Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 461: The process of transfer of cell-mediated immunity from one individual to another is?

A. Innate immunity
B. Acquired immunity
C. Herd immunity
D. Adoptive immunity (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Adoptive immunity** **Why it is correct:** Adoptive immunity refers to the transfer of **pre-sensitized immune cells** (specifically T-lymphocytes) from an immune donor to a non-immune recipient. Unlike passive immunity, which involves the transfer of pre-formed antibodies (humoral immunity), adoptive immunity specifically transfers **cell-mediated immunity (CMI)**. This process is used to provide immediate CMI to a host who is unable to mount an effective response on their own. **Why the other options are incorrect:** * **A. Innate immunity:** This is the non-specific, first line of defense present from birth (e.g., skin, mucus, phagocytes). It is not "transferred" between individuals but is genetically determined. * **B. Acquired immunity:** This is a broad term for immunity developed during an individual's lifetime. While it includes both active and passive types, it does not specifically define the *process* of transferring cells between individuals. * **C. Herd immunity:** This is an epidemiological concept referring to the indirect protection of susceptible individuals in a population when a large proportion of that population becomes immune to an infectious disease. **High-Yield Clinical Pearls for NEET-PG:** * **Passive vs. Adoptive:** Passive immunity = Transfer of **Antibodies** (Short-lived). Adoptive immunity = Transfer of **Cells** (Can be long-lasting). * **Clinical Application:** Adoptive immunotherapy is a cornerstone of modern oncology, such as **CAR-T cell therapy**, where a patient’s T-cells are modified and re-infused to target cancer cells. * **Transfer Factor:** CMI can also be transferred via "transfer factor," a low-molecular-weight extract from sensitized lymphocytes. * **Key Mediator:** T-lymphocytes are the primary mediators of adoptive immunity.

Question 462: Which component of the immune system secretes Interferon gamma, a cytokine known to enhance the cytotoxic effects of macrophages?

A. CD4 T cell (Correct Answer)
B. CD8 T cell
C. RBC
D. Neutrophils

Explanation: **Explanation:** **Interferon-gamma (IFN-γ)** is a critical Type II interferon and the primary cytokine responsible for **macrophage activation**. It is predominantly secreted by **CD4+ T helper 1 (Th1) cells**, as well as Natural Killer (NK) cells and CD8+ cytotoxic T cells. 1. **Why CD4 T cell is correct:** Upon encountering intracellular pathogens (like *M. tuberculosis*), Th1 cells secrete IFN-γ. This cytokine acts on macrophages to increase their phagocytic activity, stimulate the production of Reactive Oxygen Species (ROS) and Nitric Oxide (NO), and upregulate MHC class II expression. This "classical activation" (M1 pathway) is essential for killing intracellular bacteria. 2. **Why other options are incorrect:** * **CD8 T cells:** While they can produce IFN-γ, their primary role is direct cytotoxicity (granzymes/perforins). In the context of standard immunology exams, CD4+ Th1 cells are recognized as the "orchestrators" and the major source of IFN-γ for macrophage stimulation. * **RBCs:** These are non-immune cells lacking a nucleus and do not participate in cytokine signaling. * **Neutrophils:** These are professional phagocytes and the first responders to inflammation, but they primarily release enzymes and NETs rather than secreting IFN-γ. **High-Yield Clinical Pearls for NEET-PG:** * **The Th1-Macrophage Axis:** Deficiencies in IFN-γ or its receptor lead to increased susceptibility to atypical mycobacterial infections (Mendelian Susceptibility to Mycobacterial Disease). * **Granuloma Formation:** IFN-γ is the key cytokine required for the formation of granulomas in Tuberculosis. * **Synergy:** IFN-γ works synergistically with TNF-α to enhance the killing capacity of the immune system.

Question 463: A student has just completed her hepatitis B vaccine series. What lab finding would you expect?

