Immunology Practice Questions

Q: Which of the following laboratory assays can be used to detect the number and types of immune cells in the peripheral blood?

Flow cytometry. **Explanation:** **Flow cytometry** is the gold standard for identifying and quantifying specific cell populations in a fluid suspension. It works by passing cells in a single-file stream through a laser beam. Cells are pre-labeled with fluorescent-tagged monoclonal antibodies that bind to specific surface markers (Clusters of Differentiation or **CD markers**). As each cell passes the laser, it scatters light and emits fluorescence, allowing the machine to measure the cell's size (Forward Scatter), granularity (Side Scatter), and specific type (Fluorescence). This makes it the ideal tool for "immunophenotyping"—calculating the absolute number and percentage of T-cells (CD3), B-cells (CD19/20), and NK cells. **Why the other options are incorrect:** * **Immunofixation electrophoresis:** Used to identify specific types of monoclonal antibodies (M-protein) in conditions like Multiple Myeloma. It detects proteins, not whole cells. * **Immunoblot (Western Blot):** Used to detect specific proteins (antigens or antibodies) in a sample (e.g., confirmatory test for HIV). It involves protein separation by molecular weight. * **Enzyme immunoassay (EIA/ELISA):** Used to detect and quantify soluble antigens or antibodies in serum (e.g., screening for Hepatitis B or HIV). It cannot count or differentiate cell populations. **High-Yield Clinical Pearls for NEET-PG:** * **CD4/CD8 Ratio:** Flow cytometry is the primary method used to monitor the CD4 count in HIV/AIDS patients. * **Leukemia/Lymphoma:** It is essential for the "lineage" determination of malignant cells (e.g., distinguishing AML from ALL). * **PNH (Paroxysmal Nocturnal Hemoglobinuria):** Flow cytometry is the most sensitive test to detect the absence of CD55 and CD59 on RBCs/WBCs.

Q: What differentiates NK cells from cytotoxic T cells?

Antibody specificity.. **Explanation:** The fundamental difference between Natural Killer (NK) cells and Cytotoxic T cells ($CD8^+$) lies in their **recognition mechanism**. 1. **Why "Antibody Specificity" is correct:** Cytotoxic T cells are part of the **adaptive immune system**. They possess T-cell receptors (TCRs) that recognize specific antigens presented by MHC Class I molecules. In contrast, NK cells are part of the **innate immune system**. They do not possess antigen-specific receptors (like TCRs or antibodies) and do not require prior sensitization. Instead, they use a "balance of signals" from **Killer Immunoglobulin-like Receptors (KIRs)** to identify and kill cells that lack MHC Class I (the "missing self" hypothesis). 2. **Analysis of Incorrect Options:** * **A. Interferons:** Incorrect. Interferons (especially IFN-α and IFN-β) actually **enhance** NK cell activity, making them more effective at killing virally infected cells. * **C. Receptor for IgG:** Incorrect. Both cells can interact with antibodies, but NK cells characteristically express **CD16** (FcγRIII), which allows them to perform Antibody-Dependent Cellular Cytotoxicity (ADCC). This is a feature, not a differentiator that favors T-cells. * **D. Presence in the spleen:** Incorrect. Both NK cells and T cells are found in the spleen and other secondary lymphoid organs. **High-Yield Clinical Pearls for NEET-PG:** * **NK Cell Markers:** CD16 (Fc receptor) and CD56 (NCAM) are the definitive markers. * **MHC Restriction:** $CD8^+$ T cells are MHC-restricted (require MHC-I), while NK cells are **MHC-unrestricted** (kill cells that downregulate MHC-I to evade T cells). * **Granules:** Both use Perforins and Granzymes to induce apoptosis in target cells. * **IL-2 and IL-12:** These cytokines are potent activators of NK cells.

Q: MHC class II molecules are typically expressed on which of the following cell types?

Macrophages. **Explanation:** The Major Histocompatibility Complex (MHC) molecules are critical for the immune system to distinguish self from non-self. **MHC Class II** molecules are specifically expressed on **Professional Antigen-Presenting Cells (pAPCs)**. **1. Why Macrophages are correct:** Macrophages, along with Dendritic cells and B-cells, are the primary pAPCs. Their role is to engulf exogenous pathogens, process them into peptides, and present them via MHC Class II molecules to **CD4+ T-helper cells**. This interaction is the "first signal" required to initiate an adaptive immune response. **2. Why the other options are incorrect:** * **Platelets and Red Blood Cells (RBCs):** These are non-nucleated cells. MHC Class I is expressed on almost all nucleated cells; therefore, RBCs lack both MHC I and MHC II. Platelets also do not typically express MHC II. * **Endothelial Cells:** While they are nucleated and express **MHC Class I**, they do not constitutively express MHC Class II. They may express it only under specific inflammatory conditions (induced by Interferon-gamma), but they are not considered "professional" APCs. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** MHC I presents to CD8+ T-cells (Rule of 8: 1 × 8 = 8), while MHC II presents to CD4+ T-cells (Rule of 8: 2 × 4 = 8). * **Structure:** MHC II is a heterodimer of **α and β chains**, both encoded by the MHC locus (HLA-DP, DQ, DR). * **Invariant Chain (Ii):** This protein binds to the MHC II groove in the ER to prevent binding of self-peptides before it reaches the endosome. * **Human Analogue:** In humans, MHC is synonymous with **HLA (Human Leukocyte Antigen)**.

