Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 761: Which of the following statements is true?

A. Paul Bunnell test is used to diagnose measles
B. Rose Waaler test is a complement fixation test
C. Indirect hemagglutination test is less sensitive than gel diffusion test
D. Antigen-antibody reaction cannot occur in the absence of electrolytes (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Antigen-antibody reaction cannot occur in the absence of electrolytes** The interaction between an antigen (Ag) and an antibody (Ab) occurs in three stages. The **primary stage** involves the initial binding via non-covalent forces (hydrogen bonds, Van der Waals forces, and electrostatic forces). For these forces to operate and for the subsequent visible **secondary stage** (like precipitation or agglutination) to occur, the presence of **electrolytes** (at physiological pH and ionic strength) is mandatory. Electrolytes help reduce the electrostatic repulsion between similarly charged particles, allowing them to come close enough to form a stable lattice. #### Analysis of Incorrect Options: * **A. Paul Bunnell test:** This is a heterophile agglutination test used to diagnose **Infectious Mononucleosis** (caused by Epstein-Barr Virus), not measles. It detects antibodies that agglutinate sheep RBCs. * **B. Rose Waaler test:** This is a **passive hemagglutination test** used to detect Rheumatoid Factor (IgM against the Fc fragment of IgG). It is not a complement fixation test. * **C. Indirect Hemagglutination (IHA):** This test is significantly **more sensitive** than gel diffusion (precipitation). In immunology, agglutination tests are generally more sensitive than precipitation tests because the large size of the carrier particle (like an RBC) makes the reaction visible at much lower concentrations of Ag/Ab. #### High-Yield Clinical Pearls for NEET-PG: * **Prozone Phenomenon:** False negative results in agglutination/precipitation tests due to **antibody excess**. * **Sensitivity Hierarchy:** ELISA/RIA > Agglutination > Precipitation (Gel diffusion). * **Coombs Test:** An example of an indirect agglutination test used to detect Rh antibodies. * **Lattice Hypothesis:** Proposed by Marrack; it states that multivalent antigens and antibodies must form a cross-linked network (lattice) for a visible reaction to occur.

Question 762: What is the most potent stimulator of naive T cells?

A. T cells
B. Mature dendritic cells (Correct Answer)
C. Epithelial cells
D. Macrophages

Explanation: **Explanation:** The activation of a **naive T cell** requires two specific signals: the recognition of an antigen-MHC complex and a potent co-stimulatory signal (primarily B7-1/B7-2 binding to CD28). **Mature Dendritic Cells (DCs)** are the most potent professional Antigen-Presenting Cells (APCs) because they constitutively express high levels of **MHC Class II** and **co-stimulatory molecules (B7)**. Unlike other APCs, mature DCs are uniquely capable of migrating from peripheral tissues to secondary lymphoid organs to present antigens to naive T cells, making them the primary initiators of the adaptive immune response. **Analysis of Incorrect Options:** * **T cells:** T cells are the recipients of the stimulus, not the stimulators. They require activation by APCs to differentiate into effector cells. * **Epithelial cells:** These are non-professional APCs. While they can express MHC Class I, they lack the necessary co-stimulatory molecules to activate naive T cells and generally cannot initiate a primary immune response. * **Macrophages:** While they are professional APCs, they are less potent than DCs for *naive* cells. Macrophages primarily present antigens to already *activated* effector T cells at the site of infection to enhance microbicidal activity. **High-Yield Facts for NEET-PG:** * **Langerhans cells** are immature dendritic cells found in the epidermis; they become "mature" and potent stimulators only after migrating to regional lymph nodes. * **B cells** are also professional APCs but primarily present antigens to Helper T cells to facilitate antibody production (humoral immunity). * **Signal 1** = TCR + MHC-Peptide; **Signal 2** = CD28 (T cell) + B7/CD80/86 (APC). Absence of Signal 2 leads to **anergy**.

Question 763: Which one of the following is the best neutrophil and macrophage attractant?