A. Positive test for HBsAg
B. Anti-HBs alone (Correct Answer)
C. Anti-HBc
D. Antibody against both surface and core antigen

Explanation: ### Explanation The Hepatitis B vaccine is a **subunit vaccine** containing only the recombinant **Hepatitis B surface Antigen (HBsAg)**. It does not contain the core antigen or the intact virus. **1. Why "Anti-HBs alone" is correct:** When a person is vaccinated, their immune system recognizes the HBsAg and produces **protective antibodies (Anti-HBs)**. Since the vaccine lacks the core antigen (HBcAg), the body never produces antibodies against the core. Therefore, a successful vaccination profile is characterized by **Anti-HBs (+) and Anti-HBc (-)**. **2. Analysis of Incorrect Options:** * **A. Positive test for HBsAg:** This indicates an **active infection** (acute or chronic). While HBsAg may be transiently positive for a few days immediately following vaccination, it is not the expected long-term finding. * **C. Anti-HBc:** This antibody is only produced in response to the **actual virus** (natural infection). It is never found after vaccination. * **D. Antibody against both surface and core:** This profile (Anti-HBs + Anti-HBc) indicates **immunity due to natural infection** (recovery), not vaccination. **3. NEET-PG High-Yield Pearls:** * **Window Period:** The time when HBsAg disappears but Anti-HBs hasn't appeared yet. The only marker present is **IgM Anti-HBc**. * **Chronic Infection:** Defined by the persistence of HBsAg for >6 months. * **Infectivity Marker:** **HBeAg** indicates high viral replication and high infectivity. * **Vaccine Non-responders:** If Anti-HBs titers are **<10 mIU/mL** after a full series, the individual is considered a non-responder.

Question 464: Which of the following is NOT a superantigen?

A. Toxic Shock Syndrome Toxin (TSST)
B. Exfoliative toxin
C. Pyrogenic exotoxin
D. Cholera toxin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Cholera toxin**. To understand why, we must differentiate between the mechanisms of Superantigens and classical A-B toxins. **1. Why Cholera Toxin is the correct answer:** Cholera toxin is a classical **A-B subunit exotoxin**. It works by ADP-ribosylation of the Gs protein, leading to increased intracellular cAMP, which results in the massive secretion of water and electrolytes into the intestinal lumen. It does **not** interact directly with MHC II or T-cell receptors (TCR) to cause a cytokine storm, which is the hallmark of a superantigen. **2. Analysis of Incorrect Options (Superantigens):** Superantigens are unique because they bypass the normal "processed antigen" pathway. They bind directly to the **variable beta (Vβ) region of the TCR** and the **MHC II molecule** on Antigen Presenting Cells (APCs). This results in the non-specific activation of up to 20% of the body’s T-cells, leading to a massive release of cytokines (IL-1, IL-2, TNF-α, IFN-γ). * **TSST-1 (Staph. aureus):** The classic superantigen causing Toxic Shock Syndrome. * **Exfoliative Toxin (Staph. aureus):** Causes Staphylococcal Scalded Skin Syndrome (SSSS); it acts as a localized superantigen. * **Pyrogenic Exotoxin (Strep. pyogenes):** Specifically SpeA and SpeC, which cause Streptococcal Toxic Shock-like Syndrome and Scarlet Fever. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Superantigens bind to the **outside** of the peptide-binding groove. * **Key Examples:** TSST-1, Staphylococcal Enterotoxins (Food poisoning), Streptococcal Pyrogenic Exotoxins (SpeA, SpeC), and Mycoplasma arthritidis mitogen. * **Result:** Systemic inflammatory response syndrome (SIRS) and multi-organ failure.

Question 465: Which antigen is primarily involved in post-transplant rejection?

