Immunology Practice Questions

Q: What is the most potent stimulator of naive T cells?

Mature dendritic cells. **Explanation:** The activation of a **naive T cell** requires two specific signals: the recognition of an antigen-MHC complex and a potent co-stimulatory signal (primarily B7-1/B7-2 binding to CD28). **Mature Dendritic Cells (DCs)** are the most potent professional Antigen-Presenting Cells (APCs) because they constitutively express high levels of **MHC Class II** and **co-stimulatory molecules (B7)**. Unlike other APCs, mature DCs are uniquely capable of migrating from peripheral tissues to secondary lymphoid organs to present antigens to naive T cells, making them the primary initiators of the adaptive immune response. **Analysis of Incorrect Options:** * **T cells:** T cells are the recipients of the stimulus, not the stimulators. They require activation by APCs to differentiate into effector cells. * **Epithelial cells:** These are non-professional APCs. While they can express MHC Class I, they lack the necessary co-stimulatory molecules to activate naive T cells and generally cannot initiate a primary immune response. * **Macrophages:** While they are professional APCs, they are less potent than DCs for *naive* cells. Macrophages primarily present antigens to already *activated* effector T cells at the site of infection to enhance microbicidal activity. **High-Yield Facts for NEET-PG:** * **Langerhans cells** are immature dendritic cells found in the epidermis; they become "mature" and potent stimulators only after migrating to regional lymph nodes. * **B cells** are also professional APCs but primarily present antigens to Helper T cells to facilitate antibody production (humoral immunity). * **Signal 1** = TCR + MHC-Peptide; **Signal 2** = CD28 (T cell) + B7/CD80/86 (APC). Absence of Signal 2 leads to **anergy**.

Q: Which one of the following is the best neutrophil and macrophage attractant?

C5a. ### Explanation **Correct Option: A. C5a** C5a is a potent **anaphylatoxin** and the most powerful chemotactic factor derived from the complement system. It acts as a "chemical beacon" that attracts neutrophils, macrophages, monocytes, and eosinophils to the site of inflammation (chemotaxis). It also increases vascular permeability and triggers mast cell degranulation. While C3a and C4a are also anaphylatoxins, they lack the significant chemotactic potency of C5a. **Analysis of Incorrect Options:** * **B. Variable region of the heavy chain of IgG:** This region is responsible for **antigen binding specificity** (Fab portion). It does not possess chemotactic properties. The *Fc region* of IgG is what interacts with phagocyte receptors for opsonization, but it is not a primary attractant. * **C. J chain (Joining chain):** This is a glycoprotein found in **polymeric immunoglobulins** (Secretory IgA and IgM). Its primary role is to hold the subunits together and facilitate the transport of IgA across mucosal surfaces. * **D. HLA-A:** This is a **MHC Class I** molecule found on all nucleated cells. Its function is to present endogenous antigens to CD8+ T-cells, not to attract phagocytes. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Chemotactic Agents:** Remember the "Big Four": **C5a**, **Interleukin-8 (IL-8)**, **Leukotriene B4 (LTB4)**, and **Bacterial products** (e.g., N-formyl methionine). * **Opsonization:** While C5a attracts cells, **C3b** is the primary complement component responsible for opsonization (tagging for ingestion). * **Deficiency:** A deficiency in the C5-C9 components (Membrane Attack Complex) predisposes individuals to recurrent *Neisseria* infections.

Q: Which of the following statements about NK cells is INCORRECT?

Mediates type IV hypersensitivity. ### Explanation **Correct Answer: A. Mediates type IV hypersensitivity** **Why Option A is Incorrect (The Right Choice):** Type IV (Delayed-type) hypersensitivity is primarily mediated by **T-lymphocytes** (specifically CD4+ Th1 cells and CD8+ T cells), not NK cells. In Type IV reactions, sensitized T-cells release cytokines that activate macrophages or cause direct cytotoxicity. NK cells are part of the **innate immune system**, whereas Type IV hypersensitivity is a component of the **adaptive immune response**. **Analysis of Other Options:** * **Option B (Kills virus-infected cells):** This is a primary function of NK cells. They identify and destroy cells that have downregulated MHC-I molecules (the "missing self" hypothesis), a common tactic used by viruses to evade CD8+ T-cells. * **Option C (Large granular lymphocytes):** Morphologically, NK cells are identified as large granular lymphocytes (LGLs). They are larger than resting B or T cells and contain prominent cytoplasmic granules. * **Option D (Releases perforin and granzyme):** NK cells utilize the perforin-granzyme pathway to induce apoptosis in target cells. Perforin creates pores in the target cell membrane, allowing granzymes (proteases) to enter and trigger the caspase cascade. **High-Yield Clinical Pearls for NEET-PG:** * **Markers:** NK cells are typically identified by the presence of **CD56** and **CD16** (FcγRIII) and the **absence of CD3**. * **Antibody-Dependent Cellular Cytotoxicity (ADCC):** NK cells use their CD16 receptor to bind to the Fc portion of IgG-coated target cells, leading to cell lysis. * **Cytokine Production:** They are a major source of **IFN-gamma**, which activates macrophages. * **MHC Restriction:** Unlike T-cells, NK cells are **not MHC-restricted**; they do not require antigen presentation via MHC to function.

