Immunology Practice Questions

Q: Graft versus host reaction is mediated by which of the following?

T lymphocytes. **Explanation:** **Graft-versus-Host Disease (GVHD)** occurs when immunologically competent cells (the graft) recognize the recipient’s (the host) tissues as foreign and mount an immune attack against them. **Why T lymphocytes are correct:** The primary mediators of GVHD are **mature donor T lymphocytes**. For GVHD to occur, three conditions (Billingham’s criteria) must be met: 1. The graft must contain immunologically competent cells (specifically T cells). 2. The recipient must possess antigens lacking in the donor (making the host appear "foreign"). 3. The recipient must be immunocompromised or unable to reject the donor cells. When donor T cells (both CD4+ and CD8+) are infused, they recognize the host’s Major Histocompatibility Complex (MHC) antigens, undergo activation, and cause direct cytotoxic damage and cytokine-mediated inflammation. **Why other options are incorrect:** * **A & D (Macrophages):** While macrophages are involved in the effector phase of inflammation and tissue damage, they are not the primary mediators or initiators of the reaction. GVHD is fundamentally a T-cell-driven process. * **C (B lymphocytes):** B cells produce antibodies. While they may play a minor role in chronic GVHD, the classic graft-versus-host reaction is a **Type IV (Cell-Mediated) Hypersensitivity** response, which is the hallmark of T-cell activity. **High-Yield Clinical Pearls for NEET-PG:** * **Common Sites:** GVHD typically affects the **skin** (rash), **liver** (jaundice/elevated enzymes), and **GIT** (diarrhea). * **Common Scenarios:** Most frequently seen in **Allogeneic Bone Marrow Transplants**. It can also occur after blood transfusions in severely immunocompromised patients. * **Prevention:** To prevent transfusion-associated GVHD, blood products should be **irradiated** to inactivate donor T lymphocytes. * **Acute vs. Chronic:** Acute GVHD occurs within 100 days; Chronic GVHD occurs after 100 days.

Q: Which of the following statements about Polymerase Chain Reaction (PCR) is false?

PCR utilizes a heat-labile DNA polymerase.. ### Explanation **1. Why Option A is the Correct (False) Statement:** The Polymerase Chain Reaction (PCR) process involves a **denaturation step** where the reaction mixture is heated to approximately **94°C–96°C** to separate double-stranded DNA. A heat-labile (heat-sensitive) enzyme would be permanently denatured and inactivated at this temperature. Therefore, PCR requires a **heat-stable (thermostable) DNA polymerase**, most commonly **Taq polymerase** (derived from the bacterium *Thermus aquaticus*), which can withstand repeated heating cycles. **2. Analysis of Other Options:** * **Option B:** This is a true statement. Heat-stable polymerases are essential for the automation of PCR, as they remain functional throughout multiple cycles of heating and cooling. * **Options C & D:** These are true statements defining the core purpose of PCR. It is an *in vitro* enzymatic technique used for the exponential **amplification** of a specific DNA segment, yielding **millions of copies** from a single template. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Steps of PCR:** 1. Denaturation (~95°C), 2. Annealing (~55°C), 3. Extension (~72°C). * **Components:** Template DNA, Primers (forward and reverse), dNTPs (nucleotides), and Taq Polymerase. * **RT-PCR:** Reverse Transcriptase PCR is used to amplify **RNA** (e.g., for diagnosing COVID-19 or HIV viral load) by first converting RNA into complementary DNA (cDNA). * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes. * **Application:** PCR is the "gold standard" for diagnosing slow-growing or non-culturable organisms (e.g., *Mycobacterium tuberculosis*, *Chlamydia*).

Q: A patient presents with a painless ulcer on his penis. A Wasserman test is done and is positive. The Wasserman reaction is an example of which immunological test?