A. HLA-Antigen (Correct Answer)
B. Nuclear antigen
C. Polysaccharide
D. DHA

Explanation: **Explanation:** The correct answer is **HLA-Antigen (Human Leukocyte Antigen)**. **1. Why HLA-Antigen is correct:** The HLA system is the human version of the **Major Histocompatibility Complex (MHC)**, located on Chromosome 6. These surface glycoproteins are the primary markers used by the immune system to distinguish "self" from "non-self." In organ transplantation, if the donor’s HLA molecules do not match the recipient’s, the recipient’s T-cells recognize them as foreign antigens. This triggers a potent immune response (Cell-mediated via CD8+ T-cells and Humoral via B-cells), leading to graft destruction or **rejection**. **2. Why other options are incorrect:** * **Nuclear Antigens:** These are components found inside the cell nucleus (e.g., ANA, anti-dsDNA). They are primarily involved in **autoimmune diseases** like Systemic Lupus Erythematosus (SLE), not transplant rejection. * **Polysaccharides:** These are typically found in bacterial capsules (e.g., *Streptococcus pneumoniae*). They trigger T-independent immune responses but are not the mediators of graft rejection. * **DHA (Docosahexaenoic acid):** This is an omega-3 fatty acid essential for brain health; it has no antigenic role in transplant immunology. **Clinical Pearls for NEET-PG:** * **MHC Class I (HLA-A, B, C):** Present on all nucleated cells; recognized by CD8+ T-cells. * **MHC Class II (HLA-DR, DQ, DP):** Present on Antigen-Presenting Cells (APCs); recognized by CD4+ T-cells. * **Hyperacute Rejection:** Occurs within minutes due to pre-formed antibodies against ABO or HLA antigens (Type II Hypersensitivity). * **Acute Rejection:** Occurs within days to weeks; primarily T-cell mediated (Type IV Hypersensitivity). * **HLA-B27** is strongly associated with Ankylosing Spondylitis.

Question 466: Which portion of the MHC class I complex forms the component of the antigen-presenting peptide binding groove?

A. Between alpha1 and alpha2 domains
B. Distal part of the alpha2 chain (Correct Answer)
C. Proximal part of the alpha3 chain
D. Between alpha1 and alpha3 domains

Explanation: **Explanation:** The **MHC Class I molecule** is a heterodimer consisting of a heavy chain (alpha chain) and a non-covalently linked $\beta_2$-microglobulin. The alpha chain is divided into three domains: $\alpha_1$, $\alpha_2$, and $\alpha_3$. The **peptide-binding groove** (cleft) of MHC Class I is formed by the folding of the **$\alpha_1$ and $\alpha_2$ domains**. Specifically, these two domains form a "floor" of beta-pleated sheets and "walls" of alpha-helices. The correct option identifies the **distal part of the $\alpha_2$ chain** (along with $\alpha_1$) as the structural component of this groove. **Analysis of Options:** * **Option A (Between $\alpha_1$ and $\alpha_2$):** While the groove is formed by both, the structural "pocket" is technically located within the distal folding of these two domains. In many standardized exams, the $\alpha_1/\alpha_2$ platform is the standard answer. * **Option C (Proximal $\alpha_3$):** This is incorrect. The $\alpha_3$ domain is the **proximal/transmembrane** portion that is highly conserved and serves as the binding site for the **CD8 T-cell co-receptor**. * **Option D (Between $\alpha_1$ and $\alpha_3$):** These domains are not adjacent in the tertiary structure; $\alpha_2$ sits between them. **High-Yield NEET-PG Pearls:** 1. **MHC Class I:** Groove formed by **$\alpha_1$ and $\alpha_2$**; binds endogenous peptides (8–10 amino acids). 2. **MHC Class II:** Groove formed by **$\alpha_1$ and $\beta_1$** domains; binds exogenous peptides (13–18 amino acids). 3. **$\beta_2$-microglobulin:** Essential for the surface expression of MHC I but does **not** contribute to the peptide-binding groove. It is encoded on Chromosome 15 (unlike the MHC locus on Chromosome 6). 4. **CD8 binding:** Occurs at the **$\alpha_3$** domain of MHC I. 5. **CD4 binding:** Occurs at the **$\beta_2$** domain of MHC II.

Question 467: Which of the following is NOT a pro-inflammatory cytokine?

