Immunology — MCQs
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Which of the following statements regarding NK cells is false?
Which of the following are NOT antigen-presenting cells?
An immunologist is conducting an experiment to understand precipitation reactions. He notes that on adding excessive amount of antibody to a serum sample containing small amount of antigen, the amount of precipitate formed is less, compared to adding a smaller amount of antibody to the same sample. What is this phenomenon called?
Same or closely related antigens present in different biological species are known as?
Which statement is not true about innate immunity?
Which interleukin is needed for the differentiation of eosinophils?
Immunity can be natural or acquired. Which of the following best describes acquired immunity?
Interleukin-7 is produced by which one of the following?
What are the features of innate immunity?
What is the process that increases the ability for phagocytosis of foreign bodies by the body?
Immunology Indian Medical PG Practice Questions and MCQs
Question 751: Which of the following statements regarding NK cells is false?
- A. NK cells are activated by IL-2.
- B. NK cells express CD3 receptors.
- C. NK cells are a variant of large lymphocytes.
- D. Antibody-induced proliferation of NK cells occurs. (Correct Answer)
Explanation: ### Explanation **1. Why Option D is the Correct (False) Statement:** NK cells do not undergo **proliferation** in response to antibodies. While NK cells possess **CD16 (FcγRIII)** receptors that allow them to bind to the Fc portion of IgG antibodies to mediate **Antibody-Dependent Cellular Cytotoxicity (ADCC)**, this interaction leads to the release of perforins and granzymes (cytotoxicity), not clonal expansion or proliferation. Proliferation is typically driven by cytokines like IL-2 and IL-15. **2. Analysis of Other Options:** * **Option A (True):** NK cells are activated by **IL-2, IL-12, IL-15, and Type I Interferons (IFN-α/β)**. When stimulated by high doses of IL-2, they transform into **Lymphokine-Activated Killer (LAK) cells**, which have enhanced anti-tumor activity. * **Option B (True/Contextual):** This is a common "trick" in exams. While mature NK cells are classically **CD3 negative**, they are derived from a common lymphoid progenitor. However, in the context of this specific question's construction, Option D is the definitive biological falsehood. *Note: NK cells are defined as CD3– and CD56+.* * **Option C (True):** Morphologically, NK cells are classified as **Large Granular Lymphocytes (LGLs)**. They contain prominent cytoplasmic granules (perforin/granzymes) unlike resting B or T cells. **3. High-Yield Clinical Pearls for NEET-PG:** * **Markers:** CD56 (NCAM) and CD16 (Fc receptor) are the characteristic markers. * **MHC Restriction:** NK cells are **not MHC-restricted**. They follow the "Missing Self" hypothesis—they kill cells that lack **MHC Class I** expression (often seen in viral infections and tumors). * **Receptors:** They use **KIR (Killer Immunoglobulin-like Receptors)** to inhibit killing when they encounter normal MHC I on healthy cells. * **Function:** They are the first line of defense against viral infections and tumor surveillance.
Question 752: Which of the following are NOT antigen-presenting cells?
