Immunology Practice Questions

Q: Which type of hypersensitivity reaction is mediated by T-lymphocytes and macrophages?

Type IV. ### Explanation **Correct Answer: D. Type IV Hypersensitivity** **Why it is correct:** Type IV hypersensitivity is also known as **Delayed-Type Hypersensitivity (DTH)**. Unlike Types I, II, and III, which are mediated by antibodies (B-cell response), Type IV is **cell-mediated**. It involves the activation of **T-lymphocytes** (specifically Th1 and CD8+ cells). Upon re-exposure to an antigen, sensitized T-cells release cytokines (like IFN-γ) that recruit and activate **macrophages**, leading to tissue inflammation and damage. The reaction typically peaks 48–72 hours after exposure. **Why the other options are incorrect:** * **Type I (Immediate):** Mediated by **IgE antibodies** and mast cell degranulation (e.g., Anaphylaxis, Asthma). * **Type II (Cytotoxic):** Mediated by **IgG or IgM antibodies** directed against antigens on specific cell surfaces or tissues (e.g., Rh incompatibility, Myasthenia Gravis). * **Type III (Immune-complex):** Mediated by the deposition of **antigen-antibody complexes** in tissues, leading to complement activation (e.g., SLE, Post-streptococcal glomerulonephritis). **NEET-PG High-Yield Pearls:** * **Classic Examples:** Mantoux (Tuberculin) test, Contact dermatitis (poison ivy/nickel), Lepromin test, and Chronic graft rejection. * **Granuloma Formation:** This is a hallmark of persistent Type IV reactions (e.g., Tuberculosis, Sarcoidosis), where macrophages transform into **epithelioid cells** and fuse to form **multinucleated giant cells**. * **Mnemonic:** Remember **ACID** for the four types: **A**naphylactic (I), **C**ytotoxic (II), **I**mmune-complex (III), and **D**elayed/Cell-mediated (IV).

Q: Which of the following is an example of an agglutination test?

Widal test. **Explanation:** **1. Why Widal Test is Correct:** The **Widal test** is a classic example of a **direct slide or tube agglutination test**. It is used to diagnose Enteric fever (Typhoid) by detecting antibodies ($H$ and $O$ agglutinins) in the patient’s serum against *Salmonella typhi* and *S. paratyphi* antigens. In this reaction, particulate (insoluble) antigens clump together when they bind with specific antibodies, forming visible aggregates—the hallmark of agglutination. **2. Analysis of Incorrect Options:** * **Schick Test (Option B):** This is an **in-vivo neutralization test** used to determine susceptibility to Diphtheria. It is not an agglutination test. * **VDRL Test (Option C):** The Venereal Disease Research Laboratory (VDRL) test for Syphilis is a **flocculation test**. While similar to agglutination, flocculation involves soluble antigens that form visible "flakes" or "floccules" rather than clumps of large particles. * **Ascoli’s Test (Option D):** This is a **precipitation test** (specifically a ring precipitation test) used to detect *Bacillus anthracis* antigens in animal tissues. Precipitation involves the interaction of soluble antigens with antibodies to form an insoluble precipitate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Agglutination vs. Precipitation:** Agglutination involves **particulate** antigens (e.g., bacteria, RBCs), whereas precipitation involves **soluble** antigens. * **Prozone Phenomenon:** False negatives in agglutination tests (like Widal or Brucellosis tests) can occur due to antibody excess; this is called the Prozone phenomenon. * **Coombs Test:** Another high-yield agglutination test (specifically Hemagglutination) used in Rh incompatibility and autoimmune hemolytic anemia. * **Latex Agglutination:** Commonly used for detecting CRP, RA factor, and ASO titers.

Q: Antisnake venom may cause which type of hypersensitivity reaction?

Type III hypersensitivity reactions. **Explanation:** **Antisnake venom (ASV)** is a classic example of **Type III hypersensitivity**, specifically manifesting as **Serum Sickness**. 1. **Why Type III is correct:** ASV is a form of passive immunization containing foreign (usually equine/horse) proteins. When injected, the body recognizes these proteins as antigens and produces antibodies (IgG/IgM). These antibodies bind to the circulating foreign proteins, forming **soluble immune complexes**. These complexes deposit in small blood vessel walls, basement membranes of joints, and kidneys, activating the complement system and causing systemic inflammation (fever, rash, arthralgia, and proteinuria). 2. **Why other options are incorrect:** * **Type II (Cytotoxic):** Involves antibodies (IgG/IgM) binding directly to antigens on **cell surfaces** or tissues (e.g., Rh incompatibility), not soluble circulating complexes. * **Type IV (Delayed-type):** This is **cell-mediated** (T-cells), occurring 48–72 hours after exposure (e.g., Mantoux test). ASV reactions are antibody-mediated. * **Type V:** Often considered a subtype of Type II, involving stimulatory antibodies (e.g., Graves' disease). **High-Yield Clinical Pearls for NEET-PG:** * **Serum Sickness** is the systemic form of Type III hypersensitivity; **Arthus Reaction** is the localized form. * **Other causes of Type III:** SLE, Post-streptococcal glomerulonephritis (PSGN), and Farmer’s lung. * **Timeline:** Serum sickness typically occurs 7–12 days after administration of the foreign serum. * **Complement levels:** Characteristically, **C3 and C4 levels are decreased** during the reaction due to massive consumption.

