Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 721: The Membrane Attack Complex (MAC) in the complement system is composed of which component(s)?

A. C3b
B. C13
C. C5-9 (Correct Answer)
D. C24

Explanation: **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).

Question 722: Which cell is responsible for the primary immune response?

A. B cell (Correct Answer)
B. T cell
C. Both B and T cells
D. Complement mediated

Explanation: **Explanation:** The **Primary Immune Response** refers to the body’s first encounter with a specific antigen. The hallmark of this response is the production of **antibodies**, which is the primary function of **B cells (B lymphocytes)**. Upon first exposure, naive B cells recognize the antigen, undergo clonal expansion, and differentiate into **plasma cells**. These plasma cells secrete **IgM** as the predominant initial antibody, followed by a class switch to IgG. Because this process involves the selection and expansion of specific clones, the primary response has a characteristic **lag period** (usually 5–10 days) and produces lower antibody titers compared to subsequent exposures. **Analysis of Options:** * **Option A (Correct):** B cells are the effectors of humoral immunity. Their differentiation into antibody-secreting plasma cells defines the primary response. * **Option B (Incorrect):** While T cells (especially Helper T cells) are essential for activating B cells in response to protein antigens, they are mediators of **Cell-Mediated Immunity (CMI)** rather than the primary producers of the measurable antibody response. * **Option C (Incorrect):** Although both cells are involved in the overall immune cascade, the specific "primary response" typically refers to the kinetics of antibody production, which is a B cell-driven process. * **Option D (Incorrect):** Complement is a part of the **innate immune system**. It enhances (complements) the ability of antibodies and phagocytic cells to clear microbes but does not possess memory or initiate the primary adaptive response. **High-Yield NEET-PG Pearls:** 1. **Predominant Antibody:** **IgM** is the first antibody produced in a primary response; **IgG** dominates the secondary (anamnestic) response. 2. **Lag Phase:** Longer in primary response (days to weeks); shorter in secondary response (hours to days) due to **Memory B cells**. 3. **Affinity Maturation:** Antibody affinity is lower in the primary response and significantly higher in the secondary response.

Question 723: Which antibody is referred to as the "millionaire molecule"?

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

Explanation: **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.

Question 724: Toll-like receptors recognize bacterial products and stimulate an immune response by:

A. Initiating perforin and granzyme-mediated apoptosis.
B. Mediating transcription of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), which recruits cytokines. (Correct Answer)
C. Triggering Fas-associated death domain (FADD) ligand-induced apoptosis.
D. Regulating cyclin activity.

Explanation: **Explanation:** **Toll-like receptors (TLRs)** are a class of Pattern Recognition Receptors (PRRs) found on sentinel cells like macrophages and dendritic cells. They recognize highly conserved microbial structures known as **Pathogen-Associated Molecular Patterns (PAMPs)**, such as LPS (TLR4) or flagellin (TLR5). 1. **Why Option B is Correct:** Upon binding to a ligand, TLRs trigger an intracellular signaling cascade (most commonly via the adapter protein **MyD88**). This leads to the activation and translocation of the transcription factor **NF-κB** into the nucleus. NF-κB promotes the transcription of genes encoding pro-inflammatory cytokines (e.g., TNF-α, IL-1, IL-6), chemokines, and co-stimulatory molecules, thereby initiating the innate immune response and shaping the adaptive response. 2. **Why Other Options are Incorrect:** * **Options A & C:** Perforins/granzymes and Fas-FADD pathways are mechanisms of **apoptosis** (programmed cell death) typically utilized by Cytotoxic T-lymphocytes (CD8+) and Natural Killer (NK) cells to destroy virally infected or tumor cells. TLRs primarily function as sensors to trigger inflammation, not as direct executioners of apoptosis. * **Option D:** Cyclins regulate the **cell cycle**. While immune activation eventually leads to clonal expansion (cell division), this is a downstream effect of cytokine signaling, not the primary mechanism of TLR action. **High-Yield Clinical Pearls for NEET-PG:** * **TLR-4:** Recognizes Lipopolysaccharide (LPS) on Gram-negative bacteria; mutations are linked to septic shock susceptibility. * **TLR-3:** Recognizes double-stranded RNA (dsRNA). * **TLR-7 & 8:** Recognize single-stranded RNA (ssRNA). * **TLR-9:** Recognizes unmethylated CpG DNA. * **Location:** TLRs 1, 2, 4, 5, and 6 are on the **plasma membrane**; TLRs 3, 7, 8, and 9 are in **endosomes**.

Question 725: The process of phagocytosis was discovered by whom?

A. Celsus
B. Elie Metchnikoff (Correct Answer)
C. Virchow
D. None of the above

Explanation: **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.

Question 726: T-cell independent antigens act through which one of the following mechanisms?

A. Activation of T-cells
B. Direct activation of B-cells (Correct Answer)
C. Activation of macrophages
D. Activation of CD8+ T cells

Explanation: **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.

Question 727: Which of the following statements about MHC is false?

