Immunology Practice Questions

Q: Which of the following is an example of Type IV hypersensitivity?

Granulomatous reaction. **Explanation:** **Type IV Hypersensitivity (Delayed-type)** is a cell-mediated immune response involving T-lymphocytes (CD4+ Th1 cells and CD8+ cytotoxic T cells) rather than antibodies. **Why the Correct Answer is Right:** **D. Granulomatous reaction:** This is the classic example of Type IV hypersensitivity. It occurs when an antigen (like *M. tuberculosis*) persists within macrophages because it cannot be easily eliminated. Th1 cells secrete cytokines (IFN-γ), which activate macrophages, transforming them into epithelioid cells and multinucleated giant cells, eventually forming a granuloma. This process typically takes 48–72 hours or longer to develop. **Why the Other Options are Incorrect:** * **A. Arthus reaction:** This is a localized **Type III hypersensitivity** reaction. It involves the formation of immune complexes (antigen-antibody) in situ, leading to complement activation and necrotizing vasculitis. * **B. Serum sickness:** This is a systemic **Type III hypersensitivity** reaction. It occurs when circulating immune complexes deposit in tissues (joints, kidneys, vessels) following the administration of foreign serum or drugs. * **C. Shwartzman reaction:** This is **not a hypersensitivity reaction**. It is an exaggerated inflammatory response to endotoxins (LPS), characterized by localized or systemic hemorrhagic necrosis and intravascular coagulation. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Type IV:** "4 T's" – **T**-cells, **T**ransplant (Rejection), **T**B test (Mantoux), **T**ouch (Contact dermatitis). * **Other Examples:** Lepromin test, Nickel allergy, and Graft-versus-host disease (GVHD). * **Key Cytokine:** **Interferon-gamma (IFN-γ)** is the most critical cytokine for macrophage activation in Type IV reactions.

Q: Opsonization occurs due to all of the following except?

Endotoxin. ### Explanation **Opsonization** is the process by which pathogens are coated with specific molecules (opsonins) to enhance their recognition and ingestion by phagocytes (neutrophils and macrophages). **1. Why Endotoxin is the Correct Answer:** Endotoxin (Lipopolysaccharide/LPS), found in the outer membrane of Gram-negative bacteria, is a **Pathogen-Associated Molecular Pattern (PAMP)**. While it triggers the immune response by binding to Toll-like receptors (TLR-4), it does not act as an opsonin itself. Instead, it is the *target* that opsonins bind to. Therefore, it does not facilitate opsonization; it is a trigger for inflammation and potential septic shock. **2. Analysis of Other Options:** * **Complement (C3b):** C3b is the most potent opsonin in the complement system. Phagocytes have CR1 receptors that bind specifically to C3b coated on surfaces. * **IgM:** While IgM is a poor opsonin directly (due to the lack of Fc receptors for IgM on phagocytes), it is the most efficient activator of the **Classical Complement Pathway**. By activating complement, it leads to the deposition of C3b, thereby facilitating opsonization. * **IgG (specifically IgG1 and IgG3):** These are the primary antibodies that act as opsonins because phagocytes possess Fcγ receptors. * *Note on IgD:* While IgD's role is primarily as a B-cell surface receptor, in the context of this question, **Endotoxin** is the most definitive "non-opsonin" as it is a bacterial component, not a host-derived coating molecule. **3. High-Yield Clinical Pearls for NEET-PG:** * **Major Opsonins:** IgG (specifically the Fc portion) and C3b. * **Mnemonic:** **"I**t **G**oops **G**erms" for **IgG** and **C3b**. * **Acute Phase Reactant:** C-Reactive Protein (CRP) also acts as an opsonin. * **Receptors:** Phagocytosis is enhanced when both FcγR and CR1 receptors are engaged simultaneously.

Q: A rabbit is injected for the first time with ova-albumin. The antibodies generated in this response would be of which class?