A. C5a (Correct Answer)
B. Variable region of the heavy chain of IgG
C. J chain
D. HLA-A

Explanation: ### Explanation **Correct Option: A. C5a** C5a is a potent **anaphylatoxin** and the most powerful chemotactic factor derived from the complement system. It acts as a "chemical beacon" that attracts neutrophils, macrophages, monocytes, and eosinophils to the site of inflammation (chemotaxis). It also increases vascular permeability and triggers mast cell degranulation. While C3a and C4a are also anaphylatoxins, they lack the significant chemotactic potency of C5a. **Analysis of Incorrect Options:** * **B. Variable region of the heavy chain of IgG:** This region is responsible for **antigen binding specificity** (Fab portion). It does not possess chemotactic properties. The *Fc region* of IgG is what interacts with phagocyte receptors for opsonization, but it is not a primary attractant. * **C. J chain (Joining chain):** This is a glycoprotein found in **polymeric immunoglobulins** (Secretory IgA and IgM). Its primary role is to hold the subunits together and facilitate the transport of IgA across mucosal surfaces. * **D. HLA-A:** This is a **MHC Class I** molecule found on all nucleated cells. Its function is to present endogenous antigens to CD8+ T-cells, not to attract phagocytes. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Chemotactic Agents:** Remember the "Big Four": **C5a**, **Interleukin-8 (IL-8)**, **Leukotriene B4 (LTB4)**, and **Bacterial products** (e.g., N-formyl methionine). * **Opsonization:** While C5a attracts cells, **C3b** is the primary complement component responsible for opsonization (tagging for ingestion). * **Deficiency:** A deficiency in the C5-C9 components (Membrane Attack Complex) predisposes individuals to recurrent *Neisseria* infections.

Question 764: Which of the following statements about NK cells is INCORRECT?

A. Mediates type IV hypersensitivity (Correct Answer)
B. Kills virus-infected cells
C. They are large granular lymphocytes
D. Releases small cytoplasmic granules of proteins called perforin and granzyme

Explanation: ### Explanation **Correct Answer: A. Mediates type IV hypersensitivity** **Why Option A is Incorrect (The Right Choice):** Type IV (Delayed-type) hypersensitivity is primarily mediated by **T-lymphocytes** (specifically CD4+ Th1 cells and CD8+ T cells), not NK cells. In Type IV reactions, sensitized T-cells release cytokines that activate macrophages or cause direct cytotoxicity. NK cells are part of the **innate immune system**, whereas Type IV hypersensitivity is a component of the **adaptive immune response**. **Analysis of Other Options:** * **Option B (Kills virus-infected cells):** This is a primary function of NK cells. They identify and destroy cells that have downregulated MHC-I molecules (the "missing self" hypothesis), a common tactic used by viruses to evade CD8+ T-cells. * **Option C (Large granular lymphocytes):** Morphologically, NK cells are identified as large granular lymphocytes (LGLs). They are larger than resting B or T cells and contain prominent cytoplasmic granules. * **Option D (Releases perforin and granzyme):** NK cells utilize the perforin-granzyme pathway to induce apoptosis in target cells. Perforin creates pores in the target cell membrane, allowing granzymes (proteases) to enter and trigger the caspase cascade. **High-Yield Clinical Pearls for NEET-PG:** * **Markers:** NK cells are typically identified by the presence of **CD56** and **CD16** (FcγRIII) and the **absence of CD3**. * **Antibody-Dependent Cellular Cytotoxicity (ADCC):** NK cells use their CD16 receptor to bind to the Fc portion of IgG-coated target cells, leading to cell lysis. * **Cytokine Production:** They are a major source of **IFN-gamma**, which activates macrophages. * **MHC Restriction:** Unlike T-cells, NK cells are **not MHC-restricted**; they do not require antigen presentation via MHC to function.

Question 765: A false-negative tuberculin test is seen in all the following conditions EXCEPT:

A. After a 4-6 week history of measles attack
B. Immunodeficiency state
C. Miliary tuberculosis
D. Atypical mycobacterial infection (Correct Answer)

Explanation: The Tuberculin Skin Test (Mantoux test) is a classic example of a **Type IV (Delayed-Type) Hypersensitivity reaction**, mediated by T-lymphocytes. A false-negative result occurs when the body fails to mount an adequate cell-mediated immune (CMI) response despite being infected with *M. tuberculosis*. ### **Why "Atypical mycobacterial infection" is the correct answer:** Atypical mycobacteria (Non-Tuberculous Mycobacteria or NTM) share several common antigens with *M. tuberculosis*. Therefore, infection with NTM typically causes **cross-reactivity**, leading to a **false-positive** result (or a small induration), rather than a false-negative. ### **Explanation of Incorrect Options (Causes of False-Negatives):** * **After a 4-6 week history of measles attack:** Viral infections like measles, mumps, and chickenpox are known to cause transient **anergy** (suppression of CMI), leading to false-negative results. * **Immunodeficiency state:** Conditions such as HIV/AIDS, Hodgkin’s lymphoma, or treatment with immunosuppressants (steroids, anti-TNF agents) impair T-cell function, preventing the induration. * **Miliary tuberculosis:** In overwhelming or disseminated TB, the body’s immune system is "overloaded" or exhausted, leading to a lack of localized skin response (anergy). ### **High-Yield Clinical Pearls for NEET-PG:** * **Reading the test:** The result is read after **48–72 hours**. Only the **induration** (palpable hardness) is measured, not the erythema (redness). * **The "Window Period":** It takes 2–10 weeks after initial infection for the Mantoux test to become positive. Testing during this window yields a false-negative. * **Storage:** PPD (Purified Protein Derivative) should be stored at 2–8°C and protected from light to prevent degradation. * **BCG Vaccine:** Can cause a false-positive, though the induration is usually smaller and wanes over time.

