Immunology Practice Questions

Q: The Membrane Attack Complex (MAC) in the complement system is composed of which component(s)?

C5-9. **Explanation:** The **Membrane Attack Complex (MAC)** is the final effector functional unit of the complement system. It is formed by the sequential assembly of terminal complement proteins **C5b, C6, C7, C8, and C9**. Once the three complement pathways (Classical, Lectin, and Alternative) converge at the cleavage of C5, the C5b fragment binds to the target cell membrane. It then recruits C6, C7, and C8. Finally, multiple molecules of **C9** polymerize to form a transmembrane pore (10 nm diameter). This pore disrupts the osmotic integrity of the cell, leading to water influx and **osmotic lysis** of the pathogen. **Analysis of Options:** * **Option A (C3b):** Incorrect. C3b acts as an **opsonin** (enhancing phagocytosis) and is a component of C5 convertase, but it is not part of the structural MAC. * **Option B & D (C13, C24):** Incorrect. These are non-existent components in the human complement system, which consists of proteins numbered C1 through C9. **High-Yield Clinical Pearls for NEET-PG:** * **Neisseria Susceptibility:** Patients with deficiencies in terminal complement components (**C5-C9**) have a significantly increased risk of recurrent systemic infections with *Neisseria meningitidis* and *Neisseria gonorrhoeae*. * **Paroxysmal Nocturnal Hemoglobinuria (PNH):** Caused by a deficiency of DAF (CD55) and MIRL (CD59). CD59 normally inhibits MAC formation on host cells; its absence leads to complement-mediated hemolysis. * **The "Big Three" Functions:** Remember complement functions as **Opsonization** (C3b), **Anaphylatoxins** (C3a, C4a, C5a), and **Lysis** (MAC: C5b-9).

Q: Which antibody is referred to as the "millionaire molecule"?

IgM. **Explanation:** **IgM** is known as the **"millionaire molecule"** primarily because of its high molecular weight (approximately 900,000 Daltons). It is the largest immunoglobulin, existing in its secreted form as a **pentamer** (five basic units held together by a J-chain). Due to this pentameric structure, it has the highest valency (10 antigen-binding sites), making it exceptionally efficient at agglutination and complement activation. **Analysis of Incorrect Options:** * **IgA:** Known as the "secretory antibody." It exists as a dimer in secretions (milk, saliva, tears) and provides mucosal immunity. * **IgG:** The most abundant antibody in serum. It is the only antibody that crosses the placenta and is responsible for the secondary immune response. * **IgD:** Primarily acts as a B-cell surface receptor. Its exact systemic function remains less defined compared to others. **High-Yield Clinical Pearls for NEET-PG:** * **First Responder:** IgM is the first antibody to appear in response to an initial exposure to an antigen (Primary Immune Response). * **Intravascular Distribution:** Because of its large size, IgM is largely confined to the intravascular compartment (plasma) and cannot cross the placenta. * **Diagnostic Marker:** Presence of specific IgM in a newborn’s serum indicates **congenital infection** (e.g., TORCH), as maternal IgM cannot cross the placenta. * **Evolutionary Fact:** It is the oldest immunoglobulin class phylogenetically.

Q: The process of phagocytosis was discovered by whom?

