Immunology Practice Questions

Q: A 9-year-old girl receives an intradermal tuberculin injection and later develops an indurated, erythematous papule 12 mm in diameter. This reaction is an example of which of the following?

Type IV hypersensitivity. ### Explanation **Correct Option: D. Type IV hypersensitivity** The clinical scenario describes a classic **Mantoux (Tuberculin) test** reaction. Type IV hypersensitivity, also known as **Delayed-Type Hypersensitivity (DTH)**, is mediated by T-lymphocytes rather than antibodies. Upon intradermal injection of PPD (Purified Protein Derivative), sensitized **Th1 cells** release cytokines (IFN-γ, IL-2), which recruit and activate macrophages. This process takes **48–72 hours** to manifest as the indurated papule described. **Analysis of Incorrect Options:** * **A. Antibody-dependent cell-mediated cytotoxicity (ADCC):** This is a mechanism associated with **Type II hypersensitivity**, where NK cells or macrophages kill target cells coated with IgG antibodies (e.g., transplant rejection). * **B. Local anaphylaxis:** This refers to **Type I hypersensitivity**, which is IgE-mediated and occurs within minutes (immediate) due to mast cell degranulation. * **C. T-cell mediated cytotoxicity:** While also a form of Type IV hypersensitivity, this specifically involves **CD8+ T-cells** directly killing target cells (e.g., viral infections or contact dermatitis). The tuberculin reaction is primarily a **CD4+ Th1-mediated** inflammatory response. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hypersensitivity (Gell & Coombs):** **A**cid (Type I: **A**llergic/Anaphylactic), **C**ytotoxic (Type II), **I**mmune-Complex (Type III), **D**elayed (Type IV). * **Type IV Examples:** Mantoux test, Contact dermatitis (Nickel, Poison Ivy), Lepromin test, and Granuloma formation (Tuberculosis, Sarcoidosis). * **Key Cells:** The hallmark of Type IV is the absence of antibodies; it is entirely **cell-mediated**. * **Induration vs. Erythema:** In a Mantoux test, only the **induration** (palpable hardness) is measured, not the erythema (redness).

Q: Which antibody is the most avidly complement fixing?

IgG. **Explanation:** The ability of an antibody to fix complement via the **Classical Pathway** depends on its structure and the availability of binding sites for the **C1q** molecule. **Why IgG is the correct answer:** While a single molecule of IgM is technically more efficient at initiating the complement cascade due to its pentameric structure, **IgG** is considered the most significant complement-fixing antibody in the context of standard medical examinations like NEET-PG when discussing overall serum activity and clinical relevance. Specifically, **IgG3** is the most potent subclass, followed by IgG1 and IgG2. IgG fixes complement after binding to an antigen, which causes a conformational change in its Fc region, allowing C1q to bind. **Analysis of Incorrect Options:** * **IgM:** Often causes confusion because it is the *most efficient* on a per-molecule basis (only one pentamer is needed). However, in many textbook contexts, IgG is highlighted for its dominant role in systemic complement activation. (Note: If the question asks for the "most efficient" or "first" antibody, IgM is the choice; for "most avidly/clinically significant," IgG is often the keyed answer). * **IgA:** Primarily functions in mucosal immunity. It does not activate the classical pathway; it can only weakly activate the **alternative pathway**. * **IgE:** Involved in Type I hypersensitivity and parasitic infections. It does not fix complement. **NEET-PG High-Yield Pearls:** 1. **Order of Complement Fixation (IgG subclasses):** IgG3 > IgG1 > IgG2. (IgG4 does *not* fix complement). 2. **Pathway Trigger:** The Classical pathway is triggered by **Antigen-Antibody complexes** (IgG or IgM). 3. **C3:** This is the most abundant complement component in the serum and the point where all three pathways (Classical, Alternative, Lectin) converge.

Q: What is the marker for NK cell activity?

CD56. **Explanation:** Natural Killer (NK) cells are large granular lymphocytes that play a critical role in the innate immune system by providing a first-line defense against virally infected cells and tumor cells. **Why CD56 is the correct answer:** CD56 (Neural Cell Adhesion Molecule or NCAM) is the definitive phenotypic marker used to identify NK cells in clinical practice and flow cytometry. While NK cells are also characterized by the expression of **CD16** (an Fc receptor for IgG that mediates Antibody-Dependent Cellular Cytotoxicity or ADCC), **CD56** is the most widely used marker for their identification. NK cells are uniquely defined as being **CD3 negative** and **CD56 positive**. **Analysis of Incorrect Options:** * **CD3:** This is the pan-T cell marker. It is part of the T-cell receptor (TCR) complex. Its absence is a key feature used to distinguish NK cells from T-lymphocytes. * **CD4:** This is a marker for Helper T-cells (Th cells) and is also found on monocytes and dendritic cells. It interacts with MHC class II molecules. * **CD19:** This is a primary marker for B-cells (along with CD20 and CD21). It is involved in B-cell signaling and activation. **High-Yield Clinical Pearls for NEET-PG:** * **NK Cell Function:** They do not require prior sensitization and lack antigen-specific receptors. They kill cells that show "missing self" (downregulation of MHC-I). * **Markers:** NK cells are **CD3–, CD16+, and CD56+**. * **Cytokine Activation:** NK cell activity is significantly enhanced by **IL-2 and IL-12**. * **LAK Cells:** NK cells treated with high doses of IL-2 become Lymphokine-Activated Killer (LAK) cells, used in experimental cancer immunotherapy.