A. Interleukin 1
B. Interleukin-10 (Correct Answer)
C. Interleukin 6
D. TNF-Alpha

Explanation: ### Explanation Cytokines are signaling molecules that regulate the immune response. They are broadly categorized into **pro-inflammatory** (promoting inflammation) and **anti-inflammatory** (limiting inflammation and promoting tissue repair). **Why Interleukin-10 (IL-10) is the correct answer:** IL-10 is a potent **anti-inflammatory cytokine**. Its primary role is to terminate the inflammatory response and prevent host tissue damage. It acts by inhibiting the synthesis of pro-inflammatory cytokines (like IL-1, IL-6, and TNF-α) and suppressing the antigen-presenting capacity of macrophages and dendritic cells by downregulating MHC Class II expression. **Analysis of Incorrect Options:** * **Interleukin-1 (IL-1):** A classic pro-inflammatory cytokine produced by macrophages. It is a primary mediator of the acute phase response and is responsible for inducing **fever** (endogenous pyrogen). * **Interleukin-6 (IL-6):** A major pro-inflammatory cytokine that stimulates the liver to produce **Acute Phase Reactants** (like CRP and Fibrinogen). It also plays a key role in the transition from innate to adaptive immunity. * **TNF-Alpha (Tumor Necrosis Factor):** A "master regulator" of inflammation. It promotes leukocyte adhesion, activates endothelial cells, and is a key mediator in the pathogenesis of **septic shock**. **NEET-PG High-Yield Pearls:** * **Anti-inflammatory cytokines:** Remember the duo **IL-10 and TGF-β** (Transforming Growth Factor-beta). * **Pro-inflammatory cytokines:** The "Big Three" are **IL-1, IL-6, and TNF-α**. * **IL-8** is the primary chemotactic factor for neutrophils ("Clean up on aisle 8"). * **IL-12** drives the differentiation of Th1 cells, bridging innate and adaptive immunity.

Question 468: Which immunoglobulin is secreted by the fetus as its primary response?

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

Explanation: **Explanation:** The correct answer is **IgM**. **Why IgM is correct:** IgM is the first immunoglobulin class produced by the fetus, starting around the 20th week of gestation. It is the primary antibody synthesized in response to intrauterine infections because it does not cross the placenta. Therefore, the presence of specific IgM in a neonate’s cord blood is a definitive diagnostic marker for **congenital infections** (e.g., TORCH group), as it must have been produced by the fetus itself rather than acquired from the mother. **Why the other options are incorrect:** * **IgG:** While IgG is the most abundant immunoglobulin in the fetus, it is **passively acquired** from the mother via the placenta (starting at 12 weeks). The fetus produces very little endogenous IgG until after birth. * **IgA:** Secretory IgA is primarily found in breast milk (colostrum) and provides mucosal immunity. The fetus does not produce significant amounts of IgA; levels only begin to rise postnatally. * **IgD:** This is primarily found on the surface of B-lymphocytes as a receptor and is not a major secreted antibody in the fetal response. **High-Yield NEET-PG Pearls:** * **Placental Transfer:** Only **IgG** crosses the placenta (via neonatal Fc receptors). * **Molecular Weight:** IgM is a pentamer and the largest immunoglobulin ("Millionaire molecule"), which prevents it from crossing the placental barrier. * **Diagnostic Significance:** Elevated **cord blood IgM** is the gold standard for diagnosing intrauterine infections. * **Order of Appearance:** In an evolution of immune response, IgM appears first (primary response), followed by IgG (secondary response).

Question 469: Besides B cells and T cells, what is the third distinct type of lymphocyte?

