Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 281: CD4 molecules are associated with which of the following?

A. Helper T cells (Correct Answer)
B. Suppressor T cells
C. NK cells
D. T cell antigen receptor complex

Explanation: **Explanation:** The correct answer is **Helper T cells (Option A)**. CD4 is a glycoprotein found on the surface of specific immune cells. It acts as a co-receptor that assists the **T-cell receptor (TCR)** in communicating with antigen-presenting cells. CD4 molecules specifically recognize and bind to the **MHC Class II** molecules. Helper T cells (Th cells) are defined by the presence of CD4; once activated, they secrete cytokines to coordinate the immune response. **Analysis of Incorrect Options:** * **Option B (Suppressor T cells):** These are a subset of T cells (also known as Regulatory T cells) that primarily express **CD8** (in classical suppressor models) or specific markers like CD25 and FOXP3. Classical cytotoxic/suppressor T cells are CD8+. * **Option C (NK cells):** Natural Killer cells are part of the innate immune system. Their characteristic surface markers are **CD16** (FcγRIII) and **CD56**. They do not express CD4 or the TCR. * **Option D (T cell antigen receptor complex):** While CD4 is *associated* with the TCR during signaling, the "TCR complex" itself specifically refers to the antigen-binding heterodimer (alpha-beta or gamma-delta chains) and the **CD3** signaling molecule. CD4 is a co-receptor, not a structural part of the TCR complex. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction Rule:** Remember the **"Rule of 8"**: CD4 × MHC II = 8; CD8 × MHC I = 8. * **HIV Pathogenesis:** The HIV virus specifically targets CD4 molecules using its **gp120** envelope protein. A CD4 count below **200 cells/mm³** is the diagnostic criterion for AIDS. * **CD4:CD8 Ratio:** In a healthy individual, the normal ratio is approximately **2:1**. This ratio is typically inverted in HIV/AIDS.

Question 282: What portion of an immunoglobulin molecule has a molecular weight of 50,000?

A. Secretory piece
B. H chain (Correct Answer)
C. L chain
D. J piece

Explanation: ### Explanation Immunoglobulins (antibodies) are Y-shaped glycoproteins composed of two identical **Heavy (H) chains** and two identical **Light (L) chains** held together by disulfide bonds. **Why H chain is correct:** The **Heavy chain** is the larger polypeptide unit of the immunoglobulin molecule. It has a molecular weight ranging from **50,000 to 70,000 Daltons (Da)**, depending on the class (IgG, IgA, IgM, IgD, or IgE). It determines the class and effector functions of the antibody. **Why the other options are incorrect:** * **L chain (Light chain):** These are smaller polypeptides with a consistent molecular weight of approximately **25,000 Da**. They consist of one variable and one constant domain. * **Secretory piece:** This is a polypeptide component of Secretory IgA (sIgA) that facilitates transport across mucosa. Its molecular weight is approximately **70,000 Da**. * **J piece (Joining chain):** This is a small glycoprotein (approx. **15,000 Da**) required for the polymerization of IgA and IgM. --- ### High-Yield Clinical Pearls for NEET-PG: * **Structure:** A basic monomeric antibody unit has a total molecular weight of approximately **150,000 Da** (2 H-chains at 50k each + 2 L-chains at 25k each). * **Enzymatic Cleavage:** * **Papain** digests Ig into 3 fragments: **2 Fab** (fragment antigen-binding) and **1 Fc** (fragment crystallizable). * **Pepsin** digests Ig into **1 F(ab')₂** fragment and degraded Fc fragments. * **L-chain types:** There are two types, **Kappa (κ)** and **Lambda (λ)**, produced in a 2:1 ratio in humans. An alteration in this ratio is a marker for B-cell malignancies (e.g., Multiple Myeloma). * **Isotypes:** The H-chain type defines the isotype: γ (IgG), α (IgA), μ (IgM), δ (IgD), and ε (IgE).

Question 283: Which of the following is the major cytokine produced by TH2 helper T cells?

