Immunology Practice Questions

Q: Delayed tuberculin test response is due to:

T lymphocytes. **Explanation:** The tuberculin test (Mantoux test) is the classic clinical example of a **Type IV Hypersensitivity reaction**, also known as **Delayed-Type Hypersensitivity (DTH)**. **Why T lymphocytes are correct:** The reaction is mediated by **T lymphocytes**, specifically CD4+ Th1 cells. When tuberculin antigen (PPD) is injected intradermally in a previously sensitized individual, memory T cells recognize the antigen. These cells then release cytokines (such as IFN-γ and IL-2), which recruit and activate macrophages. This process takes time to develop, which is why the induration is read **48 to 72 hours** after the injection—hence the term "delayed." **Why other options are incorrect:** * **B lymphocytes:** These are responsible for Type I, II, and III hypersensitivity reactions via the production of antibodies. Type IV reactions are strictly cell-mediated and do not involve antibodies. * **Monocytes/Histiocytes:** While monocytes (which differentiate into macrophages/histiocytes) are the primary effector cells that cause the actual tissue swelling and induration, they are **recruited and directed** by T lymphocytes. The "response" or the immunological memory that triggers the test is fundamentally a T-cell function. **NEET-PG High-Yield Pearls:** * **Key Cytokine:** IFN-γ (Interferon-gamma) is the most critical cytokine in this reaction. * **Histology:** The hallmark of a positive test is the perivascular accumulation of "cuffing" by mononuclear cells (lymphocytes and macrophages). * **False Negatives:** Can occur in patients with miliary TB, sarcoidosis, malnutrition, or immunosuppression (e.g., HIV/AIDS) due to **anergy** (loss of T-cell reactivity). * **Other Type IV Examples:** Contact dermatitis (poison ivy, nickel), Granuloma formation, and Lepromin test.

Q: Which of the following is a lineage-specific T cell marker?

CD 3. **Explanation:** **CD3** is the definitive lineage-specific marker for **T lymphocytes**. It is a multi-protein complex covalently associated with the T-cell receptor (TCR). Its primary function is signal transduction following antigen recognition. CD3 is expressed on all mature T cells (both Helper and Cytotoxic) and is first detected during the early stages of thymocyte development, making it the most reliable marker for identifying cells of T-cell lineage. **Analysis of Incorrect Options:** * **CD1:** This is a family of glycoproteins structurally related to MHC molecules. They are primarily expressed on **Antigen Presenting Cells (APCs)**, such as dendritic cells and Langerhans cells, and are involved in presenting lipid antigens to NKT cells. * **CD19:** This is the classic lineage-specific marker for **B lymphocytes**. It is expressed from the pro-B cell stage until just before terminal differentiation into plasma cells. * **CD4:** While found on T cells, it is **not lineage-specific**. It is a co-receptor specific to the **Helper T cell subset** (MHC II restricted). Furthermore, CD4 is also expressed on the surface of monocytes and macrophages, which are not of the T-cell lineage. **High-Yield Clinical Pearls for NEET-PG:** * **Pan-T cell markers:** CD2, CD3, CD5, and CD7. * **Pan-B cell markers:** CD19, CD20, and CD21. * **NK cell markers:** CD16 (FcγRIII) and CD56. * **Flow Cytometry:** CD3 is used in clinical practice to differentiate T-cell acute lymphoblastic leukemia (T-ALL) from B-cell lineages. * **Abatacept/Muromonab-CD3:** Historically, OKT3 was a therapeutic monoclonal antibody targeting CD3 used to prevent transplant rejection.

Q: Which of the following statements accurately describes Natural Killer (NK) cells?