Q: A false-negative tuberculin test is seen in all the following conditions EXCEPT:

Atypical mycobacterial infection. The Tuberculin Skin Test (Mantoux test) is a classic example of a **Type IV (Delayed-Type) Hypersensitivity reaction**, mediated by T-lymphocytes. A false-negative result occurs when the body fails to mount an adequate cell-mediated immune (CMI) response despite being infected with *M. tuberculosis*. ### **Why "Atypical mycobacterial infection" is the correct answer:** Atypical mycobacteria (Non-Tuberculous Mycobacteria or NTM) share several common antigens with *M. tuberculosis*. Therefore, infection with NTM typically causes **cross-reactivity**, leading to a **false-positive** result (or a small induration), rather than a false-negative. ### **Explanation of Incorrect Options (Causes of False-Negatives):** * **After a 4-6 week history of measles attack:** Viral infections like measles, mumps, and chickenpox are known to cause transient **anergy** (suppression of CMI), leading to false-negative results. * **Immunodeficiency state:** Conditions such as HIV/AIDS, Hodgkin’s lymphoma, or treatment with immunosuppressants (steroids, anti-TNF agents) impair T-cell function, preventing the induration. * **Miliary tuberculosis:** In overwhelming or disseminated TB, the body’s immune system is "overloaded" or exhausted, leading to a lack of localized skin response (anergy). ### **High-Yield Clinical Pearls for NEET-PG:** * **Reading the test:** The result is read after **48–72 hours**. Only the **induration** (palpable hardness) is measured, not the erythema (redness). * **The "Window Period":** It takes 2–10 weeks after initial infection for the Mantoux test to become positive. Testing during this window yields a false-negative. * **Storage:** PPD (Purified Protein Derivative) should be stored at 2–8°C and protected from light to prevent degradation. * **BCG Vaccine:** Can cause a false-positive, though the induration is usually smaller and wanes over time.

Q: In contact dermatitis, which cells play a major role?

T-cells. **Explanation:** Contact dermatitis is a classic example of **Type IV Hypersensitivity (Delayed-type hypersensitivity)** [4]. The pathogenesis is mediated by cellular immunity rather than antibodies [1]. 1. **Why T-cells are correct:** The reaction occurs in two phases: sensitization and elicitation [1]. Upon re-exposure to an allergen (hapten), memory **T-cells (specifically CD4+ Th1 and CD8+ cytotoxic T-cells)** are activated [2], [3]. They release cytokines like IFN-γ and TNF-α, which induce inflammation and keratinocyte damage. Since T-cells are the primary effectors driving the immune response, they are the "major" cells involved. 2. **Why other options are incorrect:** * **B-cells:** These are involved in antibody-mediated (Type I, II, and III) hypersensitivity [4]. Contact dermatitis is cell-mediated and independent of antibodies. * **Langerhans cells:** While these are the professional Antigen Presenting Cells (APCs) in the skin that initiate the response by carrying the antigen to regional lymph nodes, they are "initiators" rather than the primary "effectors" of the clinical reaction. * **Macrophages:** These act as downstream effector cells that cause tissue damage after being recruited and activated by T-cell cytokines, but they are not the specific recognition cells of the reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Time frame:** Type IV reactions typically take **48–72 hours** to manifest (hence "delayed") [4]. * **Common triggers:** Nickel (jewelry), Poison ivy (Urushiol), Neomycin, and Latex. * **Diagnostic Test:** The **Patch Test** is the gold standard for identifying the causative allergen in contact dermatitis. * **Key Cytokine:** **IFN-γ** is the hallmark cytokine of the Th1 response in Type IV hypersensitivity.

Q: Which of the following statements about complement is false?

Constitutes about 20% of normal serum proteins. ### Explanation **1. Why Option B is the Correct (False) Statement:** The complement system consists of a group of approximately 30 proteins, but they constitute only about **10% of the total serum globulin fraction** (or roughly **5% of total serum proteins**). Stating that they make up 20% is a significant overestimation, making this the false statement. **2. Analysis of Other Options:** * **Option A (Group of proteins normally found in serum):** This is true. Complement proteins are synthesized primarily by the liver (and also by macrophages) and circulate in an inactive (pro-enzyme) form in the blood. * **Option C (Heat labile):** This is true. Complement is highly sensitive to heat. It is inactivated when serum is heated at **56°C for 30 minutes**. This property is used in laboratories to "inactivate" serum before certain serological tests. * **Option D (Species non-specific):** This is true. Unlike antibodies, which are specific to the antigen and the species that produced them, complement from one species (e.g., guinea pig) can react with antibodies from another species (e.g., human) to cause lysis. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Most Abundant Component:** **C3** is the most abundant complement protein in the serum. * **Pathways:** There are three pathways—Classical (triggered by Ag-Ab complexes), Alternative (triggered by endotoxins/microbial surfaces), and Lectin (triggered by mannose-binding lectin). * **C3b:** Acts as a powerful **opsonin** (enhances phagocytosis). * **C5a:** The most potent **anaphylatoxin** and chemotactic agent. * **Membrane Attack Complex (MAC):** Formed by **C5b-C9**, responsible for cell lysis. * **Deficiency:** C3 deficiency is the most severe as it is the "bottleneck" of all pathways, leading to recurrent pyogenic infections.