Complement fixation. The **Wasserman test** is a classic serological test used for the diagnosis of syphilis. It is a prime example of a **Complement Fixation Test (CFT)**. ### Why Complement Fixation is Correct In this test, the patient’s serum is mixed with a specific antigen (cardiolipin). If antibodies are present, they form an antigen-antibody complex that "fixes" or consumes the added complement. To visualize this, an indicator system (sheep RBCs coated with hemolysin) is added. * **Positive Result:** Complement is already fixed; no lysis of RBCs occurs. * **Negative Result:** Complement is free to lyse the RBCs, resulting in hemolysis. ### Why Other Options are Incorrect * **Precipitation:** This involves the interaction of soluble antigens with antibodies to form an insoluble visible precipitate (e.g., VDRL or Kahn test). While VDRL also diagnoses syphilis, it is a flocculation test, not CFT. * **Neutralization:** This test measures the ability of an antibody to inhibit the biological activity of an antigen, such as a virus or toxin (e.g., ASO titer). * **Agglutination:** This occurs when antibodies react with particulate (insoluble) antigens, causing visible clumping (e.g., Widal test or TPHA). ### NEET-PG High-Yield Pearls * **Antigen Used:** The Wasserman test uses **Cardiolipin** (extracted from beef heart) as the antigen, making it a non-specific (non-treponemal) test. * **Modern Equivalent:** The Wasserman test is largely historical and has been replaced by the **VDRL** and **RPR** tests, which are easier to perform. * **Specific Tests:** For confirmation of syphilis, treponemal tests like **FTA-ABS** and **TPHA** are used. * **Clinical Correlation:** A painless ulcer (chancre) is the hallmark of **Primary Syphilis**.

Q: The human leukocyte antigen (HLA) complex is located on which chromosome?

6. **Explanation:** The **Human Leukocyte Antigen (HLA) complex**, also known as the Major Histocompatibility Complex (MHC) in humans, is a cluster of genes located on the **short arm (p) of Chromosome 6**. These genes encode surface glycoproteins that play a critical role in antigen presentation and the regulation of the immune system. * **HLA Class I (A, B, C):** Found on all nucleated cells; presents endogenous antigens to CD8+ T-cells. * **HLA Class II (DR, DQ, DP):** Found on professional antigen-presenting cells (APCs); presents exogenous antigens to CD4+ T-cells. * **HLA Class III:** Encodes components of the complement system (C2, C4) and cytokines like TNF-α. **Analysis of Incorrect Options:** * **Option B (Chromosome 7):** Associated with the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene and the T-cell receptor (TCR) gamma chain. * **Option C (Chromosome 8):** Contains the MYC oncogene (implicated in Burkitt lymphoma). * **Option D (Chromosome 9):** Location of the ABO blood group genes and the CDKN2A tumor suppressor gene. **High-Yield Clinical Pearls for NEET-PG:** 1. **MHC Restriction:** CD4 cells recognize Class II, and CD8 cells recognize Class I (Rule of 8: 4×2=8; 8×1=8). 2. **Disease Associations:** * **HLA-B27:** Ankylosing spondylitis, Reiter’s syndrome. * **HLA-DR3/DR4:** Type 1 Diabetes Mellitus. * **HLA-DQ2/DQ8:** Celiac disease. 3. **Inheritance:** HLA genes are codominantly expressed and inherited as a haplotype (one set from each parent).

Q: Which skin test is useful in the diagnosis of Hydatid disease?

Casoni's test. **Explanation:** **Casoni’s test** is the correct answer. It is an immediate hypersensitivity (Type I) skin test used for the diagnosis of **Hydatid disease** (caused by *Echinococcus granulosus*). The test involves the intradermal injection of 0.2 ml of sterile hydatid fluid. A positive result is indicated by the formation of a large wheal (with pseudopodia) within 20 minutes. While historically significant, it is now largely replaced by serological tests (ELISA) and imaging (USG/CT) due to its low specificity and risk of anaphylaxis. **Analysis of Incorrect Options:** * **Schick test:** Used to demonstrate immunity or susceptibility to **Diphtheria** (*Corynebacterium diphtheriae*). It detects the presence of circulating antitoxin. * **Patch test:** A diagnostic tool for **Type IV (Delayed) hypersensitivity** reactions, primarily used to identify the causative agents of allergic contact dermatitis. * **Dick’s test:** Used to identify susceptibility to **Scarlet Fever**. It involves the intradermal injection of erythrogenic toxin produced by *Streptococcus pyogenes*. **High-Yield Clinical Pearls for NEET-PG:** * **Hydatid Fluid:** The antigen used in Casoni’s test is usually obtained from human or sheep hydatid cysts and sterilized by Seitz filtration. * **False Positives:** Casoni’s test can be falsely positive in cases of Taeniasis, Cirrhosis, or other helminthic infections. * **Gold Standard:** For Hydatid disease, **USG (WHO classification)** and **Serology (IgG ELISA)** are currently the preferred diagnostic modalities. * **Contraindication:** Never aspirate a hydatid cyst for diagnosis, as it may lead to fluid leakage and life-threatening anaphylaxis.

Q: Arthus reaction is an example of which type of hypersensitivity?