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: The Nitroblue Tetrazolium Test is used for assessing which process?

Phagocytosis. **Explanation:** The **Nitroblue Tetrazolium (NBT) Test** is a classic diagnostic tool used to evaluate the **metabolic burst (oxidative burst)** activity of phagocytic cells, specifically neutrophils. 1. **Why Phagocytosis is Correct:** During phagocytosis, neutrophils produce reactive oxygen species (ROS) like superoxide radicals via the enzyme **NADPH oxidase**. In the NBT test, the colorless NBT dye is added to a sample of the patient's blood. If the phagocytes are functioning correctly, the superoxide radicals reduce the yellow NBT dye into insoluble, dark blue **formazan crystals**. A positive test (blue color) indicates normal oxidative killing, while a negative test (no color change) indicates a defect in this process. 2. **Why Other Options are Incorrect:** * **Complement activation:** Assessed by tests like CH50 (classical pathway) or AH50 (alternative pathway). * **T cell function:** Evaluated via Skin Delayed-Type Hypersensitivity (DTH) tests or Flow Cytometry (CD3/CD4/CD8 counts). * **B cell proliferation:** Assessed by measuring serum immunoglobulin levels or using Mitogen stimulation tests. **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Granulomatous Disease (CGD):** This is the primary condition diagnosed using the NBT test. It is characterized by a deficiency in **NADPH oxidase**, leading to recurrent infections with **catalase-positive organisms** (e.g., *S. aureus*, *Aspergillus*, *Serratia*). * **Modern Alternative:** The **Dihydrorhodamine (DHR) 123 flow cytometry test** is now the preferred "gold standard" over NBT due to its higher sensitivity and quantitative nature. * **Inheritance:** CGD is most commonly **X-linked recessive**.

Q: Which of the following is NOT a true statement regarding Natural Killer (NK) cells?

NK cells have a significant role in cell-mediated immunity.. **Explanation:** The core concept tested here is the classification of immune cells. While **Natural Killer (NK) cells** are lymphocytes, they are fundamentally part of the **innate immune system**, not the adaptive (acquired) immune system. 1. **Why Option A is the Correct Answer (The False Statement):** "Cell-mediated immunity" (CMI) specifically refers to the adaptive immune response mediated by **T-lymphocytes** (T-helper and Cytotoxic T-cells). CMI is characterized by antigen specificity and memory. NK cells, conversely, do not possess T-cell receptors (TCR) or undergo gene rearrangement; they provide a non-specific, rapid first line of defense without requiring prior sensitization. Therefore, they are not considered primary mediators of CMI. 2. **Analysis of Incorrect Options (True Statements):** * **Option B:** NK cells are indeed part of the **innate immune system**. They recognize "stressed" cells (virally infected or tumor cells) that have downregulated MHC Class I molecules (the "missing self" hypothesis). * **Option C & D:** These are the definitive phenotypic markers for NK cells. **CD56** (NCAM) is the universal marker used to identify them, while **CD16** (FcγRIII) allows NK cells to bind to IgG-coated target cells, mediating **Antibody-Dependent Cellular Cytotoxicity (ADCC)**. **High-Yield NEET-PG Pearls:** * **Morphology:** NK cells are identified histologically as **Large Granular Lymphocytes (LGLs)**. * **Cytokines:** Their activity is significantly enhanced by **IL-2, IL-12, and IFN-α/β**. * **Receptors:** They use **KIRs** (Killer Immunoglobulin-like Receptors) to monitor MHC Class I expression on host cells. * **Granules:** They kill targets via **Perforins** (create pores) and **Granzymes** (induce apoptosis).

Question 1

One of the most remarkable aspects of the human immune system is its diversity, that is, the ability to recognize a wide range of antigens and to mount a specific antibody response. This is called clonal selection. At the cellular level, which of the following are primarily responsible for such specificity?

Accepted Answer

Hypervariable regions in domains of B cells

Answer

Cytotoxic T cells

Answer

The major histocompatibility complex

Answer

Specific T cell receptors

Question 2

A 9-year-old girl receives an intradermal tuberculin injection and later develops an indurated, erythematous papule 12 mm in diameter. This reaction is an example of which of the following?

Accepted Answer

Type IV hypersensitivity

Answer

Antibody-dependent cell-mediated cytotoxicity

Answer

Local anaphylaxis

Answer

T-cell mediated cytotoxicity

Question 3

Which antibody is the most avidly complement fixing?

Accepted Answer

IgG

Answer

IgA

Answer

IgM

Answer

IgE

Question 4

What is the marker for NK cell activity?

Accepted Answer

CD56

Answer

CD3

Answer

CD4

Answer

CD19

Question 5

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 6

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 7

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 8

The Nitroblue Tetrazolium Test is used for assessing which process?

Accepted Answer

Phagocytosis

Answer

Complement activation

Answer

T cell function

Answer

B cell proliferation

Question 9

Which of the following is NOT a true statement regarding Natural Killer (NK) cells?

Accepted Answer

NK cells have a significant role in cell-mediated immunity.

Answer

NK cells are part of the innate immune system.

Answer

NK cells are typically CD16 positive.

Answer

NK cells are typically CD56 positive.

Question 10

Which of the following is true about immunoglobulin gene rearrangement and immunoglobulin diversity?

Accepted Answer

Somatic mutations theory

Answer

One loop and two loop joining theory

Answer

DNA rearrangement

Answer

Appropriate class switching

Immunology — MCQs

Immunology — MCQs

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