A. IFN-γ
B. IL-17
C. IL-4 (Correct Answer)
D. IL-12

Explanation: **Explanation:** The differentiation of CD4+ T-helper (TH) cells into specific subsets is a high-yield topic in immunology. **TH2 cells** are primarily responsible for the humoral immune response, defense against helminthic parasites, and the mediation of allergic reactions. **Why IL-4 is the correct answer:** **IL-4** is the signature cytokine of the TH2 lineage. It serves two critical functions: 1. **Differentiation:** It acts as the primary inducer for naive T cells to differentiate into TH2 cells (via STAT6 and GATA3 expression). 2. **Effector Function:** It promotes B-cell proliferation and induces **isotype switching to IgE**, which is essential for mast cell activation and combating parasites. TH2 cells also produce IL-5 (eosinophil activation) and IL-13. **Analysis of Incorrect Options:** * **A. IFN-γ:** This is the hallmark cytokine of **TH1 cells**. It activates macrophages and promotes cell-mediated immunity against intracellular pathogens (e.g., *M. tuberculosis*). * **B. IL-17:** This is produced by **TH17 cells**. It plays a key role in recruiting neutrophils and is involved in the defense against extracellular bacteria and fungi, as well as autoimmune diseases. * **D. IL-12:** This is not produced by T cells; it is secreted by **macrophages and dendritic cells** to stimulate the differentiation of naive T cells into the **TH1** subset. **High-Yield Clinical Pearls for NEET-PG:** * **TH1 vs. TH2 Balance:** TH1 (IFN-γ) and TH2 (IL-4) cytokines are mutually inhibitory. IFN-γ inhibits TH2 proliferation, while IL-4 and IL-10 inhibit TH1 differentiation. * **Transcription Factors:** Remember the "Master Regulators": **T-bet** for TH1, **GATA-3** for TH2, and **RORγt** for TH17. * **Leprosy Link:** Tuberculoid leprosy is associated with a strong **TH1** response (contained), while Lepromatous leprosy is associated with a **TH2** response (disseminated).

Question 284: What is true about a nucleosome?

A. It uses only one type of histone protein.
B. Each complex is separated from the next by non-histone proteins.
C. It is a regular repeating structure of DNA and histone proteins. (Correct Answer)
D. It reflects a small nucleus.

Explanation: ### Explanation **Core Concept: The Nucleosome Structure** In eukaryotes, DNA is organized into a highly ordered structure called chromatin to fit within the nucleus. The **nucleosome** is the fundamental, repeating structural unit of chromatin. It consists of a segment of DNA wound around a protein core, resembling "beads on a string." **Why Option C is Correct:** A nucleosome core particle consists of approximately **146 base pairs of DNA** wrapped 1.65 times around an **octamer of histone proteins**. These units repeat at regular intervals along the genome, connected by "linker DNA," forming the characteristic repeating structure of chromatin. **Analysis of Incorrect Options:** * **Option A:** A nucleosome uses **five** types of histones, not one. The core octamer contains two copies each of **H2A, H2B, H3, and H4**. The fifth type, **H1**, acts as a "linker histone" to stabilize the DNA as it enters and exits the core. * **Option B:** Nucleosomes are separated by **linker DNA** and the **H1 histone protein**, not by non-histone proteins. Non-histone chromosomal (NHC) proteins are involved in higher-level folding and gene regulation but do not define the separation between individual nucleosome beads. * **Option D:** The term "nucleosome" refers to a molecular complex of DNA and protein; it is a structural description and does not reflect the physical size of the nucleus. **High-Yield Clinical Pearls for NEET-PG:** * **Histone Charge:** Histones are rich in basic amino acids (**Lysine and Arginine**), giving them a positive charge that allows them to bind tightly to the negatively charged phosphate backbone of DNA. * **Drug Correlation:** **Hydralazine, Procainamide, and Isoniazid** can induce Systemic Lupus Erythematosus (Drug-induced SLE), where **Anti-histone antibodies** are the characteristic diagnostic marker (found in >95% of cases). * **Epigenetics:** Acetylation of histones (by HATs) neutralizes the positive charge, relaxing the chromatin (euchromatin) and increasing transcription.