NK cells display a cytotoxic effect on tumor cells.. **Explanation:** **Natural Killer (NK) cells** are a distinct lineage of large granular lymphocytes that play a critical role in the innate immune system. Unlike T or B cells, they do not possess antigen-specific receptors (TCR or BCR). **Why Option C is correct:** NK cells are specialized to identify and destroy "stressed" cells, such as **tumor cells** and **virus-infected cells**, without prior sensitization. They function via the "Missing Self" hypothesis: they express **Inhibitory Receptors** (e.g., KIR) that recognize MHC Class I molecules on healthy cells. Since many tumor cells downregulate MHC Class I to evade T-cells, NK cells lose this inhibitory signal, trigger their **Activating Receptors**, and release perforins and granzymes to induce apoptosis. **Why other options are incorrect:** * **Options A & B:** NK cells belong to the **Lymphoid lineage** (derived from the Common Lymphoid Progenitor), but they are distinct from B-cells and T-cells. They are often referred to as "Null cells" because they lack traditional B-cell (CD19/20) or T-cell (CD3) surface markers. * **Option D:** NK cells are part of the **innate immune system**. They do not require previous antigen exposure, clonal expansion, or memory formation for their primary activation, allowing for a rapid first-line response. **High-Yield Clinical Pearls for NEET-PG:** * **Surface Markers:** NK cells are characteristically **CD56+** and **CD16+** (the FcγRIII receptor used for Antibody-Dependent Cellular Cytotoxicity - ADCC). They are **CD3 negative**. * **Cytokine Activation:** Their activity is significantly enhanced by **IL-2, IL-12, and Interferon-α/β**. * **Chediak-Higashi Syndrome:** A classic immunodeficiency where NK cell function is impaired due to defective vesicle trafficking.

Q: Which of the following features is not shared between T cells and B cells?

Positive selection during development. **Explanation:** The core difference in the development of T and B lymphocytes lies in their **selection processes** within primary lymphoid organs. **1. Why Option A is correct:** **Positive selection** is a process unique to **T cell development** in the thymus. It ensures that T cells can recognize self-MHC molecules; those that cannot bind are eliminated via apoptosis. **B cells do not undergo positive selection.** They only undergo **negative selection** in the bone marrow, where self-reactive B cells are eliminated or undergo receptor editing to ensure self-tolerance. **2. Why other options are incorrect:** * **Option B (Class I MHC expression):** All nucleated cells in the human body express MHC Class I molecules. Since both B cells and T cells are nucleated lymphocytes, they both express MHC Class I. (Note: B cells also express MHC Class II as they are professional antigen-presenting cells). * **Option C (Antigen-specific receptors):** Both cell types possess highly specific receptors generated by V(D)J recombination. B cells have **BCRs** (membrane-bound antibodies), and T cells have **TCRs**. Both are designed to recognize specific antigens. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Maturation:** B cells mature in the **B**one marrow; T cells mature in the **T**hymus. * **Negative Selection:** Occurs in **both** B and T cells to prevent autoimmunity (Central Tolerance). * **MHC Restriction:** T cells are "MHC restricted" (require MHC to see antigen), whereas B cells can recognize free, native antigens directly. * **Double Positive Stage:** Only T cells go through a CD4+ CD8+ stage during thymic maturation.

Q: What type of receptor is present on T cells?

CD4. ### Explanation **Correct Answer: C. CD4** **Underlying Medical Concept:** T cells (T lymphocytes) are defined by the presence of the **T-cell receptor (TCR)** complex. However, they are further categorized by specific surface glycoproteins known as **Cluster of Differentiation (CD)** molecules. **CD4** is a co-receptor found on Helper T cells ($T_H$). It plays a crucial role in immune recognition by binding to the non-polymorphic region of **MHC Class II** molecules on Antigen-Presenting Cells (APCs), facilitating the activation of the T cell. **Analysis of Incorrect Options:** * **A & B (IgG and IgD):** These are types of Immunoglobulins (antibodies). While **IgD** (along with IgM) serves as the **B-cell receptor (BCR)** on naive B cells, they are not found as receptors on T cells. T cells recognize antigens via TCRs, not surface immunoglobulins. * **D (Prostaglandins):** These are lipid-derived inflammatory mediators (eicosanoids) produced by the cyclooxygenase pathway. They act as local hormones/signaling molecules but are not structural receptors defining T cell populations. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** Remember the **"Rule of 8"**: CD4 cells recognize MHC II ($4 \times 2 = 8$), while CD8 cells (Cytotoxic T cells) recognize MHC I ($8 \times 1 = 8$). * **HIV Pathogenesis:** The CD4 molecule is the primary receptor for the **HIV gp120** envelope protein. A decline in CD4+ T cell count is the hallmark of progression to AIDS. * **Pan-T Cell Marker:** **CD3** is the universal marker present on all mature T cells, as it is part of the TCR complex required for signal transduction. * **Th1 vs Th2:** CD4+ cells differentiate into Th1 (cell-mediated immunity, secretes IFN-$\gamma$) or Th2 (humoral immunity, secretes IL-4, IL-5).