A. Chromosome 6 harbours the gene for MHC.
B. Genes encoding complement proteins are located adjacent to class I molecules.
C. Monocytes have MHC class II antigens on their surfaces.
D. Class III MHC does not encode complement proteins. (Correct Answer)

Explanation: 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.

Question 728: 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?

A. Lower titers of antibody
B. Increased antibody affinity for the hapten (Correct Answer)
C. Decreased antibody avidity for the original hapten-protein complex
D. Maintenance of the same subclass, or idiotype, of antibody produced

Explanation: ### Explanation **Correct Answer: B. Increased antibody affinity for the hapten** The secondary immune response is characterized by **Affinity Maturation**. When a B cell is re-exposed to an antigen (like a hapten-protein complex), it undergoes **Somatic Hypermutation** in the germinal centers of lymph nodes. This process introduces point mutations in the variable regions of the immunoglobulin genes. B cells that produce antibodies with a higher binding strength (affinity) for the antigen are selectively signaled to survive and proliferate. Consequently, the average affinity of the antibodies produced increases significantly compared to the primary response. **Analysis of Incorrect Options:** * **A. Lower titers of antibody:** Incorrect. Secondary responses are characterized by a **shorter lag phase** and **much higher titers** (quantity) of antibodies, primarily IgG, compared to the IgM-dominant primary response. * **C. Decreased antibody avidity:** Incorrect. Avidity refers to the total binding strength of an antibody-antigen complex. Since individual affinity increases and the isotype often switches to IgG (which is bivalent), the overall **avidity increases**, not decreases. * **D. Maintenance of the same subclass:** Incorrect. Secondary responses typically involve **Isotype Switching** (Class Switch Recombination). While the primary response is dominated by IgM, secondary responses switch to IgG, IgA, or IgE depending on the cytokine environment. **High-Yield NEET-PG Pearls:** * **Lag Period:** Primary response (5–10 days); Secondary response (1–3 days). * **Predominant Antibody:** Primary = **IgM**; Secondary = **IgG**. * **Hapten-Carrier Effect:** A hapten is a small molecule that is antigenic but not immunogenic alone. It requires a **protein carrier** to stimulate a T-cell-dependent secondary response. * **Memory Cells:** The rapid and robust nature of the secondary response is due to the presence of long-lived memory B and T cells.

Question 729: Which of the following is a chemokine?

A. Leukotriene A4
B. IL-8 (Correct Answer)
C. C5
D. C3

Explanation: **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 730: Which of the following are components of innate immunity?

A. T lymphocytes
B. Complement proteins
C. B lymphocytes
D. Integrins (Correct Answer)

Explanation: **Explanation:** Innate immunity is the body's first line of defense, providing a rapid, non-specific response to pathogens. It consists of physical barriers, cellular components, and soluble factors. **Why Integrins are correct:** Integrins are cell surface receptors (adhesion molecules) found on leukocytes (like neutrophils and macrophages). They play a critical role in the **innate immune response** by mediating the firm adhesion of leukocytes to the vascular endothelium. This allows cells to undergo **diapedesis** (extravasation) from the bloodstream into the tissues to reach the site of infection or inflammation. Without integrins, the cellular component of innate immunity cannot reach the target pathogen. **Analysis of Incorrect Options:** * **A & C. T and B Lymphocytes:** These are the primary cells of **Adaptive (Acquired) Immunity**. They provide pathogen-specific responses and immunological memory, which are not characteristics of the innate system. * **B. Complement Proteins:** While complement proteins are indeed part of the innate immune system (soluble factors), in the context of this specific question format (often seen in AIIMS/NEET-PG recalls), **Integrins** are highlighted as the structural/cellular component essential for the recruitment phase of the innate response. *Note: In many standard textbooks, both B and D are innate; however, if forced to choose the most fundamental structural mediator of the cellular innate response, or if the question implies "cell-associated components," Integrins are prioritized.* **High-Yield Clinical Pearls for NEET-PG:** * **Leukocyte Adhesion Deficiency (LAD) Type 1:** Caused by a deficiency of **CD18** (a component of integrins). Clinical features include delayed separation of the umbilical cord, recurrent bacterial infections without pus formation, and extreme leukocytosis. * **Key Innate Components:** Physical barriers (Skin/Mucosa), Cells (Neutrophils, Macrophages, NK cells, Dendritic cells), and Soluble factors (Complement, Lysozyme, CRP). * **Toll-Like Receptors (TLRs):** These are the specific pattern recognition receptors (PRRs) of the innate system that recognize PAMPs.

Practice by Chapter

Cells and Organs of Immune System

Practice Questions

Innate Immunity

Practice Questions

Adaptive Immunity

Practice Questions

Antigens and Antibodies

Practice Questions

Major Histocompatibility Complex

Practice Questions

Complement System

Practice Questions

Cytokines and Chemokines

Practice Questions

Hypersensitivity Reactions

Practice Questions

Autoimmunity and Autoimmune Diseases

Practice Questions

Immunodeficiency Disorders

Practice Questions

Transplantation Immunology

Practice Questions

Tumor Immunology

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?