Immunology Practice Questions

Q: T-cell independent antigens act through which one of the following mechanisms?

Direct activation of B-cells. **Explanation:** **1. Why the Correct Answer is Right:** T-cell independent (TI) antigens are typically non-protein molecules, such as **bacterial polysaccharides**, lipids, or nucleic acids. Unlike proteins, these antigens cannot be processed and presented via MHC-II molecules to T-helper cells. Instead, they possess repetitive epitopes that cross-link multiple B-cell receptors (BCRs) simultaneously. This extensive cross-linking provides a strong enough signal to **directly activate B-cells** to proliferate and differentiate into plasma cells without the "second signal" usually provided by CD4+ T-helper cells. **2. Why the Incorrect Options are Wrong:** * **Option A & D:** By definition, TI antigens bypass T-cell involvement. They do not bind to MHC molecules, which is a prerequisite for the activation of both CD4+ (Helper) and CD8+ (Cytotoxic) T-cells. * **Option C:** While macrophages act as Antigen Presenting Cells (APCs) for T-cell dependent antigens, the hallmark of the TI response is the circumvention of the APC-T cell-B cell axis. **3. High-Yield Facts for NEET-PG:** * **Nature of Response:** TI antigens primarily produce **IgM** antibodies. They show poor isotype switching, little to no affinity maturation, and **no memory cell production**. * **Clinical Relevance:** The capsules of *Streptococcus pneumoniae*, *Haemophilus influenzae*, and *Neisseria meningitidis* are TI antigens. * **Age Factor:** Children under 2 years of age respond poorly to TI antigens (polysaccharides) because their splenic marginal zone B-cells are immature. This is why **conjugate vaccines** (linking a polysaccharide to a protein carrier) are used to convert a TI response into a T-cell dependent one, ensuring long-term immunity in infants.

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

Class III MHC does not encode complement proteins.. The Major Histocompatibility Complex (MHC), known as the Human Leukocyte Antigen (HLA) system in humans, is a cluster of genes located on the **short arm of Chromosome 6**. ### **Explanation of Options** * **Why Option D is the correct (False) statement:** Class III MHC genes **do** encode several components of the complement system, specifically **C2, C4 (C4A and C4B), and Factor B**. They also encode non-immune proteins like Tumor Necrosis Factor (TNF) and heat shock proteins. Unlike Class I and II, Class III molecules are secreted proteins and do not function as antigen-presenting surface receptors. * **Why Option A is True:** The HLA gene complex is located on the short arm of **Chromosome 6**. This is a high-yield fact frequently tested in NEET-PG. * **Why Option B is True:** The MHC locus is organized linearly as **Class II — Class III — Class I**. Because Class III (which contains complement genes) sits between Class I and Class II, these genes are physically adjacent to the Class I and Class II loci. * **Why Option C is True:** MHC Class II molecules are constitutively expressed on **Professional Antigen Presenting Cells (APCs)**, which include Monocytes, Macrophages, B-cells, and Dendritic cells. ### **High-Yield Clinical Pearls for NEET-PG** * **MHC Class I:** Found on all nucleated cells (not RBCs); presents endogenous antigens to **CD8+ T-cells**. * **MHC Class II:** Found only on APCs; presents exogenous antigens to **CD4+ T-cells**. * **HLA Associations:** * **B27:** Ankylosing spondylitis, Reiter’s syndrome. * **DR3/DR4:** Type 1 Diabetes Mellitus. * **DQ2/DQ8:** Celiac disease. * **Rule of 8:** MHC I × CD8 = 8; MHC II × CD4 = 8.

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.

Question 1

T-cell independent antigens act through which one of the following mechanisms?

Accepted Answer

Direct activation of B-cells

Answer

Activation of T-cells

Answer

Activation of macrophages

Answer

Activation of CD8+ T cells

Question 2

Which of the following statements about MHC is false?

Accepted Answer

Class III MHC does not encode complement proteins.

Answer

Chromosome 6 harbours the gene for MHC.

Answer

Genes encoding complement proteins are located adjacent to class I molecules.

Answer

Monocytes have MHC class II antigens on their surfaces.

Question 3

Relative to the primary immunological response, secondary and later booster responses to a given hapten-protein complex can be associated with which one of the following?

Accepted Answer

Increased antibody affinity for the hapten

Answer

Lower titers of antibody

Answer

Decreased antibody avidity for the original hapten-protein complex

Answer

Maintenance of the same subclass, or idiotype, of antibody produced

Question 4

Opsonization occurs due to all of the following except?

Accepted Answer

Endotoxin

Answer

Complement

Answer

IgM

Answer

IgD

Question 5

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 6

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 7

Which bacterium is frequently used as an immunomodulator?

Accepted Answer

Corynebacterium parvum

Answer

Mycobacterium marinum

Answer

Chromobacterium violasium

Answer

Flavobacterium meningosepticum

Question 8

In which zone are large antigen-antibody complexes formed?

Accepted Answer

Zone of equivalence

Answer

Prozone

Answer

Postzone

Answer

None of the above

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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