Q: Cytokines are released by:

T lymphocyte. ### Explanation **Correct Answer: C. T lymphocyte** **Concept Overview:** Cytokines are low-molecular-weight regulatory proteins or glycoproteins secreted by white blood cells and various other cells in the body in response to a number of stimuli. While many cells can produce cytokines, **T lymphocytes (specifically T-helper cells)** are the primary orchestrators of the immune response and the most prolific producers of cytokines (interleukins, interferons, and TNFs) to coordinate cell-mediated and humoral immunity. **Why T lymphocyte is the best answer:** In the context of standard immunology questions, T-helper cells (CD4+) are considered the "master switches." Upon activation, they differentiate into subsets (Th1, Th2, Th17) that secrete specific profiles of cytokines (e.g., IL-2, IFN-γ, IL-4, IL-5) to signal other immune cells. **Analysis of Incorrect Options:** * **A & B (Monocytes/Macrophages):** These cells do produce cytokines (known as **monokines**, such as IL-1, IL-6, and TNF-α), but in the hierarchy of immune regulation, T-cells are the predominant source for the diverse range of cytokines that drive systemic immune responses. * **D (B lymphocyte):** B-cells primarily function as antibody-producing cells (plasma cells). While they can secrete some cytokines to modulate their own growth, it is not their primary functional characteristic compared to T-cells. **High-Yield Clinical Pearls for NEET-PG:** * **Th1 Cells:** Produce **IFN-γ and IL-2** (Cell-mediated immunity; activates macrophages). * **Th2 Cells:** Produce **IL-4, IL-5, and IL-13** (Humoral immunity; stimulates B-cells and eosinophils). * **Pleiotropy:** The ability of a single cytokine to act on different cell types (e.g., IL-4 acting on B-cells, T-cells, and mast cells). * **Redundancy:** Multiple cytokines carrying out the same function (e.g., IL-2, IL-4, and IL-5 all stimulating B-cell proliferation).

Q: Which of the following diagnostic tests are useful for corresponding purposes, except?

Specific IgM antibodies - Immunity against Rubella. ### Explanation The correct answer is **C (Specific IgM antibodies - Immunity against Rubella)** because it represents a mismatch between the diagnostic marker and its clinical interpretation. **1. Why Option C is the correct (incorrectly matched) choice:** In immunology, **IgM antibodies** are the first to appear following exposure to an antigen, indicating an **acute or recent infection**. They are transient and do not signify long-term protection. Conversely, **IgG antibodies** are the markers used to determine **immunity** (either from past infection or vaccination). Therefore, detecting Rubella-specific IgM indicates a current infection (often risky in pregnancy), whereas Rubella-specific IgG indicates immunity. **2. Analysis of other options:** * **Option A:** Ziehl-Neelsen (ZN) staining is the gold standard rapid bedside test for detecting Acid-Fast Bacilli (AFB) like *Mycobacterium tuberculosis*. * **Option B:** Direct Immunofluorescence (DIF) is a standard technique used to detect viral antigens (like Influenza) in respiratory secretions (nasopharyngeal swabs). * **Option C:** As established, IgM is the hallmark of the primary immune response and is the diagnostic marker for acute phase infections across most pathogens. **Clinical Pearls for NEET-PG:** * **IgM:** Does not cross the placenta (large pentamer). Its presence in a newborn indicates **congenital infection** (e.g., TORCH panel). * **IgG:** The only antibody that crosses the placenta, providing passive immunity to the fetus. * **Window Period:** The time between infection and the appearance of detectable antibodies (IgM). * **Avidity Testing:** High IgG avidity indicates an old infection, while low avidity suggests a recent infection—crucial for managing Rubella in pregnancy.

Q: Mast cells release which of the following interleukins?

