Immunology Practice Questions

Q: Mixed lymphocyte culture is used to identify which of the following?

MHC class II antigen. **Explanation:** **Mixed Lymphocyte Reaction (MLR)**, also known as Mixed Lymphocyte Culture, is a functional assay used to determine the degree of histocompatibility between a donor and a recipient. 1. **Why MHC Class II is correct:** The MLR primarily detects differences in **MHC Class II (HLA-D)** antigens. When lymphocytes from two different individuals are cultured together, the T-cells of one individual recognize the foreign MHC Class II molecules on the surface of the other individual's cells (specifically on B-cells or monocytes). This recognition triggers T-cell activation and proliferation. The degree of proliferation is proportional to the disparity in MHC Class II antigens. 2. **Why other options are incorrect:** * **MHC Class I:** These are typically identified using **Serological methods** (Lymphocytotoxicity tests) rather than culture-based proliferation assays. * **B lymphocytes & T helper cells:** While these cells are *participants* in the reaction (B-cells act as stimulators and T-cells as responders), the test is designed to identify the **antigenic compatibility** of the tissue, not to quantify or identify the cell types themselves. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Use:** MLR was traditionally the "gold standard" for predicting **Graft-versus-Host Disease (GVHD)** in bone marrow transplants. * **One-way vs. Two-way MLR:** In clinical practice, a "one-way" MLR is used where the donor cells are irradiated (treated with Mitomycin C) so they cannot divide, allowing only the recipient's T-cell response to be measured. * **Modern Alternative:** MLR has largely been replaced by rapid **PCR-based DNA sequencing** for HLA typing, but it remains a classic exam topic for understanding cellular immunity.

Q: Which is the primary secretory immunoglobulin?

IgA. **Explanation:** **IgA (Immunoglobulin A)** is the primary secretory antibody because it is the most abundant immunoglobulin found in mucosal secretions (tears, saliva, colostrum, and gastrointestinal/respiratory tracts). In its secretory form, it exists as a **dimer** held together by a J-chain and a **secretory component**. This secretory component is crucial as it protects the antibody from enzymatic degradation by proteases present in the gut and other secretions, allowing it to provide local "mucosal immunity" by preventing the attachment of pathogens to epithelial surfaces. **Why other options are incorrect:** * **IgG:** This is the most abundant antibody in the **serum** (not secretions) and the only one that crosses the placenta. It is the primary mediator of the secondary immune response. * **IgM:** This is the largest antibody (pentamer) and the first to appear in the **primary immune response**. While it also contains a J-chain, it is not the primary secretory antibody. * **IgE:** This is involved in **Type I hypersensitivity** reactions and provides defense against helminthic parasitic infections. It is found in very low concentrations in the serum. **High-Yield NEET-PG Pearls:** * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients are prone to recurrent sinopulmonary infections and Giardiasis. * **Colostrum:** Rich in IgA, providing passive immunity to the neonate. * **Half-life:** IgG has the longest half-life (approx. 23 days). * **Valency:** Secretory IgA is tetravalent (4 antigen-binding sites).

Q: Which of the following antibacterial substances is present in eggs and saliva?

Lysozyme. **Explanation:** **Lysozyme** (also known as muramidase) is a key component of the innate immune system. It is an enzyme that exerts antibacterial activity by hydrolyzing the **β-1,4 glycosidic bonds** between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in the **peptidoglycan** layer of bacterial cell walls. It is found in high concentrations in egg whites and various human secretions, including **saliva, tears, nasal secretions, and sweat**, as well as within the granules of neutrophils. It is particularly effective against Gram-positive bacteria, which have a thick, exposed peptidoglycan layer. **Analysis of Incorrect Options:** * **Secretozyme:** This is a fictitious term and does not represent a recognized biological enzyme or antibacterial substance. * **Albumin:** While the major protein in egg whites (ovalbumin) and human serum, its primary functions are maintaining oncotic pressure and transporting molecules; it does not possess direct enzymatic antibacterial properties. * **Isozyme:** This is a general biochemical term for enzymes that differ in amino acid sequence but catalyze the same chemical reaction. It is not a specific antibacterial substance. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Lysozyme acts as a "natural antibiotic" by targeting the cell wall. * **Tears:** Lysozyme in tears is the primary reason the ocular surface remains remarkably resistant to infection despite constant exposure. * **Deficiency:** Low levels of lysozyme in secretions can predispose individuals to increased mucosal infections. * **Other Innate Factors:** Remember other substances like **Lactoferrin** (sequesters iron) and **Defensins** (pore-forming peptides) which often appear alongside lysozyme in exam questions regarding innate immunity.

