Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 911: Which of the following best defines an autoantigen?

A. An antigen present on red blood cells that determines blood group.
B. A non-human antigen that can elicit an immune response.
C. Both a and b.
D. Neither a nor b. (Correct Answer)

Explanation: ### Explanation **Core Concept: Autoantigen** An **autoantigen** is a normal constituent of the body (a "self" protein, DNA, or RNA) that is mistakenly recognized as foreign by the immune system, leading to an autoimmune response. Under normal physiological conditions, the immune system maintains **self-tolerance**, but when this fails, autoantigens become the targets of autoantibodies or T-cell-mediated attacks. **Why Option D is Correct:** Neither Option A nor Option B defines an autoantigen. An autoantigen must be an endogenous "self" molecule that triggers an abnormal immune response against one's own tissues. **Analysis of Incorrect Options:** * **Option A:** Antigens on red blood cells (like A, B, or Rh) are **isoantigens** (or alloantigens). These are present in some individuals of a species but not others. They do not trigger an immune response in the host who possesses them; they only cause reactions during mismatched transfusions (isoimmunity). * **Option B:** Non-human antigens (e.g., bacterial toxins, viral proteins) are **heteroantigens** (or xenoantigens). These are foreign to the human species and are the primary targets of a healthy immune system. **High-Yield NEET-PG Pearls:** * **Sequestrated Antigens:** Some autoantigens are hidden from the immune system (e.g., lens protein of the eye, sperm, myelin basic protein). If these enter circulation due to injury, they are treated as foreign because the body never developed tolerance to them. * **Hapten:** A small molecule that is not immunogenic by itself but can become an antigen when conjugated with a carrier protein. * **Adjuvant:** A substance (like Alum) added to vaccines to enhance the immunogenicity of an antigen without being antigenic itself. * **Superantigens:** These bypass normal processing and bind directly to MHC II and TCR, causing massive cytokine release (e.g., Toxic Shock Syndrome Toxin).

Question 912: Which of the following is not a heterophile reaction?

A. Weil Felix test
B. Paul Bunnell test
C. Frie's test (Correct Answer)
D. Cold agglutinin test

Explanation: ### Explanation **Concept of Heterophile Antigens:** Heterophile antigens are closely related antigens found in unrelated species (e.g., humans, animals, plants, or bacteria). A **heterophile reaction** occurs when antibodies produced against an antigen from one species cross-react with antigens from a completely different species. **Why Frie’s Test is the Correct Answer:** **Frie’s test** is a **Type IV hypersensitivity (delayed-type)** skin test formerly used to diagnose *Lymphogranuloma Venereum (LGV)* caused by *Chlamydia trachomatis*. It involves the intradermal injection of the antigen into the patient; a positive result is indicated by a skin papule. Since it is a specific antigen-antibody/T-cell interaction rather than a cross-reaction between unrelated species, it is **not** a heterophile reaction. **Analysis of Incorrect Options:** * **Weil-Felix Test:** A classic heterophile reaction where antibodies produced against *Rickettsia* cross-react with specific strains of *Proteus* (OX-19, OX-2, OX-K). * **Paul Bunnell Test:** Used to diagnose Infectious Mononucleosis (EBV). It detects heterophile antibodies in human serum that agglutinate sheep or horse red blood cells. * **Cold Agglutinin Test:** Used in *Mycoplasma pneumoniae* infections. The antibodies produced cross-react with the **I-antigen** on human RBCs at low temperatures (4°C). **High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Patterns:** * Epidemic Typhus: OX-19 (+). * Scrub Typhus: OX-K (+). * Q Fever: No Weil-Felix reaction (Negative). * **Frie's Test Status:** It is now obsolete and replaced by more specific NAAT (Nucleic Acid Amplification Tests). * **Other Skin Tests (Type IV):** Remember **Mitsuda test** (Leprosy), **Mantoux test** (TB), and **Casoni’s test** (Hydatid disease - though it has an immediate component).

Question 913: What type of receptors are present on T cells?