A. MHC cell
B. NK cell (Correct Answer)
C. Macrophage
D. Neutrophil

Explanation: **Explanation:** Lymphocytes are a subtype of white blood cells essential for the immune response. While B cells and T cells comprise the adaptive immune system, **Natural Killer (NK) cells** are the third major type of lymphocyte. Unlike B and T cells, NK cells are part of the **innate immune system**. They are large granular lymphocytes that do not express antigen-specific receptors (TCR or BCR) but instead use a system of activating and inhibitory receptors to identify and kill virally infected or tumor cells. **Analysis of Options:** * **NK Cell (Correct):** These are bone marrow-derived lymphocytes that provide a rapid response. They are characterized by the surface markers **CD56 and CD16** and the absence of CD3. * **MHC Cell (Incorrect):** Major Histocompatibility Complex (MHC) is not a cell type; it is a set of surface proteins (HLA in humans) found on cells that help the immune system recognize foreign substances. * **Macrophage (Incorrect):** These are myeloid-derived mononuclear phagocytes, not lymphocytes. They function as professional antigen-presenting cells (APCs). * **Neutrophil (Incorrect):** These are granulocytes belonging to the myeloid lineage. They are the most abundant leukocytes and are the primary responders in acute inflammation. **High-Yield Clinical Pearls for NEET-PG:** * **Markers:** NK cells are typically **CD3 negative**, **CD56 positive**, and **CD16 positive** (CD16 allows for Antibody-Dependent Cellular Cytotoxicity or ADCC). * **Mechanism:** They utilize **perforins and granzymes** to induce apoptosis in target cells. * **Inhibitory Signal:** NK cells recognize **MHC Class I** molecules on healthy cells via KIR (Killer Immunoglobulin-like Receptors); the absence of MHC-I (common in tumors/viruses) triggers NK cell activation ("Missing Self" hypothesis).

Question 470: A 1-year-old boy presents with a history of recurrent pneumonia, eczema, and prolonged bleeding after circumcision. A peripheral smear shows thrombocytopenia with small platelets. Which of the following is most likely to be deficient?

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

Explanation: ### Explanation The clinical triad of **recurrent infections (pneumonia), eczema, and thrombocytopenia with small platelets (microthrombocytes)** is pathognomonic for **Wiskott-Aldrich Syndrome (WAS)**. This is an X-linked recessive disorder caused by a mutation in the *WASP* gene, which affects the actin cytoskeleton of hematopoietic cells. **Why IgM is the correct answer:** In Wiskott-Aldrich Syndrome, the characteristic immunoglobulin pattern is: * **Low IgM:** This is the most consistent finding and leads to an increased susceptibility to polysaccharide-encapsulated bacteria (e.g., *S. pneumoniae*, *H. influenzae*). * **Normal to Low IgG.** * **Elevated IgA and IgE.** **Analysis of Incorrect Options:** * **Option A (IgA):** In WAS, IgA levels are typically **elevated**, not deficient. Isolated IgA deficiency presents with mucosal infections and anaphylaxis during blood transfusions, but not with thrombocytopenia or eczema. * **Option B (IgE):** IgE levels are typically **elevated** in WAS, contributing to the severe atopic dermatitis (eczema) seen in these patients. * **Option D (IgG):** IgG levels are usually normal or only slightly decreased. A primary IgG deficiency (like CVID) would cause recurrent infections but would not explain the small platelets or eczema. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for WAS:** **TIE** (Thrombocytopenia, Infections, Eczema). * **Platelet Morphology:** WAS is the *only* condition characterized by **microthrombocytes** (small platelets). In contrast, ITP or Bernard-Soulier syndrome features large platelets. * **Genetics:** X-linked recessive; mutation in the **WASP gene** (presents on the short arm of the X chromosome). * **Complications:** Increased risk of autoimmune diseases and B-cell lymphomas. * **Definitive Treatment:** Hematopoietic stem cell transplant.

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Cells and Organs of Immune System

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Innate Immunity

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Adaptive Immunity

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Antigens and Antibodies

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Major Histocompatibility Complex

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Complement System

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Cytokines and Chemokines

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Hypersensitivity Reactions

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Autoimmunity and Autoimmune Diseases

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Immunodeficiency Disorders

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Transplantation Immunology

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Tumor Immunology

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Immunology — MCQs

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