Q: Which component of the complement system is an anaphylotoxin?

C5a. ### Explanation The complement system is a crucial part of innate immunity, consisting of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens. **1. Why C5a is the Correct Answer:** Anaphylatoxins are small peptide fragments (C3a, C4a, and C5a) released during complement activation. They trigger degranulation of mast cells and basophils, releasing histamine and increasing vascular permeability. Among these, **C5a is the most potent anaphylatoxin**. It also acts as a powerful **chemotactic factor**, recruiting neutrophils and macrophages to the site of inflammation. **2. Analysis of Incorrect Options:** * **A. C3b:** This is primarily an **opsonin**. It coats the surface of pathogens, making them more "tasty" for phagocytes (opsonization). It also helps in the formation of C5 convertase. * **B. C5b-9:** This represents the **Membrane Attack Complex (MAC)**. It forms a pore in the lipid bilayer of the target cell, leading to osmotic lysis and cell death. * **C. C1-3:** This refers to the early components of the Classical pathway. While C3 is cleaved to produce the anaphylatoxin C3a, the complex C1-3 as a whole does not function as an anaphylatoxin. **3. High-Yield Clinical Pearls for NEET-PG:** * **Potency Order:** C5a > C3a > C4a. * **Opsonization:** C3b and iC3b are the major opsonins. * **Chemotaxis:** C5a is the primary complement-derived chemoattractant for neutrophils. * **Deficiency:** Deficiency of C5-C9 (MAC) predisposes individuals to recurrent **Neisserial infections** (Meningitis and Gonorrhea). * **Inhibitor:** **C1 esterase inhibitor** deficiency leads to Hereditary Angioedema.

Q: Which type of cells are most dependent on IFN-gamma?

IFN-gamma. **Explanation:** The question focuses on the cytokine profile of **Th1 cells**. Th1 cells are primarily responsible for cell-mediated immunity and are most dependent on **IFN-gamma** for both their differentiation and their primary effector functions. 1. **Why IFN-gamma is correct:** IFN-gamma acts in a positive feedback loop. It induces the transcription factor **T-bet**, which commits naive T-cells to the Th1 lineage. Once activated, Th1 cells secrete IFN-gamma to activate **macrophages**, enhancing their microbicidal activity (killing of intracellular pathogens like *M. tuberculosis*). 2. **Why the other options are incorrect:** * **IL-4:** This is the signature cytokine for **Th2 cells**. It promotes B-cell differentiation into plasma cells and stimulates the isotype switch to **IgE**. * **IL-5:** Also produced by Th2 cells, its primary role is the activation and chemotaxis of **eosinophils**, crucial in the defense against helminthic infections. * **IL-6:** This is a pro-inflammatory cytokine involved in the acute phase response and the differentiation of **Th17 cells** (along with TGF-beta). **High-Yield Clinical Pearls for NEET-PG:** * **Th1 Profile:** IFN-gamma, IL-2, TNF-beta (Targets: Intracellular pathogens). * **Th2 Profile:** IL-4, IL-5, IL-10, IL-13 (Targets: Extracellular parasites, Allergens). * **Key Transcription Factors:** Th1 = **T-bet**; Th2 = **GATA-3**; Th17 = **ROR-gamma-t**. * **Leprosy Link:** Tuberculoid leprosy (strong CMI) shows a Th1 response, while Lepromatous leprosy (poor CMI) shows a Th2 response.