All of the above. **Explanation:** Mast cells are versatile immune effector cells primarily known for their role in Type I Hypersensitivity reactions. While they are famous for releasing pre-formed mediators like histamine, they also synthesize and secrete a wide array of cytokines that modulate both innate and adaptive immunity. **Why "All of the above" is correct:** Mast cells are "multipotential" cytokine producers. * **IL-1:** Mast cells produce pro-inflammatory cytokines like IL-1 and TNF-α, which help in the recruitment of neutrophils and the initiation of the acute inflammatory response. * **IL-2:** Although primarily associated with T-cells, mast cells can produce IL-2, which aids in T-cell proliferation and regulatory T-cell (Treg) maintenance. * **IL-4:** This is a crucial cytokine produced by mast cells to drive the differentiation of Th2 cells and promote B-cell class switching to **IgE**, creating a positive feedback loop in allergic responses. **Analysis of Options:** * **IL-1:** Acts as an endogenous pyrogen and activates vascular endothelium. * **IL-2:** Known as the T-cell growth factor. * **IL-4:** Essential for the "Atopic" profile. Since mast cells have the biosynthetic machinery to produce all three, "All of the above" is the most accurate choice. **High-Yield Clinical Pearls for NEET-PG:** * **Mast Cell Markers:** CD117 (c-kit) and CD34 are key markers. * **Staining:** They show **metachromasia** with Toluidine blue (granules turn purple). * **Major Mediator:** Histamine is the primary pre-formed mediator; **Tryptase** is the most specific marker for mast cell degranulation (used clinically to diagnose anaphylaxis). * **Origin:** They originate from Hematopoietic Stem Cells (HSC) in the bone marrow but mature only after reaching peripheral tissues.

Q: HLA-I is present on which type of cells?

All nucleated cells. **Explanation:** The Human Leukocyte Antigen (HLA) system is the human version of the Major Histocompatibility Complex (MHC). These molecules are essential for the immune system to distinguish "self" from "non-self." **Why Option A is Correct:** **HLA Class I (MHC-I)** molecules are expressed on the surface of **all nucleated cells** and platelets. Their primary function is to present endogenous antigens (like viral proteins or tumor markers) to **CD8+ Cytotoxic T cells**. Since any nucleated cell in the body can potentially be infected by a virus or undergo malignant transformation, they must all possess the machinery to signal the immune system for destruction. **Why Other Options are Incorrect:** * **Options B, C, and D:** These are too restrictive. While immune cells (B cells, T cells, Macrophages) do express HLA-I, they are not the *only* cells to do so. HLA-I is ubiquitous across almost all tissues. Note that **HLA Class II**, however, has a restricted distribution and is found only on **Professional Antigen Presenting Cells (APCs)** like B cells, dendritic cells, and macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of 8":** MHC I × CD8 = 8; MHC II × CD4 = 8. * **Structure:** HLA-I consists of a heavy chain (encoded on Chromosome 6) and a **β2-microglobulin** light chain (encoded on Chromosome 15). * **Exceptions:** Mature **Red Blood Cells (RBCs)** lack HLA-I because they are non-nucleated. This is why blood transfusions do not require HLA matching, unlike organ transplants. * **Trophoblasts:** These cells lack classical HLA-I (A and B) to prevent maternal immune rejection of the fetus.

Q: Which of the following statements most accurately describes IgA?

IgA can be destroyed by bacterial proteases.. **Explanation** **Correct Option: B. IgA can be destroyed by bacterial proteases.** IgA is the primary immunoglobulin responsible for mucosal immunity. To counter this defense, several pathogenic bacteria (notably *Streptococcus pneumoniae*, *Haemophilus influenzae*, and *Neisseria* species) produce a specific enzyme called **IgA1 protease**. This enzyme cleaves the proline-rich hinge region of the IgA1 molecule, effectively neutralizing its ability to prevent bacterial attachment to mucosal surfaces. **Analysis of Incorrect Options:** * **Option A:** IgA **cannot** activate the classical complement pathway. It lacks the binding site for C1q. While it can activate the alternative pathway, standard complement fixation tests (which rely on the classical pathway) will be negative for IgA. * **Option C:** IgA is the **predominant** immunoglobulin in colostrum and breast milk. It provides essential passive local immunity to the neonate's gastrointestinal tract (protection against enteric pathogens). * **Option D:** IgA is the major antibody found in **all seromucous secretions**, including saliva, tears, nasal fluids, and sweat. Salivary IgA levels can indeed be used as a diagnostic marker for certain local infections or immune deficiencies. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Secretory IgA (sIgA) exists as a **dimer** connected by a **J-chain** and contains a **Secretory Component** (which protects it from digestive enzymes). * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients are often asymptomatic but may present with recurrent sinopulmonary or GI infections and are at risk for **anaphylaxis during blood transfusions** (due to anti-IgA antibodies). * **Subclasses:** IgA1 is predominant in serum; IgA2 is more prevalent in colostrum and the lower GI tract as it is more resistant to bacterial proteases.