Type III. ### Explanation **Correct Answer: C. Type III Hypersensitivity** The **Arthus reaction** is a classic example of **Type III (Immune-complex mediated) hypersensitivity**. It is a localized inflammatory response that occurs when an antigen is injected into the skin of an individual who already has high levels of circulating IgG antibodies. * **Mechanism:** The injected antigen reacts with pre-formed antibodies to form **insoluble immune complexes** locally. These complexes deposit in the walls of small blood vessels, leading to the activation of the **Complement system** (C5a, C3a). This recruits neutrophils, causing vasculitis, edema, and localized tissue necrosis. * **Clinical Presentation:** Typically appears within 4–10 hours after vaccination (e.g., Tetanus or Diphtheria boosters). **Why other options are incorrect:** * **Type I (Immediate):** Mediated by **IgE** and mast cell degranulation (e.g., Anaphylaxis, Asthma). * **Type II (Cytotoxic):** Mediated by **IgG or IgM** binding to antigens on cell surfaces, leading to cell lysis (e.g., Rh incompatibility, Myasthenia Gravis). * **Type IV (Delayed):** Mediated by **T-cells**, not antibodies. It takes 48–72 hours to develop (e.g., Mantoux test, Contact dermatitis). **High-Yield NEET-PG Pearls:** * **Arthus Reaction vs. Serum Sickness:** Arthus is **localized** (Type III), whereas Serum Sickness is **systemic** (Type III). * **Key Mediator:** Neutrophils and Complement (C5a). * **Mnemonic for Hypersensitivity (Gell & Coombs):** **ACID** * **A** – **A**naphylactic (Type I) * **C** – **C**ytotoxic (Type II) * **I** – **I**mmune-Complex (Type III) * **D** – **D**elayed-type (Type IV)

Q: Paul Bunnell reaction is a type of?

Agglutination. The **Paul-Bunnell test** is a classic diagnostic tool used to detect **Infectious Mononucleosis** caused by the Epstein-Barr Virus (EBV). ### Why Agglutination is Correct The test is based on the principle of **Heterophile Agglutination**. Patients with EBV infection produce "heterophile antibodies" (IgM) that have the unique property of cross-reacting with antigens on the surface of red blood cells (RBCs) from different species. In this specific reaction, the patient's serum is mixed with **sheep erythrocytes**. If heterophile antibodies are present, they bind to the sheep RBCs, causing visible clumping or **agglutination**. ### Why Other Options are Incorrect * **Complement Fixation:** This involves the consumption of complement proteins during an antigen-antibody reaction. While used for other viral titers, it is not the mechanism for the Paul-Bunnell test. * **Precipitation:** This occurs when soluble antigens react with antibodies to form an insoluble precipitate. The Paul-Bunnell test uses whole cells (particulate antigen), making it an agglutination reaction, not precipitation. * **Flocculation:** This is a specific type of precipitation where the antigen-antibody complex remains suspended as "flakes" (e.g., VDRL test for Syphilis). ### High-Yield Clinical Pearls for NEET-PG * **Specificity:** The Paul-Bunnell test is not 100% specific. To differentiate EBV from serum sickness or Forssman antibodies, the **Davidsohn Differential Test** (using guinea pig kidney and beef RBCs) is performed. * **Modern Alternative:** The **Monospot test** (latex agglutination) is the faster, modern version of this reaction. * **Key Association:** Always associate "Heterophile antibodies," "Sheep RBCs," and "Agglutination" with the Paul-Bunnell test. * **Clinical Note:** This test is usually negative in "Mononucleosis-like syndrome" caused by CMV.

Q: Antigens processed by the exogenous antigen presentation pathway are presented in association with which of the following?

MHC class II molecules. ### Explanation **Correct Answer: D. MHC class II molecules** The immune system utilizes two distinct pathways for antigen presentation based on the source of the antigen: 1. **Exogenous Pathway (MHC II):** Extracellular pathogens (bacteria, toxins) are internalized by **Antigen-Presenting Cells (APCs)** like macrophages, B cells, and dendritic cells via phagocytosis or endocytosis. These antigens are degraded in endolysosomes and then loaded onto **MHC Class II** molecules. These complexes are presented to **CD4+ T-helper cells**. 2. **Endogenous Pathway (MHC I):** Intracellular antigens (viruses, tumor proteins) are processed by proteasomes in the cytosol and loaded onto **MHC Class I** molecules in the endoplasmic reticulum. These are presented to **CD8+ Cytotoxic T cells**. #### Why Incorrect Options are Wrong: * **A. Fc receptors:** These are found on the surface of various immune cells (like NK cells and mast cells) and bind to the Fc portion of antibodies; they do not present antigens to T cells. * **B. IgG heavy chains:** These are structural components of the immunoglobulin molecule, responsible for determining the antibody class, not for antigen presentation. * **C. MHC class I molecules:** These present **endogenous** antigens (derived from within the cell) to CD8+ T cells. #### High-Yield Clinical Pearls for NEET-PG: * **Rule of 8:** MHC **II** × CD**4** = 8; MHC **I** × CD**8** = 8. * **Invariant Chain (Ii):** In the exogenous pathway, the invariant chain prevents premature binding of self-peptides to MHC II in the ER. * **Cross-presentation:** A unique process where dendritic cells can present exogenous antigens via MHC I to activate CD8+ T cells (essential for anti-tumor immunity). * **MHC II Distribution:** Unlike MHC I (found on all nucleated cells), MHC II is restricted to professional APCs.