Q: Which of the following is NOT a Type I hypersensitivity reaction?

Contact dermatitis. **Explanation:** Hypersensitivity reactions are classified by the Gell and Coombs system based on the immune mechanism involved. **Why Contact Dermatitis is the correct answer:** **Contact dermatitis** is a classic example of **Type IV (Delayed-type) Hypersensitivity**. Unlike Type I, it is mediated by **T-lymphocytes** (CD4+ and CD8+ cells) rather than antibodies. Upon re-exposure to an allergen (like nickel or poison ivy), sensitized T-cells release cytokines that recruit macrophages, leading to inflammation typically 48–72 hours after exposure. **Why the other options are incorrect:** All other options are examples of **Type I (Immediate) Hypersensitivity**, which is mediated by **IgE antibodies** and mast cell degranulation: * **Casoni’s test:** An immediate skin test used to diagnose Hydatid disease. Injection of hydatid fluid causes a wheal-and-flare response within 20 minutes in sensitized individuals. * **Food allergy:** Ingestion of allergens (e.g., peanuts, shellfish) triggers IgE-mediated mast cell activation, leading to symptoms ranging from urticaria to systemic anaphylaxis. * **Prausnitz-Küstner (PK) reaction:** A historical method used to demonstrate "reaginic" (IgE) antibodies. Serum from an allergic person is injected into a healthy person's skin; subsequent challenge at that site produces a local Type I reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Type I:** IgE-mediated; "Immediate"; Examples: Anaphylaxis, Atopy, Bronchial Asthma. * **Type II:** Cytotoxic (IgG/IgM); Examples: Erythroblastosis fetalis, Goodpasture syndrome, Myasthenia gravis. * **Type III:** Immune-complex mediated; Examples: SLE, Arthus reaction, Serum sickness. * **Type IV:** Cell-mediated; "Delayed"; Examples: Mantoux test, Lepromin test, Graft rejection.

Q: Which method is used to detect antigen-antibody reactions?

Northern Blot. **Explanation:** The question asks for a method to detect antigen-antibody reactions. However, there is a technical discrepancy in the provided key: **ELISA (Option A)** and **Western Blot (Option D)** are the standard methods for detecting antigen-antibody interactions. **Northern Blot (Option C)** is traditionally used for RNA detection. If we follow standard medical microbiology: 1. **ELISA (Enzyme-Linked Immunosorbent Assay):** This is the gold standard for detecting antigen-antibody reactions. It uses enzyme-labeled antibodies to quantify specific antigens or antibodies in a patient's serum (e.g., screening for HIV or Hepatitis B). 2. **Western Blot:** This method detects specific **proteins** (antigens) using labeled antibodies after gel electrophoresis. It is the classic confirmatory test for HIV. 3. **Southern Blot:** Used for the detection of specific **DNA** sequences using DNA probes. (Mnemonic: **S**outhern = **D**NA). 4. **Northern Blot:** Used for the detection of specific **RNA** sequences (mRNA) to study gene expression. (Mnemonic: **N**orthern = **R**NA). **Note on the Correct Answer:** While the key indicates Northern Blot, in standard NEET-PG curriculum, Northern Blot detects RNA, not antigen-antibody reactions. If the question intended to identify a "Blotting" technique for proteins, **Western Blot** would be the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **SNOW DROP Mnemonic:** **S**outhern-**D**NA, **N**orthern-**R**NA, **O**-O, **W**estern-**P**rotein. * **ELISA** is the most common screening test; **Western Blot** is the confirmatory test. * **Eastern Blot** is used to detect post-translational modifications of proteins (carbohydrates/lipids).

Q: What type of hypersensitivity reaction is seen in myasthenia gravis?