IgM. ### Explanation The correct answer is **B. IgM**. **1. Why IgM is the correct answer:** This question tests the concept of the **Primary Immune Response**. When an antigen (like ova-albumin) is introduced into a host for the very first time, the immune system undergoes a primary response. The first immunoglobulin class to be synthesized by naive B cells following initial antigenic challenge is always **IgM**. This is because IgM can be produced without the need for T-cell-mediated "isotype switching." It serves as the body's first line of adaptive humoral defense. **2. Why the other options are incorrect:** * **A. IgG:** This is the predominant antibody in the **Secondary (Anamnestic) Immune Response**. While IgG is eventually produced after the primary response via class switching, it is not the *initial* antibody generated. * **C. IgD:** This class primarily acts as a surface receptor on mature B cells. It is not secreted in significant quantities during a systemic immune response to an injected antigen. * **D. IgE:** This antibody is specifically associated with Type I Hypersensitivity reactions and parasitic infections. It is not the default primary response antibody for a general protein antigen like albumin. **3. High-Yield NEET-PG Pearls:** * **Primary Response:** Characterized by a long lag phase (5–10 days), low antibody titer, and **IgM** predominance. * **Secondary Response:** Characterized by a short lag phase (1–3 days), high antibody titer, and **IgG** predominance due to memory B cells. * **Structure:** IgM is a **pentamer** (highest valency) in secretion, making it highly efficient at agglutination and complement activation, despite having lower affinity than IgG. * **Fetal Infection:** Since IgM cannot cross the placenta, its presence in a newborn indicates an *in utero* (congenital) infection.

Q: Which bacterium is frequently used as an immunomodulator?

Corynebacterium parvum. **Explanation:** **Corynebacterium parvum** (also known as *Propionibacterium acnes*) is a potent **non-specific immunomodulator**. It acts primarily by stimulating the reticuloendothelial system, leading to the activation of macrophages and natural killer (NK) cells. In clinical oncology and experimental immunology, it has been used as an adjuvant to enhance the immune response against tumors and to increase resistance to various bacterial and viral infections. **Analysis of Options:** * **A. Corynebacterium parvum (Correct):** It is a classic example of a bacterial immunomodulator (along with BCG). It induces cytokine production (like IL-12 and IFN-γ), which shifts the immune response toward a Th1 phenotype. * **B. Mycobacterium marinum:** This is a non-tuberculous mycobacterium (NTM) responsible for "Fish Tank Granuloma." While *Mycobacterium bovis* (BCG) is used as an immunomodulator, *M. marinum* is strictly a pathogen. * **C. Chromobacterium violaceum:** A rare opportunistic pathogen found in tropical soil and water, known for producing a violet pigment called violacein. It is not used for immunomodulation. * **D. Flavobacterium meningosepticum (now Elizabethkingia):** An environmental gram-negative rod known for causing neonatal meningitis and nosocomial outbreaks; it is highly multidrug-resistant and has no role in immunomodulation. **NEET-PG High-Yield Pearls:** * **Bacterial Immunomodulators:** The two most frequently tested are **BCG** (used intravesically for bladder cancer) and **Corynebacterium parvum**. * **Mechanism:** They act as "biological response modifiers" by stimulating innate immunity. * **Levamisole:** An anti-helminthic drug often mentioned alongside these bacteria as a chemical immunomodulator.

Q: The Frei test is used to identify which type of hypersensitivity reaction?

Type IV. **Explanation:** The **Frei test** is a skin test historically used for the diagnosis of **Lymphogranuloma Venereum (LGV)**, caused by *Chlamydia trachomatis* (serotypes L1, L2, and L3). It is a classic example of a **Type IV (Delayed-type) Hypersensitivity reaction**. 1. **Why Type IV is correct:** The test involves the intradermal injection of the "Frei antigen" (inactivated LGV agent). In a positive case, sensitized T-lymphocytes recognize the antigen, leading to the release of cytokines and the recruitment of macrophages. This results in an inflammatory papule or induration at the injection site, which peaks at **48 to 72 hours**—the hallmark timing of a cell-mediated (Type IV) response. 2. **Why other options are incorrect:** * **Type I (Immediate):** Mediated by IgE and mast cell degranulation (e.g., Anaphylaxis, Atopy). These occur within minutes. * **Type II (Cytotoxic):** Mediated by IgG/IgM against cell surface antigens (e.g., Rh incompatibility, Myasthenia Gravis). * **Type III (Immune-complex):** Involves deposition of antigen-antibody complexes in tissues (e.g., SLE, Post-streptococcal glomerulonephritis). **High-Yield Clinical Pearls for NEET-PG:** * **Status:** The Frei test is now largely obsolete, replaced by more sensitive Nucleic Acid Amplification Tests (NAAT) and serology. * **Cross-reactivity:** It is not highly specific as it can show cross-reactivity with other Chlamydia species (e.g., *C. psittaci*). * **Other Type IV Skin Tests:** Tuberculin (Mantoux) test, Lepromin test, Casoni’s test (immediate and delayed), and Montenegro test (Leishmaniasis). * **LGV Clinical Sign:** Look for the **"Groove sign"** (enlargement of inguinal lymph nodes above and below the inguinal ligament).