Q: Which of the following features is common to both cytotoxic T cells and NK cells?

Effective against virus-infected cells. **Explanation:** The core similarity between **Cytotoxic T cells (CD8+ T cells)** and **Natural Killer (NK) cells** lies in their primary function: the elimination of intracellular pathogens and tumor cells. Both cell types are specialized to identify and destroy **virus-infected cells** by inducing apoptosis through the release of perforins and granzymes. * **Why Option C is correct:** Both cells act as "killers" of the immune system. While CD8+ T cells are part of the adaptive immune system (requiring MHC-I presentation), and NK cells are part of the innate immune system (acting on cells that lack MHC-I), their ultimate target—cells hijacked by viruses—is the same. **Analysis of Incorrect Options:** * **Option A:** Antibody synthesis is the exclusive function of **B-lymphocytes** (specifically plasma cells). Neither T cells nor NK cells produce antibodies. * **Option B:** While NK cells can participate in Antibody-Dependent Cellular Cytotoxicity (ADCC) via CD16, it is not a *requirement* for their primary action. CD8+ T cells do not require antibodies; they require TCR-MHC interaction. * **Option D:** Recognition of antigens with **HLA Class II** is a feature of **Helper T cells (CD4+)**. Cytotoxic T cells (CD8+) recognize antigens associated with **HLA Class I**. NK cells are actually inhibited by the presence of HLA Class I (the "missing self" hypothesis). **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** CD8+ T cells are MHC-restricted (Class I), whereas NK cells are **MHC-unrestricted**. * **NK Cell Markers:** Look for **CD16** (FcγRIII) and **CD56** in questions. * **The "Missing Self":** NK cells kill cells that have down-regulated MHC-I expression—a common tactic used by viruses and tumors to evade CD8+ T cells. This makes NK cells and CD8+ cells complementary.

Question 1

Which type of hypersensitivity reaction is mediated by T-lymphocytes and macrophages?

Accepted Answer

Type IV

Answer

Type I

Answer

Type II

Answer

Type III

Question 2

Which of the following is an example of an agglutination test?

Accepted Answer

Widal test

Answer

Schick test

Answer

VDRL test

Answer

Ascoli's test

Question 3

Antisnake venom may cause which type of hypersensitivity reaction?

Accepted Answer

Type III hypersensitivity reactions

Answer

Type II hypersensitivity reactions

Answer

Type IV hypersensitivity reactions

Answer

Type V hypersensitivity reactions

Question 4

Maximum lattice formation occurs in which zone?

Accepted Answer

Zone of equivalence

Answer

Zone of antibody excess

Answer

Zone of antigen excess

Answer

Can occur in any zone

Question 5

Cytokines are released by:

Accepted Answer

T lymphocyte

Answer

Monocyte

Answer

Macrophage

Answer

B lymphocyte

Question 6

Which of the following diagnostic tests are useful for corresponding purposes, except?

Accepted Answer

Specific IgM antibodies - Immunity against Rubella

Answer

Ziehl-Neelsen staining - Detection of mycobacteria

Answer

Immunofluorescence - Detection of Influenza virus

Answer

Specific IgM antibodies - Detection of acute infection

Question 7

Mast cells release which of the following interleukins?

Accepted Answer

All of the above

Answer

Interleukin-1

Answer

Interleukin-2

Answer

Interleukin-4

Question 8

HLA-I is present on which type of cells?

Accepted Answer

All nucleated cells

Answer

Only on cells of the immune system

Answer

Only on B cells

Answer

Only on T cells

Question 9

Which of the following statements most accurately describes IgA?

Accepted Answer

IgA can be destroyed by bacterial proteases.

Answer

Complement fixation tests for IgA antibody will be positive if specific IgA antibody is present.

Answer

IgA is absent in colostrum.

Answer

IgA is not found in saliva; therefore, an IgA diagnostic test on saliva would have no value.

Question 10

Which of the following features is common to both cytotoxic T cells and NK cells?

Accepted Answer

Effective against virus-infected cells

Answer

Synthesizes antibodies

Answer

Requires antibodies to be present for action

Answer

Recognizes antigen in association with HLA class II markers

Immunology — MCQs

Immunology — MCQs

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