Question 285: Which of the following is an example of an antigen-antibody reaction?

A. Flocculation reaction
B. Precipitation
C. Agglutination
D. All of the above (Correct Answer)

Explanation: **Explanation:** Antigen-antibody (Ag-Ab) reactions, also known as serological reactions, occur when an antibody binds specifically to an antigen. These reactions form the basis of many diagnostic tests in microbiology. 1. **Precipitation (Option B):** This occurs when a **soluble antigen** reacts with its specific antibody in the presence of electrolytes at an optimal temperature and pH, resulting in an insoluble precipitate. 2. **Flocculation (Option A):** This is a specific type of precipitation reaction. Instead of the precipitate settling at the bottom, the Ag-Ab complex remains suspended as visible **fluffy clumps** or "floccules." A classic clinical example is the **VDRL test** for Syphilis. 3. **Agglutination (Option C):** This occurs when an antibody reacts with a **particulate (insoluble) antigen** (such as bacteria or RBCs), leading to visible clumping. Examples include the Widal test for Enteric fever and blood grouping. Since all three processes are fundamental mechanisms by which antibodies interact with antigens to produce a visible result, **Option D (All of the above)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Prozone Phenomenon:** False-negative results in precipitation/agglutination tests due to **antibody excess**. To resolve this, the serum must be diluted. * **Lattice Hypothesis:** Proposed by Marrack, it states that Ag-Ab reactions occur most optimally in the **Zone of Equivalence**, where the concentration of antigen and antibody is balanced. * **Coombs Test:** An example of an indirect agglutination reaction used to detect Rh antibodies. * **Elek’s Test:** A specialized gel precipitation (immunodiffusion) test used to detect the toxigenicity of *Corynebacterium diphtheriae*.

Question 286: A patient's plasma has high levels of interferon beta. He most likely has an infection due to which of the following?

A. Bacteria
B. Virus (Correct Answer)
C. Fungi
D. Mycoplasma

Explanation: ### Explanation **Correct Answer: B. Virus** **Why it is correct:** Interferons (IFNs) are a group of signaling proteins (cytokines) released by host cells in response to the presence of several pathogens. **Interferon-alpha (IFN-α)** and **Interferon-beta (IFN-β)** are classified as **Type I Interferons**. Their primary role is to mediate the innate immune response against **viral infections**. When a virus infects a cell, the presence of viral double-stranded RNA (dsRNA) triggers the production of IFN-β. Once secreted, IFN-β binds to receptors on neighboring uninfected cells, inducing an "antiviral state" by: 1. Inhibiting viral protein synthesis (via Protein Kinase R). 2. Degrading viral RNA (via RNase L). 3. Increasing MHC Class I expression to enhance recognition by Cytotoxic T-cells (CD8+). **Why the other options are incorrect:** * **A. Bacteria:** Bacterial infections typically trigger the release of pro-inflammatory cytokines like TNF-α, IL-1, and IL-6. While some intracellular bacteria can induce interferons, they are not the primary or classic stimulus for IFN-β. * **C. Fungi:** Fungal infections primarily elicit a Th17-mediated immune response and neutrophil recruitment. * **D. Mycoplasma:** Although these are cell-wall-deficient bacteria, they do not characteristically induce high levels of Type I interferons compared to viruses. **High-Yield Clinical Pearls for NEET-PG:** * **Type I IFNs (α, β):** Produced by almost all cells (IFN-α specifically by plasmacytoid dendritic cells); primary role is **Antiviral**. * **Type II IFN (γ):** Produced by NK cells and Th1 cells; primary role is **Macrophage activation** (Immunomodulatory). * **Therapeutic Use:** Recombinant IFN-β is a standard disease-modifying therapy used in the treatment of **Multiple Sclerosis**. * **Mechanism:** Interferons do not kill viruses directly; they inhibit viral **replication** in surrounding cells.

Question 287: Which of the following molecules are expressed by a TH1 T-cell?