Elie Metchnikoff. **Explanation:** **Correct Answer: B. Elie Metchnikoff** The process of **phagocytosis** (the ingestion and destruction of foreign particles by cells) was discovered by the Russian scientist **Elie Metchnikoff** in 1882. While observing starfish larvae under a microscope, he noticed specialized cells "eating" carmine dye particles. This discovery laid the foundation for our understanding of **Cellular Immunity**. For his work on immunity, he shared the Nobel Prize in Physiology or Medicine in 1908 with Paul Ehrlich (who focused on humoral immunity). **Why the other options are incorrect:** * **A. Celsus:** A Roman encyclopedist known for describing the **four cardinal signs of inflammation**: *Rubor* (redness), *Tumor* (swelling), *Calor* (heat), and *Dolor* (pain). * **C. Virchow:** Known as the "Father of Modern Pathology," Rudolf Virchow is famous for the cell theory (*Omnis cellula e cellula*) and describing the pathophysiology of thrombosis (Virchow’s Triad), but he did not discover phagocytosis. **NEET-PG High-Yield Pearls:** * **Father of Natural Immunity:** Elie Metchnikoff. * **Professional Phagocytes:** Primarily include Neutrophils (microphages) and Macrophages (monocytes). * **Opsonization:** The process that enhances phagocytosis by coating antigens with opsonins (mainly **C3b** and **IgG**). * **Phagosome + Lysosome:** These fuse to form a **Phagolysosome**, where respiratory burst (via NADPH oxidase) occurs to kill the pathogen.

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: Which of the following is a chemokine?

IL-8. **Explanation:** **Correct Answer: B. IL-8** Chemokines are a specific subfamily of cytokines that act as chemoattractants to recruit inflammatory cells to the site of infection or injury. **Interleukin-8 (IL-8)**, also known as CXCL8, is the prototypical chemokine. It is primarily produced by macrophages and endothelial cells to induce **neutrophil chemotaxis** and activation. In the NEET-PG context, remember the mnemonic: *"Clean up on Aisle 8"* (IL-8 recruits neutrophils to clean up the site). **Analysis of Incorrect Options:** * **A. Leukotriene A4:** This is an intermediate in the arachidonic acid metabolism pathway. While its derivative, **LTB4**, is a potent neutrophil chemoattractant, LTA4 itself is an unstable precursor and not classified as a chemokine (which are protein signaling molecules). * **C & D. C5 and C3:** These are components of the complement system. While their cleavage products, **C5a** (and to a lesser extent C3a), are powerful "anaphylatoxins" and chemoattractants, they are plasma proteins/fragments, not cytokines or chemokines. **High-Yield Clinical Pearls for NEET-PG:** * **Potent Chemoattractants:** The "Big Four" frequently tested are **IL-8, LTB4, C5a, and Bacterial products (N-formyl methionine).** * **Chemokine Receptors:** CCR5 and CXCR4 are critical co-receptors for **HIV entry** into CD4+ T cells and macrophages. * **Classification:** Chemokines are classified into four groups based on the arrangement of cysteine residues: **CC, CXC, C, and CX3C.** IL-8 belongs to the **CXC** family.

Question 1

The Membrane Attack Complex (MAC) in the complement system is composed of which component(s)?

Accepted Answer

C5-9

Answer

C3b

Answer

C13

Answer

C24

Question 2

Which cell is responsible for the primary immune response?

Accepted Answer

B cell

Answer

T cell

Answer

Both B and T cells

Answer

Complement mediated

Question 3

Which antibody is referred to as the "millionaire molecule"?

Accepted Answer

IgM

Answer

IgA

Answer

IgG

Answer

IgD

Question 4

Toll-like receptors recognize bacterial products and stimulate an immune response by:

Accepted Answer

Mediating transcription of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), which recruits cytokines.

Answer

Initiating perforin and granzyme-mediated apoptosis.

Answer

Triggering Fas-associated death domain (FADD) ligand-induced apoptosis.

Answer

Regulating cyclin activity.

Question 5

The process of phagocytosis was discovered by whom?

Accepted Answer

Elie Metchnikoff

Answer

Celsus

Answer

Virchow

Answer

None of the above

Question 6

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 7

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 8

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 9

Which of the following is a chemokine?

Accepted Answer

IL-8

Answer

Leukotriene A4

Answer

C5

Answer

C3

Question 10

Which of the following are components of innate immunity?

Accepted Answer

Integrins

Answer

T lymphocytes

Answer

Complement proteins

Answer

B lymphocytes

Immunology — MCQs

Immunology — MCQs

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