Question 766: In contact dermatitis, which cells play a major role?

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

Explanation: **Explanation:** Contact dermatitis is a classic example of **Type IV Hypersensitivity (Delayed-type hypersensitivity)** [4]. The pathogenesis is mediated by cellular immunity rather than antibodies [1]. 1. **Why T-cells are correct:** The reaction occurs in two phases: sensitization and elicitation [1]. Upon re-exposure to an allergen (hapten), memory **T-cells (specifically CD4+ Th1 and CD8+ cytotoxic T-cells)** are activated [2], [3]. They release cytokines like IFN-γ and TNF-α, which induce inflammation and keratinocyte damage. Since T-cells are the primary effectors driving the immune response, they are the "major" cells involved. 2. **Why other options are incorrect:** * **B-cells:** These are involved in antibody-mediated (Type I, II, and III) hypersensitivity [4]. Contact dermatitis is cell-mediated and independent of antibodies. * **Langerhans cells:** While these are the professional Antigen Presenting Cells (APCs) in the skin that initiate the response by carrying the antigen to regional lymph nodes, they are "initiators" rather than the primary "effectors" of the clinical reaction. * **Macrophages:** These act as downstream effector cells that cause tissue damage after being recruited and activated by T-cell cytokines, but they are not the specific recognition cells of the reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Time frame:** Type IV reactions typically take **48–72 hours** to manifest (hence "delayed") [4]. * **Common triggers:** Nickel (jewelry), Poison ivy (Urushiol), Neomycin, and Latex. * **Diagnostic Test:** The **Patch Test** is the gold standard for identifying the causative allergen in contact dermatitis. * **Key Cytokine:** **IFN-γ** is the hallmark cytokine of the Th1 response in Type IV hypersensitivity.

Question 767: Which of the following statements about complement is false?

A. Group of proteins normally found in serum
B. Constitutes about 20% of normal serum proteins (Correct Answer)
C. Heat labile
D. Species non-specific

Explanation: ### Explanation **1. Why Option B is the Correct (False) Statement:** The complement system consists of a group of approximately 30 proteins, but they constitute only about **10% of the total serum globulin fraction** (or roughly **5% of total serum proteins**). Stating that they make up 20% is a significant overestimation, making this the false statement. **2. Analysis of Other Options:** * **Option A (Group of proteins normally found in serum):** This is true. Complement proteins are synthesized primarily by the liver (and also by macrophages) and circulate in an inactive (pro-enzyme) form in the blood. * **Option C (Heat labile):** This is true. Complement is highly sensitive to heat. It is inactivated when serum is heated at **56°C for 30 minutes**. This property is used in laboratories to "inactivate" serum before certain serological tests. * **Option D (Species non-specific):** This is true. Unlike antibodies, which are specific to the antigen and the species that produced them, complement from one species (e.g., guinea pig) can react with antibodies from another species (e.g., human) to cause lysis. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Most Abundant Component:** **C3** is the most abundant complement protein in the serum. * **Pathways:** There are three pathways—Classical (triggered by Ag-Ab complexes), Alternative (triggered by endotoxins/microbial surfaces), and Lectin (triggered by mannose-binding lectin). * **C3b:** Acts as a powerful **opsonin** (enhances phagocytosis). * **C5a:** The most potent **anaphylatoxin** and chemotactic agent. * **Membrane Attack Complex (MAC):** Formed by **C5b-C9**, responsible for cell lysis. * **Deficiency:** C3 deficiency is the most severe as it is the "bottleneck" of all pathways, leading to recurrent pyogenic infections.

Question 768: The Mantoux test is an indicator of which type of hypersensitivity reaction?

A. Immediate hypersensitivity
B. Delayed hypersensitivity (Correct Answer)
C. Cell-mediated hypersensitivity
D. Immune complex-mediated hypersensitivity

