation: The **Coombs test** (Antiglobulin test) is a cornerstone of immunohematology used to detect non-agglutinating (incomplete) IgG antibodies. ### **Explanation of the Correct Answer** **Option B (Free antibodies in the serum)** is correct because the **Indirect Coombs Test (ICT)** is designed to detect **unbound** antibodies circulating in the patient’s serum. In this procedure, the patient’s serum is incubated with known O-positive reagent RBCs. If specific antibodies are present in the serum, they will bind to these RBCs in vitro. After washing, Coombs reagent (antihuman globulin) is added to bridge these sensitized cells, resulting in visible agglutination. ### **Analysis of Incorrect Options** * **Option A:** Antibodies already **bound to red blood cells** are detected by the **Direct Coombs Test (DCT)**. This is used when the sensitization has already occurred *in vivo* (inside the body). * **Option C:** Agglutinated red blood cells are the *result* of a positive test, not the target of detection. The test is specifically used to identify "incomplete" antibodies that are too small to cause agglutination on their own. ### **NEET-PG High-Yield Pearls** * **Clinical Uses of ICT:** 1. **Antenatal Screening:** To check for Rh-antibodies in an Rh-negative mother (detecting potential Rh incompatibility). 2. **Cross-matching:** To check for compatibility before blood transfusion. * **Clinical Uses of DCT:** 1. **Hemolytic Disease of the Newborn (HDN):** Testing the baby’s cord blood. 2. **Autoimmune Hemolytic Anemia (AIHA):** Detecting autoantibodies on the patient's own RBCs. 3. **Drug-induced hemolysis.** * **The Reagent:** Coombs reagent is **Antihuman Globulin (AHG)**, typically produced by immunizing rabbits against human IgG or complement.

Question 419: Which complement component is primarily synthesized by the liver?

A. C1
B. C5
C. C3 (Correct Answer)
D. C4

Explanation: **Explanation:** The complement system consists of over 30 plasma and cell-surface proteins. The **liver** is the primary site for the synthesis of the majority of complement components, including C3, C4, C5, C6, C7, C8, C9, and Factor B. **Why C3 is the correct answer:** **C3** is the most abundant complement protein in the serum and serves as the central hub where the classical, lectin, and alternative pathways converge. While many components are produced in the liver, C3 is the **primary** and most quantitatively significant component synthesized by **hepatocytes**. It is also an acute-phase reactant, meaning its hepatic production increases during inflammation. **Analysis of Incorrect Options:** * **C1:** Unlike most other components, **C1 (specifically C1q, C1r, and C1s)** is primarily synthesized by **intestinal epithelial cells** and macrophages, rather than hepatocytes. * **C5 and C4:** While these are also synthesized in the liver, C3 is the "best" answer in the context of NEET-PG because it is the most abundant, the most clinically significant (central component), and the classic representative of hepatic complement synthesis in standard textbooks. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Most are made in the liver, EXCEPT **C1** (Intestinal epithelium) and **Factor D** (Adipose tissue). * **C3 Deficiency:** Leads to severe, recurrent pyogenic infections (e.g., *S. pneumoniae, H. influenzae*) because it impairs opsonization and the formation of the Membrane Attack Complex (MAC). * **C3a and C5a:** Known as **Anaphylatoxins**; they trigger mast cell degranulation. * **C3b:** The primary **Opsonin** (facilitates phagocytosis).

Question 420: Tube agglutination test is used for the serological diagnosis of which condition?

A. Enteric fever (Correct Answer)
B. Rabies antigen
C. HIV
D. Syphilis

Explanation: **Explanation:** The **Tube Agglutination Test** is a classical serological technique where antibodies in the patient's serum react with particulate antigens (like bacteria) in a test tube, resulting in visible clumping or sedimentation. **Why Enteric Fever is Correct:** The most iconic example of a tube agglutination test is the **Widal Test**, used to diagnose Enteric (Typhoid) fever. It detects antibodies against the *Salmonella Typhi* and *Paratyphi* antigens (O and H). In this test, serial dilutions of the patient's serum are mixed with specific bacterial suspensions in Dreyer’s (for H antigen) or Felix (for O antigen) tubes. A rising titer or a significant single titer (usually >1:80 for O and >1:160 for H) indicates infection. **Why Other Options are Incorrect:** * **Rabies Antigen:** Diagnosis is typically made via **Direct Fluorescent Antibody (DFA)** testing of brain tissue or skin biopsies, or by detecting Negri bodies. Agglutination is not used. * **HIV:** Screening is performed using **ELISA** (an enzyme immunoassay), and confirmation is done via **Western Blot** (detecting specific proteins) or PCR for viral load. * **Syphilis:** Screening uses **flocculation tests** (a variation of precipitation, not tube agglutination) like **VDRL** and **RPR**. Confirmation uses specific treponemal tests like TPHA or FTA-ABS. **High-Yield Clinical Pearls for NEET-PG:** * **Standard Tube Agglutination Tests:** Widal (Enteric fever), Standard Agglutination Test (SAT) for **Brucellosis**, and the Weil-Felix test (for **Rickettsial** infections). * **Prozone Phenomenon:** A false-negative result in tube agglutination due to antibody excess; solved by diluting the serum. * **Widal Timing:** It usually becomes positive only after the **first week** of fever (best results in the 2nd and 3rd weeks).

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Cells and Organs of Immune System

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Innate 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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Immunology — MCQs

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