Q: Which immunoglobulin is implicated in atopy and anaphylaxis?

IgE. **Explanation:** **Correct Option: A (IgE)** IgE is the primary mediator of **Type I Hypersensitivity reactions**, which include atopy (allergic rhinitis, asthma, eczema) and systemic anaphylaxis. The mechanism involves the binding of IgE to high-affinity receptors (**FcεRI**) on the surface of **mast cells and basophils**. Upon re-exposure to an allergen, cross-linking of these IgE molecules triggers degranulation, releasing potent inflammatory mediators like histamine, leukotrienes, and prostaglandins. **Why other options are incorrect:** * **IgM:** This is the first antibody produced in a primary immune response. It is a pentamer and is primarily involved in complement activation and agglutination, not allergic reactions. * **IgG:** The most abundant immunoglobulin in serum. It provides long-term immunity, crosses the placenta, and mediates Type II and Type III hypersensitivity, but not the immediate Type I response. * **IgA:** Found predominantly in secretions (tears, saliva, colostrum, GI tract). Its main role is mucosal immunity by preventing the attachment of pathogens to epithelial surfaces. **High-Yield Clinical Pearls for NEET-PG:** * **Prausnitz-Küstner (PK) Reaction:** A classic test used to demonstrate the presence of IgE in serum (though now replaced by in-vitro tests like RAST). * **Heat Lability:** IgE is the most heat-labile immunoglobulin (inactivated at 56°C for 30 minutes). * **Parasitic Infections:** IgE levels also rise significantly during helminthic infestations (e.g., Ascariasis) to facilitate eosinophil-mediated killing. * **Receptor:** IgE binds to mast cells via the **Fc portion**, leaving the Fab portion free to bind allergens.

Question 1

Graft versus host reaction is mediated by which of the following?

Accepted Answer

T lymphocytes

Answer

Macrophages

Answer

B lymphocytes

Answer

T lymphocytes and macrophages

Question 2

Division and differentiation of B cells leading to production of plasma cells both require:

Accepted Answer

IL-4 and IL-6

Answer

CD4 and CD8

Answer

Interleukin (IL)-1 and IL-5

Answer

IL-5 only

Question 3

Which of the following statements about Polymerase Chain Reaction (PCR) is false?

Accepted Answer

PCR utilizes a heat-labile DNA polymerase.

Answer

PCR utilizes a heat-stable DNA polymerase.

Answer

PCR is a technique for DNA amplification.

Answer

PCR is used to yield multiple copies of DNA.

Question 4

A patient presents with a painless ulcer on his penis. A Wasserman test is done and is positive. The Wasserman reaction is an example of which immunological test?

Accepted Answer

Complement fixation

Answer

Precipitation

Answer

Neutralization

Answer

Agglutination

Question 5

The human leukocyte antigen (HLA) complex is located on which chromosome?

Accepted Answer

6

Answer

7

Answer

8

Answer

9

Question 6

Which skin test is useful in the diagnosis of Hydatid disease?

Accepted Answer

Casoni's test

Answer

Schick test

Answer

Patch test

Answer

Dick's test

Question 7

Arthus reaction is an example of which type of hypersensitivity?

Accepted Answer

Type III

Answer

Type I

Answer

Type II

Answer

Type IV

Question 8

Paul Bunnell reaction is a type of?

Accepted Answer

Agglutination

Answer

Complement fixation

Answer

Precipitation

Answer

Flocculation test

Question 9

Antigens processed by the exogenous antigen presentation pathway are presented in association with which of the following?

Accepted Answer

MHC class II molecules

Answer

Fc receptors

Answer

IgG heavy chains

Answer

MHC class I molecules

Question 10

Which immunoglobulin is implicated in atopy and anaphylaxis?

Accepted Answer

IgE

Answer

IgM

Answer

IgG

Answer

IgA

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?