Explanation: **Explanation:** The **Mantoux test** (Tuberculin Skin Test) is the classic clinical example of a **Type IV Hypersensitivity reaction**, also known as **Delayed-Type Hypersensitivity (DTH)**. **Why it is correct:** When Purified Protein Derivative (PPD) is injected intradermally, it triggers a response in individuals previously sensitized to *Mycobacterium tuberculosis*. This reaction is mediated by **T-lymphocytes** (specifically Th1 cells) rather than antibodies. These cells release cytokines (like IFN-γ) that recruit macrophages, leading to local inflammation and induration. The reaction is termed "delayed" because it takes **48–72 hours** to reach its peak, reflecting the time required for T-cell recruitment and activation. **Analysis of Incorrect Options:** * **Option A (Immediate):** Refers to Type I hypersensitivity (IgE-mediated), such as anaphylaxis or asthma, which occurs within minutes. * **Option C (Cell-mediated):** While Type IV is indeed cell-mediated, in the context of the Mantoux test, "Delayed hypersensitivity" is the more specific and standard terminology used in clinical examinations to describe the temporal nature of the skin reaction. * **Option D (Immune complex-mediated):** Refers to Type III hypersensitivity (e.g., Arthus reaction, SLE), involving Ag-Ab complexes depositing in tissues. **High-Yield Clinical Pearls for NEET-PG:** * **Key Mediator:** CD4+ T-lymphocytes (Th1). * **Reading the test:** Only the **induration** (palpable hardness) is measured, not the erythema (redness). * **False Negative:** Can occur in miliary TB, sarcoidosis, malnutrition, or AIDS (due to **anergy**). * **False Positive:** Common in individuals who have received the **BCG vaccine** or have atypical mycobacterial infections.

Question 769: Autoimmunity is caused by which of the following mechanisms?

A. Pressure of forbidden clones
B. Expression of cryptic antigens
C. Failure of negative selection of T cells in the thymus (Correct Answer)
D. Inappropriate expression of MHC proteins

Explanation: **Explanation:** The core mechanism of autoimmunity is the **breakdown of self-tolerance**. Central tolerance occurs during lymphocyte development, where the body eliminates self-reactive cells. **Why Option C is Correct:** Central tolerance in the thymus involves **Negative Selection**. During this process, developing T cells (thymocytes) are exposed to self-antigens. T cells that bind with high affinity to these self-antigens undergo apoptosis. If this mechanism fails, these autoreactive T cells escape into the peripheral circulation, where they can attack the body's own tissues, leading to autoimmune diseases. This is considered a fundamental failure in the "education" of the immune system. **Analysis of Incorrect Options:** * **Option A:** This is a distractor based on Burnet’s **Forbidden Clone Theory**. The theory states that autoimmunity arises from the *persistence* (not "pressure") of clones that should have been deleted. * **Option B:** While the **release of sequestered (cryptic) antigens** (e.g., lens protein, sperm) can cause autoimmunity after trauma, it is a specific trigger rather than the broad underlying mechanism of immune failure described in Option C. * **Option D:** While certain MHC (HLA) alleles are strongly *associated* with autoimmune diseases (e.g., HLA-B27 and Ankylosing Spondylitis), the "inappropriate expression" is usually a result of inflammation rather than the primary cause of the autoimmune state itself. **NEET-PG High-Yield Pearls:** * **AIRE Gene:** The *Autoimmune Regulator* gene is essential for expressing peripheral tissue antigens in the thymus for negative selection. Mutations lead to **APECED** syndrome. * **Peripheral Tolerance:** If self-reactive cells escape the thymus, they are managed in the periphery via **Anergy** (functional inactivation), **Suppression** by T-regs (CD4+ CD25+ FoxP3+), or **Deletion** (Fas-FasL mediated apoptosis). * **Molecular Mimicry:** A classic example is Rheumatic Fever, where antibodies against Group A Streptococcal M-protein cross-react with cardiac myosin.

Question 770: An allograft is a graft transplanted between:

A. Individuals of the same genetic constitution
B. Individuals of the same species but different genetic identity (Correct Answer)
C. Genetically identical individuals (twins)
D. Members of different species

Explanation: **Explanation:** The classification of transplants is based on the genetic relationship between the donor and the recipient. This is a high-yield concept in immunology, as it determines the risk of graft rejection. **1. Why Option B is Correct:** An **Allograft** (or homograft) is a transplant between two members of the **same species** who are **genetically non-identical**. This is the most common type of clinical transplant (e.g., a kidney transplant from a deceased donor or a non-twin relative). Because the Human Leukocyte Antigens (HLA) differ between the donor and recipient, allografts trigger an immune response, necessitating the use of lifelong immunosuppression. **2. Analysis of Incorrect Options:** * **Options A & C (Isograft/Syngeneic graft):** These refer to grafts between individuals with the **same genetic constitution**, such as monozygotic (identical) twins. Since the HLA molecules are identical, there is no immune recognition or rejection. * **Option D (Xenograft):** This is a transplant between members of **different species** (e.g., a porcine/pig heart valve transplanted into a human). These carry the highest risk of hyperacute rejection. * **Autograft (Not listed):** A graft taken from one part of an individual's body and transplanted to another part of the same individual (e.g., skin grafting or CABG). **NEET-PG High-Yield Pearls:** * **Order of Rejection Risk:** Autograft = Isograft < Allograft < Xenograft. * **MHC/HLA:** The primary targets of allograft rejection are the **MHC Class I and II** molecules. * **Hyperacute Rejection:** Occurs within minutes due to pre-formed antibodies; most common in ABO incompatibility or Xenografts.

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