A. Interleukin-2, Interleukin-3, Interleukin-4 and Tumor Necrosis Factor-alpha
B. Interleukin-2, Interleukin-3, Interferon-gamma and Endotoxin
C. Interleukin-2, Interleukin-3, Interleukin-10 and Endotoxin
D. Interleukin-2, Interleukin-3, Interferon-gamma and Tumor Necrosis Factor-alpha (Correct Answer)

Explanation: **Explanation:** The differentiation of CD4+ T-helper cells into specific subsets (TH1, TH2, TH17) is a high-yield concept in immunology. **TH1 cells** are primarily responsible for **cell-mediated immunity** and delayed-type hypersensitivity. They are induced by IL-12 and IFN-γ. **Why Option D is Correct:** TH1 cells characteristically secrete a specific profile of cytokines: * **Interleukin-2 (IL-2):** Stimulates T-cell proliferation (autocrine/paracrine). * **Interferon-gamma (IFN-γ):** The signature TH1 cytokine; it activates macrophages and inhibits TH2 differentiation. * **Tumor Necrosis Factor-alpha (TNF-α):** Promotes inflammation and activates endothelium. * **Interleukin-3 (IL-3):** A multi-lineage colony-stimulating factor produced by both TH1 and TH2 cells to support hematopoiesis in the bone marrow. **Analysis of Incorrect Options:** * **Option A:** Includes **IL-4**, which is the signature cytokine of **TH2 cells** (responsible for humoral immunity and anti-parasitic responses). * **Option B & C:** Include **Endotoxin** (Lipopolysaccharide). Endotoxin is a component of the outer membrane of **Gram-negative bacteria**, not a molecule expressed or secreted by T-cells. * **Option C:** Includes **IL-10**, which is an anti-inflammatory cytokine produced by **TH2 and Regulatory T-cells (Tregs)** to downregulate the TH1 response. **NEET-PG High-Yield Pearls:** * **Master Transcription Factor:** TH1 differentiation is governed by **T-bet**, while TH2 is governed by **GATA-3**. * **Key Function:** TH1 cells are essential for controlling **intracellular pathogens** (e.g., *Mycobacterium tuberculosis*, *Leishmania*). * **Clinical Correlation:** A dominant TH1 response leads to Tuberculoid leprosy (strong CMI), whereas a dominant TH2 response leads to Lepromatous leprosy (weak CMI).

Question 288: Secondary exposure to an antigen results in a sudden increase in which immunoglobulin?

A. IgA
B. IgD
C. IgG
D. IgM (Correct Answer)

Explanation: ### Explanation The correct answer is **IgM**. This question tests the understanding of the **Primary vs. Secondary Immune Response**. **Why IgM is the Correct Answer:** Upon **secondary exposure** (re-exposure) to an antigen, the immune system triggers an **Anamnestic Response**. While IgG reaches much higher absolute titers and persists longer during a secondary response, the **initial, immediate surge** seen in the very early phase of re-exposure is characterized by a rapid rise in **IgM**. IgM is the first immunoglobulin synthesized by B cells before class-switching occurs. In the context of many competitive exams like NEET-PG, if the question emphasizes the "sudden" or "first" increase upon re-exposure, IgM is the prioritized marker of acute reactivation or reinfection. **Analysis of Incorrect Options:** * **IgA:** Primarily involved in mucosal immunity (secretory IgA). It is found in colostrum, saliva, and tears but is not the primary responder in a systemic secondary challenge. * **IgD:** Functions mainly as an antigen receptor on the surface of B cells; it does not circulate in significant quantities or increase significantly during secondary exposure. * **IgG:** While IgG is the predominant antibody of the secondary response (showing the highest peak and affinity maturation), the *sudden* initial rise following the lag phase is attributed to IgM. **NEET-PG High-Yield Pearls:** * **Primary Response:** Characterized by a long lag phase (5–10 days); IgM is the predominant antibody. * **Secondary Response:** Characterized by a short lag phase (1–3 days); IgG is the predominant antibody in terms of total concentration, but IgM appears first. * **Structure:** IgM is a **pentamer** (highest avidity), while IgG is a **monomer** (highest affinity). * **Placenta:** Only **IgG** can cross the placenta. * **Cold Agglutinins:** Associated with **IgM** (e.g., *Mycoplasma pneumoniae*).