Q: The Mantoux test is an indicator of which type of hypersensitivity reaction?

Delayed hypersensitivity. **Explanation:** The **Mantoux test** (Tuberculin Skin Test) is the classic clinical example of a **Type IV Hypersensitivity reaction**, also known as **Delayed-Type Hypersensitivity (DTH)**. **Why it is correct:** When Purified Protein Derivative (PPD) is injected intradermally, it triggers a response in individuals previously sensitized to *Mycobacterium tuberculosis*. This reaction is mediated by **T-lymphocytes** (specifically Th1 cells) rather than antibodies. These cells release cytokines (like IFN-γ) that recruit macrophages, leading to local inflammation and induration. The reaction is termed "delayed" because it takes **48–72 hours** to reach its peak, reflecting the time required for T-cell recruitment and activation. **Analysis of Incorrect Options:** * **Option A (Immediate):** Refers to Type I hypersensitivity (IgE-mediated), such as anaphylaxis or asthma, which occurs within minutes. * **Option C (Cell-mediated):** While Type IV is indeed cell-mediated, in the context of the Mantoux test, "Delayed hypersensitivity" is the more specific and standard terminology used in clinical examinations to describe the temporal nature of the skin reaction. * **Option D (Immune complex-mediated):** Refers to Type III hypersensitivity (e.g., Arthus reaction, SLE), involving Ag-Ab complexes depositing in tissues. **High-Yield Clinical Pearls for NEET-PG:** * **Key Mediator:** CD4+ T-lymphocytes (Th1). * **Reading the test:** Only the **induration** (palpable hardness) is measured, not the erythema (redness). * **False Negative:** Can occur in miliary TB, sarcoidosis, malnutrition, or AIDS (due to **anergy**). * **False Positive:** Common in individuals who have received the **BCG vaccine** or have atypical mycobacterial infections.

Question 1

Which of the following statements is true?

Accepted Answer

Antigen-antibody reaction cannot occur in the absence of electrolytes

Answer

Paul Bunnell test is used to diagnose measles

Answer

Rose Waaler test is a complement fixation test

Answer

Indirect hemagglutination test is less sensitive than gel diffusion test

Question 2

What is the most potent stimulator of naive T cells?

Accepted Answer

Mature dendritic cells

Answer

T cells

Answer

Epithelial cells

Answer

Macrophages

Question 3

Which one of the following is the best neutrophil and macrophage attractant?

Accepted Answer

C5a

Answer

Variable region of the heavy chain of IgG

Answer

J chain

Answer

HLA-A

Question 4

Which of the following statements about NK cells is INCORRECT?

Accepted Answer

Mediates type IV hypersensitivity

Answer

Kills virus-infected cells

Answer

They are large granular lymphocytes

Answer

Releases small cytoplasmic granules of proteins called perforin and granzyme

Question 5

A false-negative tuberculin test is seen in all the following conditions EXCEPT:

Accepted Answer

Atypical mycobacterial infection

Answer

After a 4-6 week history of measles attack

Answer

Immunodeficiency state

Answer

Miliary tuberculosis

Question 6

In contact dermatitis, which cells play a major role?

Accepted Answer

T-cells

Answer

B-cells

Answer

Langerhans cells

Answer

Macrophages

Question 7

Which of the following statements about complement is false?

Accepted Answer

Constitutes about 20% of normal serum proteins

Answer

Group of proteins normally found in serum

Answer

Heat labile

Answer

Species non-specific

Question 8

Autoimmunity is caused by which of the following mechanisms?

Accepted Answer

Failure of negative selection of T cells in the thymus

Answer

Pressure of forbidden clones

Answer

Expression of cryptic antigens

Answer

Inappropriate expression of MHC proteins

Question 9

The Mantoux test is an indicator of which type of hypersensitivity reaction?

Accepted Answer

Delayed hypersensitivity

Answer

Immediate hypersensitivity

Answer

Cell-mediated hypersensitivity

Answer

Immune complex-mediated hypersensitivity

Question 10

An allograft is a graft transplanted between:

Accepted Answer

Individuals of the same species but different genetic identity

Answer

Individuals of the same genetic constitution

Answer

Genetically identical individuals (twins)

Answer

Members of different species

Immunology — MCQs

Immunology — MCQs

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