A. IgG
B. IgD
C. CD4 (Correct Answer)
D. Prostaglandins

Explanation: **Explanation:** The correct answer is **CD4**. T cells are characterized by the presence of specific surface markers known as **Cluster of Differentiation (CD)** molecules. Mature T cells express either **CD4** (Helper T cells) or **CD8** (Cytotoxic T cells) in association with the T-cell receptor (TCR) complex. CD4 acts as a co-receptor that recognizes and binds to **MHC Class II** molecules on antigen-presenting cells, facilitating the immune response. **Analysis of Incorrect Options:** * **A & B (IgG and IgD):** These are types of immunoglobulins (antibodies). While **IgD** (along with IgM) serves as a B-cell receptor (BCR) on the surface of naive B cells, **IgG** is primarily a secreted antibody involved in secondary immune responses. T cells do not express surface immunoglobulins. * **D (Prostaglandins):** These are lipid-derived inflammatory mediators, not structural receptors. While T cells have receptors *for* prostaglandins (which can modulate their function), prostaglandins themselves are not characteristic T-cell receptors. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** Remember the "Rule of 8": CD4 cells bind to MHC II (4 × 2 = 8), and CD8 cells bind to MHC I (8 × 1 = 8). * **Pan-T Cell Marker:** **CD3** is the definitive marker present on all mature T cells and is part of the TCR complex. * **HIV Pathogenesis:** The HIV virus specifically targets the **CD4 molecule** as its primary entry receptor, leading to a depletion of Helper T cells. * **Th1 vs Th2:** CD4+ cells further differentiate into Th1 (cell-mediated immunity) and Th2 (humoral immunity) based on cytokine exposure.

Question 914: Which of the following statements about immunoglobulins is false?

A. They are Y-shaped heterodimers composed of four polypeptide chains.
B. All heavy and light chains are bound to each other by disulfide bonds.
C. An antibody molecule contains two types of light chains and one type of heavy chain. (Correct Answer)
D. There are 5 classes of heavy chains and two classes of light chains.

Explanation: ### Explanation **Why Option C is the Correct (False) Statement:** The fundamental rule of immunoglobulin structure is **symmetry**. A single antibody molecule consists of two identical heavy (H) chains and two identical light (L) chains. Therefore, an antibody molecule contains only **one type of light chain** (either two Kappa or two Lambda) and **one type of heavy chain** (e.g., two Gamma chains in IgG). It can never be a hybrid containing both Kappa and Lambda chains simultaneously. **Analysis of Incorrect Options:** * **Option A (True):** Immunoglobulins are indeed Y-shaped heterodimers. They consist of four polypeptide chains: two heavy (~50 kDa) and two light (~25 kDa). * **Option B (True):** The four chains are held together by **interchain disulfide bonds**. Specifically, H-chains are linked to each other, and each L-chain is linked to an H-chain. * **Option D (True):** There are 5 heavy chain isotypes (Gamma-γ, Alpha-α, Mu-μ, Delta-δ, Epsilon-ε) which determine the class (IgG, IgA, IgM, IgD, IgE). There are 2 light chain types: **Kappa (κ)** and **Lambda (λ)**. **High-Yield Clinical Pearls for NEET-PG:** * **Kappa:Lambda Ratio:** In a healthy individual, the ratio of κ to λ light chains is approximately **2:1**. A significant deviation (e.g., 10:1) suggests a monoclonal gammopathy like **Multiple Myeloma**. * **Bence-Jones Proteins:** These are free monoclonal light chains found in the urine of Multiple Myeloma patients. * **Variable vs. Constant:** The N-terminal ends (Fab fragment) contain the variable regions responsible for antigen binding, while the C-terminal ends (Fc fragment) determine biological activity (e.g., complement fixation).

Question 915: What type of receptor is present on T cells?

A. IgG
B. IgD
C. CD4 (Correct Answer)
D. Prostaglandins

Explanation: ### 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).

Question 916: The Prausnitz-Küstner reaction detects which immunoglobulin?

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

Explanation: **Explanation:** The **Prausnitz-Küstner (PK) reaction** is a classic immunological test used to demonstrate the presence of **reaginic antibodies**, now known as **IgE**. In this reaction, serum from an allergic individual (containing specific IgE) is injected intradermally into a non-allergic recipient. After a latent period (allowing the IgE to bind to the mast cells of the recipient via the Fcε receptor), the corresponding allergen is injected at the same site. This triggers mast cell degranulation, resulting in a **Type I Hypersensitivity** reaction (Wheal and Flare). This test historically proved that allergy is mediated by a serum factor (IgE). **Why other options are incorrect:** * **IgG:** While IgG is the most abundant antibody and can cross the placenta, it is not the primary mediator of immediate hypersensitivity or the PK reaction. * **IgA:** This is the primary secretory immunoglobulin found in colostrum, saliva, and mucosal surfaces; it does not bind to mast cells to trigger the PK reaction. * **IgM:** This is the first antibody produced in a primary immune response and exists as a pentamer. It is involved in complement activation but not in Type I hypersensitivity. **High-Yield Clinical Pearls for NEET-PG:** * **Heat Lability:** IgE is heat-labile (inactivated at 56°C for 30 minutes), a property used to distinguish it from other antibodies in early research. * **Safety Note:** The PK reaction is **no longer used clinically** due to the high risk of transmitting blood-borne pathogens like Hepatitis B and HIV. It has been replaced by *in vitro* tests like **RAST** (Radioallergosorbent Test) and **ELISA**. * **Homocytotropism:** IgE is "homocytotropic," meaning it has a high affinity for cells of the same species (specifically mast cells and basophils).