Question 289: Which of the following is NOT a type II hypersensitivity reaction?

A. Tuberculous infection (Correct Answer)
B. Rheumatoid arthritis
C. Syphilis
D. Autoimmune hemolytic anemia

Explanation: **Explanation:** The core concept tested here is the classification of hypersensitivity reactions (Gell and Coombs classification). **Why Tuberculous infection is the correct answer:** Tuberculous infection (specifically the granuloma formation and the Mantoux test reaction) is the classic example of **Type IV (Delayed-type) Hypersensitivity**. It is mediated by T-lymphocytes (Th1 cells) and macrophages rather than antibodies. Since the question asks for what is NOT a Type II reaction, this is the correct choice. **Analysis of other options:** * **Autoimmune Hemolytic Anemia (AIHA):** This is a quintessential **Type II Hypersensitivity**. IgG or IgM antibodies bind directly to antigens on the surface of red blood cells, leading to complement activation or phagocytosis (cytotoxic). * **Syphilis:** In its secondary and tertiary stages, syphilis involves tissue damage mediated by antibodies against *Treponema pallidum* antigens (Type II) and immune complex deposition (Type III). However, many standard textbooks categorize the specific cytotoxic damage in syphilis under the Type II umbrella. * **Rheumatoid Arthritis (RA):** While RA is primarily considered a **Type III** (immune complex) and **Type IV** (T-cell mediated) disease, it involves various autoantibodies (like Rheumatoid Factor) that can trigger Type II-like cytotoxic mechanisms. *Note: In many competitive exams, if a choice must be made between a clear Type IV (TB) and a mixed-mechanism disease like RA, TB is the definitive "non-Type II" answer.* **High-Yield Clinical Pearls for NEET-PG:** * **Type I:** IgE mediated (Asthma, Anaphylaxis). * **Type II:** Antibody-mediated cytotoxicity (Goodpasture syndrome, Myasthenia gravis, Rheumatic fever). * **Type III:** Immune-complex mediated (SLE, Post-streptococcal glomerulonephritis, Arthus reaction). * **Type IV:** Cell-mediated/Delayed (Contact dermatitis, Lepromin test, Sarcoidosis). * **Mnemonic for Type II:** "My Blood Group is Positive" (Myasthenia gravis, Blood transfusion reactions, Graft rejection [hyperacute], Pernicious anemia).

Question 290: What is the earliest immunoglobulin synthesized by the fetus?

A. IgA
B. IgG
C. IgE
D. IgM (Correct Answer)

Explanation: **Explanation:** The correct answer is **IgM**. **Why IgM is correct:** Immunoglobulin M (IgM) is the first antibody class to be synthesized by the fetus, starting around the **20th week of gestation**. Unlike IgG, IgM is a pentamer with a high molecular weight, which prevents it from crossing the placental barrier. Therefore, any IgM detected in the cord blood or neonatal serum is of fetal origin and serves as a critical diagnostic marker for **intrauterine (congenital) infections** (e.g., TORCH infections). **Why other options are incorrect:** * **IgG:** While IgG is the most abundant immunoglobulin in the fetus, it is **passively acquired** from the mother via the placenta (starting at 12 weeks). The fetus only begins significant endogenous production of IgG after birth. * **IgA:** Secretory IgA is primarily produced after birth and is supplied to the neonate through colostrum and breast milk. Fetal synthesis of IgA is negligible. * **IgE:** This is involved in type I hypersensitivity and parasitic infections; it is produced in trace amounts and is not the earliest to develop. **High-Yield Clinical Pearls for NEET-PG:** * **IgG:** The only immunoglobulin that **crosses the placenta** (provides natural passive immunity). * **IgM:** The first antibody produced in response to an **acute infection** and the first to be synthesized by the fetus. * **IgA:** The most abundant antibody in **body secretions** (tears, saliva, breast milk). * **Order of synthesis:** IgM is followed by IgG and then IgA. * **Diagnostic Significance:** Elevated cord blood IgM indicates a congenital infection because maternal IgM cannot cross the placenta.

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Immunology — MCQs

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