Type 2 hypersensitivity reaction. **Explanation:** **Myasthenia Gravis (MG)** is a classic example of a **Type 2 Hypersensitivity reaction**, which is characterized by antibody-mediated cytotoxicity. In MG, the body produces autoantibodies (IgG) specifically directed against the **post-synaptic nicotinic acetylcholine receptors (AChR)** at the neuromuscular junction. These antibodies do not cause cell lysis; instead, they block the receptors and trigger their internalization and degradation. This leads to impaired neuromuscular transmission, resulting in the hallmark symptom of fluctuating muscle weakness. **Why other options are incorrect:** * **Type 1 (Immediate):** Mediated by IgE and mast cell degranulation (e.g., Anaphylaxis, Asthma). * **Type 3 (Immune-complex):** Involves deposition of antigen-antibody complexes in tissues (e.g., SLE, Post-streptococcal glomerulonephritis). * **Type 4 (Delayed):** Cell-mediated immunity involving T-lymphocytes, not antibodies (e.g., Mantoux test, Contact dermatitis). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Type 2 reactions can be cytotoxic (e.g., Autoimmune Hemolytic Anemia) or **non-cytotoxic/cell-stimulating/blocking** (e.g., MG and Graves' disease). * **Antibody Profile:** 85% of MG patients are positive for **anti-AChR antibodies**. If negative, check for **anti-MuSK** (Muscle-Specific Kinase) antibodies. * **Associated Pathology:** MG is frequently associated with **Thymic hyperplasia (65%)** or **Thymoma (15%)**. * **Diagnosis:** The **Edrophonium (Tensilon) test** is a classic bedside test (though largely replaced by serology and EMG). * **Treatment:** Acetylcholinesterase inhibitors (Pyridostigmine) are the first-line symptomatic treatment.

Question 1

Mixed lymphocyte culture is used to identify which of the following?

Accepted Answer

MHC class II antigen

Answer

MHC class I antigen

Answer

B lymphocytes

Answer

T helper cells

Question 2

During an infection with Streptococcus pyogenes, an individual generated sufficiently high levels of IgM and IgG antibodies against Streptococcus pyogenes antigen with structural similarities to the heart, causing cardiac damage. In this example, what process explains how the microbe contributed to autoimmunity?

Accepted Answer

Molecular mimicry

Answer

Anergy

Answer

Central tolerance

Answer

Epitope spreading

Question 3

Both B and T cell defect is present in all EXCEPT:

Accepted Answer

X-linked Agammaglobulinemia

Answer

Severe combined immunodeficiency (SCID)

Answer

Common variable immunodeficiency (CVID)

Answer

Wiskott-Aldrich syndrome

Question 4

Which is the primary secretory immunoglobulin?

Accepted Answer

IgA

Answer

IgG

Answer

IgM

Answer

IgE

Question 5

Which of the following antibacterial substances is present in eggs and saliva?

Accepted Answer

Lysozyme

Answer

Secretozyme

Answer

Albumin

Answer

Isozyme

Question 6

Which component of the complement system is an anaphylotoxin?

Accepted Answer

C5a

Answer

C3b

Answer

C5b-9

Answer

C1-3

Question 7

Which type of cells are most dependent on IFN-gamma?

Accepted Answer

IFN-gamma

Answer

IL-4

Answer

IL-5

Answer

IL-6

Question 8

Which of the following is NOT a Type I hypersensitivity reaction?

Accepted Answer

Contact dermatitis

Answer

Casoni's test

Answer

Food allergy

Answer

Prausnitz-Küstner reaction

Question 9

Which method is used to detect antigen-antibody reactions?

Accepted Answer

Northern Blot

Answer

ELISA

Answer

Southern Blot

Answer

Western Blot

Question 10

What type of hypersensitivity reaction is seen in myasthenia gravis?

Accepted Answer

Type 2 hypersensitivity reaction

Answer

Type 1 hypersensitivity reaction

Answer

Type 3 hypersensitivity reaction

Answer

Type 4 hypersensitivity reaction

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?