Question 917: In respiratory and gastrointestinal infections, which immunoglobulin is most affected?

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

Explanation: **Explanation:** The correct answer is **IgA** because it is the primary immunoglobulin involved in **mucosal immunity**. **1. Why IgA is correct:** IgA exists in two forms: monomeric (in serum) and dimeric (secretory). **Secretory IgA (sIgA)** is specifically synthesized by plasma cells in the lamina propria of mucosal surfaces, such as the respiratory, gastrointestinal, and genitourinary tracts. It contains a **J-chain** (joining chain) and a **secretory component** that protects it from enzymatic degradation by proteases found in gut and lung secretions. Its primary role is "immune exclusion"—preventing the attachment of pathogens (viruses and bacteria) to epithelial cells, thereby acting as the first line of defense against infections in these systems. **2. Why other options are incorrect:** * **IgG:** This is the most abundant antibody in the **serum** and provides systemic immunity. It crosses the placenta but is not the primary defender at mucosal surfaces. * **IgM:** This is the first antibody produced in a primary immune response and exists as a pentamer. While it has a J-chain, it is not the predominant secretory antibody. * **IgD:** Found primarily on the surface of B-cells as a receptor; its systemic or mucosal anti-infective function is negligible. **Clinical Pearls for NEET-PG:** * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients present with recurrent sinopulmonary and GI infections (e.g., *Giardia lamblia*). * **Breast Milk:** IgA is the predominant antibody in colostrum, providing passive mucosal immunity to the neonate’s gut. * **Structure:** Remember that the **Secretory Component** is derived from the epithelial cells (poly-Ig receptor), not the plasma cell.

Question 918: Which of the following is very difficult to induce an antibody response against?

A. Polysaccharide (Correct Answer)
B. Protein
C. Antigen
D. Effector

Explanation: **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.

Question 919: What is the sedimentation coefficient of IgE?

A. 7S
B. 8S (Correct Answer)
C. 11S
D. 20S

Explanation: **Explanation:** The sedimentation coefficient (measured in Svedberg units, S) reflects the size and shape of an immunoglobulin molecule during ultracentrifugation. **1. Why 8S is Correct:** IgE is a monomeric antibody characterized by an extra constant domain ($\text{C}_H4$) in its heavy chains ($\epsilon$). While most monomeric antibodies like IgG have a sedimentation coefficient of 7S, the additional domain in IgE increases its molecular weight to approximately 190,000 Daltons, resulting in a higher sedimentation coefficient of **8S**. **2. Analysis of Incorrect Options:** * **7S (Option A):** This is the sedimentation coefficient for monomeric **IgG, IgD, and serum IgA**. These lack the extra domain found in IgE. * **11S (Option C):** This is characteristic of **Secretory IgA (sIgA)**. sIgA exists as a dimer held together by a J-chain and a secretory component, increasing its size and sedimentation rate. * **20S (Option D):** This is characteristic of **IgM**. Because IgM typically exists as a pentamer (five units joined by a J-chain), it is the largest antibody (Macroglobulin), resulting in the highest sedimentation coefficient (19S–20S). **High-Yield Clinical Pearls for NEET-PG:** * **Heat Lability:** IgE is the most heat-labile antibody (inactivated at 56°C for 30 minutes). * **Prausnitz-Küstner (PK) Reaction:** Historically used to detect IgE (reaginic antibody) by passive transfer in skin. * **Receptors:** IgE binds to high-affinity receptors ($\text{Fc}\epsilon\text{RI}$) on **mast cells and basophils**, triggering Type I Hypersensitivity. * **Role:** Essential for defense against helminthic parasites (via Eosinophils).

Question 920: Which antibody is the first to appear and acts fastest during a primary (first) immune response to an infection?

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

Explanation: ***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.

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Immunology — MCQs

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