Q: Which immunoglobulin is primarily responsible for complement-mediated lysis?

IgM. ### Explanation **1. Why IgM is the Correct Answer:** IgM is the most potent activator of the **Classical Complement Pathway**. Its high efficiency stems from its **pentameric structure** (five monomer units joined by a J-chain). For the first component of the complement (C1q) to be activated, it must bind to at least two Fc portions of antibodies. Because IgM is a pentamer, a single molecule of IgM bound to an antigen provides multiple closely spaced Fc sites, easily triggering the complement cascade leading to the formation of the **Membrane Attack Complex (MAC)** and subsequent cell lysis. In contrast, IgG requires at least two separate molecules to be in close proximity to achieve the same effect. **2. Why Other Options are Incorrect:** * **IgE:** Primarily involved in Type I hypersensitivity (allergic) reactions and defense against helminthic parasites. It binds to mast cells and basophils via high-affinity Fcε receptors. * **IgA:** The primary secretory antibody found in colostrum, saliva, and mucosal surfaces. It is a poor activator of the classical complement pathway; it primarily neutralizes pathogens and prevents mucosal attachment. * **IgD:** Found mainly on the surface of B-cells as an antigen receptor. Its precise systemic function is less defined, but it does not play a significant role in complement-mediated lysis. **3. Clinical Pearls for NEET-PG:** * **Potency:** IgM is **100–1000 times** more effective than IgG in mediating complement-dependent hemolysis. * **IgG Subclasses:** Among IgG, the order of complement activation efficiency is **IgG3 > IgG1 > IgG2**. IgG4 does not activate complement. * **Structure:** IgM is a pentamer in secretions/serum but a monomer when acting as a B-cell receptor (BCR). * **Nature:** IgM is the first antibody produced in response to an infection (Primary immune response) and indicates acute infection.

Question 1

Which of the following is an example of Type IV hypersensitivity?

Accepted Answer

Granulomatous reaction

Answer

Arthus reaction

Answer

Serum sickness

Answer

Shwartzman reaction

Question 2

Opsonization occurs due to all of the following except?

Accepted Answer

Endotoxin

Answer

Complement

Answer

IgM

Answer

IgD

Question 3

A rabbit is injected for the first time with ova-albumin. The antibodies generated in this response would be of which class?

Accepted Answer

IgM

Answer

IgG

Answer

IgD

Answer

IgE

Question 4

The protection against smallpox by previous infection with cowpox represents which of the following?

Accepted Answer

Antigenic cross-reactivity

Answer

Antigenic specificity

Answer

Passive immunity

Answer

Innate immunity

Question 5

Which bacterium is frequently used as an immunomodulator?

Accepted Answer

Corynebacterium parvum

Answer

Mycobacterium marinum

Answer

Chromobacterium violasium

Answer

Flavobacterium meningosepticum

Question 6

In which zone are large antigen-antibody complexes formed?

Accepted Answer

Zone of equivalence

Answer

Prozone

Answer

Postzone

Answer

None of the above

Question 7

The Frei test is used to identify which type of hypersensitivity reaction?

Accepted Answer

Type IV

Answer

Type I

Answer

Type II

Answer

Type III

Question 8

Which immunoglobulin is primarily responsible for complement-mediated lysis?

Accepted Answer

IgM

Answer

IgE

Answer

IgA

Answer

IgD

Question 9

Which of the following statements about MHC is false?

Accepted Answer

MHC class I molecules are presented to CD8 T cells.

Answer

MHC class I is present on all nucleated cells.

Answer

MHC class I antigen presentation involves the cytosolic pathway.

Answer

MHC class I peptides are typically 8-10 amino acids long.

Question 10

Which condition exhibits a mechanism similar to the Schwartzman reaction?

Accepted Answer

Waterhouse Friderichsen syndrome

Answer

Fitz Hugh Curtis syndrome

Answer

Eichwald-Silberberg phenomenon

Answer

Anaphylactoid reaction

Immunology — MCQs

Immunology — MCQs

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