Q: A woman with infertility receives an ovary transplant from her sister who is an identical twin. What type of graft is it?

Isograft. ### Explanation The correct answer is **Isograft (Option D)**. **1. Why Isograft is correct:** An **Isograft** (also known as a syngeneic graft) is a tissue or organ transplant between two genetically identical individuals of the same species. In humans, this occurs exclusively between **monozygotic (identical) twins**. Because the donor and recipient share identical Major Histocompatibility Complex (MHC/HLA) molecules, the recipient’s immune system does not recognize the graft as foreign. Consequently, there is no immune response, and the graft is accepted without the need for long-term immunosuppression. **2. Why other options are incorrect:** * **Allograft (A):** This is a transplant between genetically different members of the same species (e.g., non-identical siblings, parent-to-child, or unrelated donors). This is the most common type of clinical transplant and requires immunosuppression to prevent rejection. * **Autograft (B):** This involves moving tissue from one site to another on the **same individual** (e.g., a skin graft from the thigh to the arm or a CABG using the saphenous vein). * **Xenograft (C):** This is a transplant between members of **different species** (e.g., a porcine/pig heart valve transplanted into a human). These are subject to rapid hyperacute rejection. **3. NEET-PG High-Yield Pearls:** * **Order of Immunogenicity:** Xenograft > Allograft > Isograft = Autograft. * **MHC/HLA:** The primary barrier to transplantation is the MHC (HLA in humans) located on **Chromosome 6**. * **Identical Twins vs. Fraternal Twins:** A transplant between fraternal (dizygotic) twins is considered an **Allograft**, not an isograft, because they are genetically distinct. * **Clinical Note:** Isografts and Autografts do not require immunosuppressive therapy because they do not trigger T-cell mediated rejection.

Q: Which of the following antigens is most immunogenic?

H antigen. **Explanation:** The immunogenicity of an antigen is primarily determined by its chemical complexity. The **H antigen (Flagellar antigen)** is composed of **proteins (flagellin)**. Proteins are the most potent immunogens because they possess complex tertiary structures and are processed via the MHC-II pathway, leading to a robust T-cell dependent immune response. This results in high antibody titers and the formation of memory cells. **Analysis of Options:** * **B. H antigen (Correct):** Being proteinaceous in nature, it is highly immunogenic compared to polysaccharide-based antigens. * **A & D. O antigen / Somatic antigen (Incorrect):** These refer to the same structure—the outer membrane Lipopolysaccharide (LPS). While O antigens are important for serotyping, they are heat-stable polysaccharides. Polysaccharides are generally less immunogenic than proteins as they often trigger T-cell independent responses. * **C. Vi antigen (Incorrect):** This is a capsular polysaccharide (Virulence antigen) found in *Salmonella Typhi*. Like the O antigen, its carbohydrate nature makes it less immunogenic than the protein-based H antigen. **NEET-PG High-Yield Pearls:** * **Chemical Nature:** H = Protein (Heat labile); O = Phospholipid-Polysaccharide complex (Heat stable); Vi = Polysaccharide (Heat labile). * **Widal Test Application:** In Enteric fever, anti-H antibodies appear later but persist longer and reach higher titers than anti-O antibodies due to the superior immunogenicity of the H antigen. * **Hierarchy of Immunogenicity:** Proteins > Polysaccharides > Lipids/Nucleic acids.

Q: Which of the following is a non-organ-specific (systemic) autoimmune disease?

Systemic lupus erythematosus. **Explanation:** Autoimmune diseases are broadly classified into two categories: **Organ-specific**, where the immune response is directed against antigens restricted to a single organ, and **Non-organ-specific (Systemic)**, where antibodies react with antigens widespread throughout the body. **Why Systemic Lupus Erythematosus (SLE) is correct:** SLE is the prototype of systemic autoimmune diseases. It is characterized by the production of **Antinuclear Antibodies (ANA)** and antibodies against various self-antigens (like dsDNA and Smith antigen) found in almost all nucleated cells. This leads to immune complex deposition and inflammation across multiple systems, including the skin, joints, kidneys, heart, and central nervous system. **Why the other options are incorrect:** * **Myasthenia Gravis:** Organ-specific; antibodies specifically target **acetylcholine receptors (AChR)** at the neuromuscular junction, affecting skeletal muscle activation. * **Hashimoto’s Thyroiditis:** Organ-specific; the immune attack (via anti-TPO and anti-thyroglobulin antibodies) is localized strictly to the **thyroid gland**, leading to hypothyroidism. * **Pernicious Anemia:** Organ-specific; antibodies target **gastric parietal cells** or **intrinsic factor**, localized to the stomach lining, resulting in Vitamin B12 deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **SLE Marker:** Anti-dsDNA is highly specific and correlates with disease activity (especially lupus nephritis). * **Drug-Induced Lupus:** Associated with **Anti-Histone antibodies** (Common drugs: Hydralazine, Procainamide, Isoniazid). * **Type of Hypersensitivity:** Most systemic autoimmune diseases (like SLE) involve **Type III hypersensitivity** (immune complexes), whereas many organ-specific diseases (like Hashimoto's) involve **Type II or Type IV**.