Q: Which of the following is very difficult to induce an antibody response against?

Polysaccharide. **Explanation:** The ability of a substance to induce an immune response is known as **immunogenicity**. Proteins are the most potent immunogens, whereas pure polysaccharides are notoriously poor at inducing a robust antibody response, especially in children. **1. Why Polysaccharides are difficult (Correct Answer):** Polysaccharides are **T-independent antigens**. Unlike proteins, they cannot be processed and presented via MHC molecules to T-helper cells. Consequently, they fail to activate the T-cell-dependent pathway, leading to: * No isotype switching (mostly IgM is produced, little IgG). * No affinity maturation. * **No memory B-cell formation**, making the immune response weak and short-lived. **2. Analysis of Incorrect Options:** * **B. Protein:** Proteins are the most powerful immunogens. They are **T-dependent antigens** that trigger T-cell help, leading to high-affinity IgG production and long-term memory. * **C. Antigen:** This is a general term for any substance that binds to an antibody. While some antigens are weak, the term itself does not specify a chemical class. * **D. Effector:** This refers to cells (like Plasma cells or Cytotoxic T-cells) that carry out the immune response, not the substance inducing it. **Clinical Pearls for NEET-PG:** * **Conjugate Vaccines:** To overcome the poor immunogenicity of polysaccharides (e.g., *H. influenzae* type b, *S. pneumoniae*), they are chemically linked to a **carrier protein**. This converts them into T-dependent antigens, allowing for an effective immune response in infants. * **Haptens:** These are small molecules that are antigenic but not immunogenic unless attached to a large carrier protein. * **Hierarchy of Immunogenicity:** Proteins > Polysaccharides > Lipids/Nucleic acids.

Q: Which antibody is the first to appear and acts fastest during a primary (first) immune response to an infection?

IgM. ***IgM***- **IgM** is the first antibody isotype produced and secreted by **plasma B cells** following initial exposure to an antigen (primary immune response).- Because it is a **pentamer** (five Y-shaped units), IgM has 10 binding sites, giving it superior efficiency in complement activation, agglutination, and acting as the immediate responder.*IgG*- **IgG** is the most abundant immunoglobulin in the serum and generally appears later than IgM during the primary response.- It is the predominant antibody during the **secondary immune response**, providing long-term immunity and crossing the **placenta**.*IgE*- **IgE** is associated primarily with the initiation of **Type I hypersensitivity reactions** (allergies) by binding to mast cells and basophils.- It is also critical for defense against **helminthic parasites** but is not the first antibody in standard acute infection.*IgA*- **IgA** functions primarily in **mucosal immunity**, being secreted as a dimer into milk, saliva, tears, and mucosal linings (e.g., gut and respiratory tract).- While vital for preventing pathogen entry, it does not lead the kinetics of the initial systemic antibody response.

Q: For the diagnosis of typhus, an antigen of Proteus is used. What is this type of antigen called?

Heterophile antigen. ***Heterophile antigen*** - These are antigens shared between different species, such as the antigenic determinants shared by *Rickettsia* (causing typhus) and certain strains of **Proteus** (used in the Weil-Felix test). - The diagnosis of typhus relies on the **Weil-Felix reaction**, which detects anti-rickettsial antibodies that cross-react and agglutinate with the non-motile *Proteus* antigens (e.g., **OX-19**). *Sequestered antigen* - This term refers to self-antigens that are normally hidden from the immune system (e.g., in the eye or testicles) but can cause **autoimmunity** if released (e.g., post-trauma). - They are involved in the loss of **self-tolerance** and are not used in diagnostic agglutination tests for external infections like typhus. *Super antigen* - These are powerful microbial toxins, such as **Toxic Shock Syndrome Toxin-1**, that cause massive, non-specific activation of T cells. - They bind directly to the outside of the MHC Class II molecules and the T-cell receptor (TCR) Vβ chain, leading to a dangerous **cytokine storm**, unrelated to the agglutination reaction. *Iso antigen* - Also called **alloantigens**, these are specific antigens found in some individuals of a species but not others (e.g., **blood group antigens**). - They are relevant to transfusion reactions and graft rejection but do not describe the cross-species reactivity utilized in the Weil-Felix test for typhus.

Q: Which of the following immunoglobulins shows the highest concentration in serum?

IgG. ***IgG*** - **IgG** accounts for approximately **75-80%** of the total immunoglobulins in human serum, making it the highest in concentration.- Its high levels reflect its crucial roles in providing **long-term humoral immunity**, neutralizing toxins, and being the only class able to cross the **placenta** to confer passive immunity to the fetus.*IgM* - **IgM** is present at much lower concentrations (about 5-10%); it is primarily known for being the first antibody produced during a **primary immune response**. - Although it is the largest immunoglobulin (a pentamer) and effective at complement activation, its total serum concentration is substantially less than IgG.*IgA* - **IgA** constitutes about 10-15% of serum immunoglobulins, ranking second to IgG in concentration. - While abundant in serum, its primary functional significance lies in its dimeric form (**Secretory IgA**) found in mucosal secretions, protecting epithelial surfaces.*IgE* - **IgE** is the immunoglobulin present at the lowest serum concentration (trace amounts). - It is mainly bound to **mast cells** and **basophils** and is responsible for mediating immediate **Type I hypersensitivity** reactions (allergies).

Question 1

Delayed tuberculin test response is due to:

Accepted Answer

T lymphocytes

Answer

B lymphocytes

Answer

Monocytes

Answer

Histiocytes

Question 2

Which of the following is a lineage-specific T cell marker?

Accepted Answer

CD 3

Answer

CD 1

Answer

CD 19

Answer

CD 4

Question 3

Which of the following statements accurately describes Natural Killer (NK) cells?

Accepted Answer

NK cells display a cytotoxic effect on tumor cells.

Answer

NK cells belong to the B-cell lineage.

Answer

NK cells belong to the T-cell lineage.

Answer

NK cells require previous antigen exposure for activation.

Question 4

Which of the following features is not shared between T cells and B cells?

Accepted Answer

Positive selection during development

Answer

Class I MHC expression

Answer

Antigen-specific receptors

Answer

All of the above

Question 5

Which of the following statements about immunoglobulins is false?

Accepted Answer

An antibody molecule contains two types of light chains and one type of heavy chain.

Answer

They are Y-shaped heterodimers composed of four polypeptide chains.

Answer

All heavy and light chains are bound to each other by disulfide bonds.

Answer

There are 5 classes of heavy chains and two classes of light chains.

Question 6

What type of receptor is present on T cells?

Accepted Answer

CD4

Answer

IgG

Answer

IgD

Answer

Prostaglandins

Question 7

Which of the following is very difficult to induce an antibody response against?

Accepted Answer

Polysaccharide

Answer

Protein

Answer

Antigen

Answer

Effector

Question 8

Which antibody is the first to appear and acts fastest during a primary (first) immune response to an infection?

Accepted Answer

IgM

Answer

IgE

Answer

IgA

Answer

IgG

Question 9

For the diagnosis of typhus, an antigen of Proteus is used. What is this type of antigen called?

Accepted Answer

Heterophile antigen

Answer

Iso antigen

Answer

Super antigen

Answer

Sequestered antigen

Question 10

Which of the following immunoglobulins shows the highest concentration in serum?

Accepted Answer

IgG

Answer

IgM

Answer

IgA

Answer

IgE

Immunology — MCQs

Immunology — MCQs

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