Q: Which of the following principally mediates delayed type hypersensitivity?

Cytotoxic T cells, TH1 cells, and macrophages. ### Explanation **Delayed-Type Hypersensitivity (DTH)**, also known as **Type IV Hypersensitivity**, is the only hypersensitivity reaction that is **cell-mediated** rather than antibody-mediated. **Why Option A is Correct:** The process occurs in two phases: 1. **Sensitization Phase:** CD4+ T cells (specifically **TH1 cells**) recognize antigens presented by APCs and undergo clonal expansion. 2. **Effector Phase:** Upon re-exposure, TH1 cells release cytokines (IFN-γ, TNF-β, and IL-2). **IFN-γ** is the most critical cytokine as it activates **macrophages**, enhancing their phagocytic and microbicidal activity. In some cases (like graft rejection or contact dermatitis), **CD8+ Cytotoxic T cells** directly destroy target cells. Thus, TH1 cells, macrophages, and Cytotoxic T cells are the primary mediators. **Why Other Options are Incorrect:** * **Option B:** Describes **Type I (Immediate) Hypersensitivity**, mediated by IgE binding to mast cells and basophils (e.g., Anaphylaxis, Asthma). * **Option C:** Describes **Type II (Cytotoxic) Hypersensitivity**, where IgG/IgM antibodies bind to cell surface antigens, leading to complement activation (e.g., Rh incompatibility, Myasthenia Gravis). * **Option D:** Describes **Type III (Immune-Complex) Hypersensitivity**, involving circulating Ag-Ab complexes that deposit in tissues, attracting neutrophils (e.g., SLE, Post-streptococcal glomerulonephritis). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Type IV is "**D**elayed" (starts after 24–48 hours). * **Classic Examples:** Mantoux Test (Tuberculin reaction), Contact Dermatitis (Poison ivy, Nickel), Lepromin test, and Graft rejection. * **Granuloma Formation:** Chronic Type IV reactions lead to granulomas (seen in Tuberculosis and Sarcoidosis) due to persistent macrophage activation by TH1 cells.

Question 1

The variable region of an antibody molecule is located where?

Accepted Answer

N-terminal

Answer

C-terminal

Answer

Intermediate region

Answer

Carbohydrate moiety

Question 2

Which of the following laboratory assays can be used to detect the number and types of immune cells in the peripheral blood?

Accepted Answer

Flow cytometry

Answer

Immunofixation electrophoresis

Answer

Immunoblot

Answer

Enzyme immunoassay (EIA)

Question 3

What differentiates NK cells from cytotoxic T cells?

Accepted Answer

Antibody specificity.

Answer

Interferons reduce NK cell activity.

Answer

Receptor for IgG.

Answer

Presence in the spleen.

Question 4

MHC class II molecules are typically expressed on which of the following cell types?

Accepted Answer

Macrophages

Answer

Platelets

Answer

Red blood cells

Answer

Endothelial cells

Question 5

A woman with infertility receives an ovary transplant from her sister who is an identical twin. What type of graft is it?

Accepted Answer

Isograft

Answer

Allograft

Answer

Autograft

Answer

Xenograft

Question 6

Which of the following antigens is most immunogenic?

Accepted Answer

H antigen

Answer

O antigen

Answer

Vi antigen

Answer

Somatic antigen

Question 7

Human immunoglobulin A can be described by which of the following statements?

Accepted Answer

It can prevent attachment of microorganisms to epithelial cell membranes.

Answer

It is the predominant immunoglobulin in plasma.

Answer

It exists in four subclasses, of which IgA2 is predominant.

Answer

It is prominent early in the immune response and is the major class of antibody in cold agglutinins.

Question 8

Which of the following is considered an immune privilege site?

Accepted Answer

Seminiferous tubule

Answer

Optic nerve

Answer

Area postrema

Answer

Spinal cord

Question 9

Which of the following is a non-organ-specific (systemic) autoimmune disease?

Accepted Answer

Systemic lupus erythematosus

Answer

Myasthenia gravis

Answer

Hashimoto's thyroiditis

Answer

Pernicious anemia

Question 10

Which of the following principally mediates delayed type hypersensitivity?

Accepted Answer

Cytotoxic T cells, TH1 cells, and macrophages

Answer

IgE, basophils, and mast cells

Answer

IgG and complement

Answer

IgG, IgM, neutrophils, and macrophages

Immunology — MCQs

Immunology — MCQs

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