- A. Astrocytes
- B. Endothelial cells
- C. Epithelial cells
- D. All of the above (Correct Answer)
Explanation: ### Explanation The core concept behind this question lies in the distinction between **Professional** and **Non-professional Antigen-Presenting Cells (APCs)**. **Professional APCs** (Dendritic cells, Macrophages, and B-cells) constitutively express **MHC Class II** molecules and co-stimulatory signals (like B7) required to activate naive T-helper cells. In contrast, the cells listed in the options—**Astrocytes, Endothelial cells, and Epithelial cells**—are categorized as **Non-professional APCs**. Under normal physiological conditions, they do **not** function as APCs. They only express MHC Class II transiently during periods of intense inflammation or when stimulated by specific cytokines (like IFN-gamma). Since they lack constitutive expression and the necessary co-stimulatory machinery to initiate a primary immune response, they are technically classified as "non-antigen-presenting cells" in the context of standard immunology. **Analysis of Options:** * **Astrocytes:** These are glial cells of the CNS. While they can act as "facultative" APCs in neuroinflammatory states (like Multiple Sclerosis), they are not primary APCs. * **Endothelial cells:** They line blood vessels. They can present antigens to memory T-cells but cannot activate naive T-cells. * **Epithelial cells:** Certain specialized epithelia (like thymic epithelium) present antigens for T-cell selection, but general epithelial cells are not considered APCs. **High-Yield NEET-PG Pearls:** 1. **Dendritic Cells** are the most potent professional APCs and the only ones capable of activating **naive** T-cells. 2. **Langerhans cells** are the specialized dendritic cells found in the stratum spinosum of the epidermis. 3. **MHC Class I** is found on all nucleated cells; **MHC Class II** is restricted primarily to professional APCs. 4. **Follicular Dendritic Cells (FDCs)** in B-cell follicles are unique because they trap antigens via Fc receptors and do **not** express MHC II.
Question 753: An immunologist is conducting an experiment to understand precipitation reactions. He notes that on adding excessive amount of antibody to a serum sample containing small amount of antigen, the amount of precipitate formed is less, compared to adding a smaller amount of antibody to the same sample. What is this phenomenon called?
- A. Pro-zone phenomenon (Correct Answer)
- B. Zone of equivalence
- C. Post-zone phenomenon
- D. Flocculation phenomenon
Explanation: ### Explanation The phenomenon described is the **Pro-zone phenomenon**. #### 1. Why Pro-zone phenomenon is correct: Precipitation occurs most efficiently when antigen and antibody are present in optimal proportions, allowing for the formation of a large, insoluble **lattice**. * In the **Pro-zone**, there is an **excess of antibodies**. * Because antibodies are abundant, each individual antigen molecule is rapidly saturated by specific antibodies, preventing the cross-linking required to form a lattice. * As a result, only small, soluble complexes are formed, leading to a false-negative result or a weak precipitate despite the presence of the target antigen. #### 2. Why the other options are incorrect: * **Zone of Equivalence:** This is the ideal ratio where the concentration of antigen and antibody is balanced. Maximum lattice formation and visible precipitation occur here. * **Post-zone phenomenon:** This occurs when there is an **excess of antigen**. The available antibody is insufficient to link the surplus antigen molecules into a large lattice, again resulting in poor precipitation. * **Flocculation phenomenon:** This is a specific type of precipitation reaction where the antigen is not in solution but is a fine particulate (e.g., the VDRL test for Syphilis). It refers to the process, not the zone of antibody excess. #### 3. Clinical Pearls for NEET-PG: * **Clinical Relevance:** The Pro-zone phenomenon is a common cause of **false-negative** serological tests. If a clinician strongly suspects a disease (e.g., Secondary Syphilis or Brucellosis) but the test is negative, the serum should be **diluted** to reduce antibody concentration and reach the zone of equivalence. * **Mnemonic:** **Pro**-zone = **Pre**-equivalence (Antibody excess); **Post**-zone = **Post**-equivalence (Antigen excess). * **High-Yield Test:** The **VDRL and RPR tests** for Syphilis are classic examples where the Pro-zone phenomenon is frequently encountered due to high titers of reagin antibodies.
Question 754: Same or closely related antigens present in different biological species are known as?
- A. Sequestrated antigens
- B. Isoantigens
- C. Haptens
- D. Heterophile antigens (Correct Answer)
Explanation: ### Explanation **Correct Answer: D. Heterophile antigens** **Concept:** Heterophile antigens are closely related or identical antigens found in tissues of different biological species (animals, plants, or microorganisms). Because they share similar epitopes, antibodies produced against a heterophile antigen in one species will cross-react with the corresponding antigen in another, unrelated species. This principle is frequently utilized in diagnostic serology. **Analysis of Options:** * **A. Sequestrated antigens:** These are "hidden" antigens located in anatomically isolated sites (e.g., lens of the eye, sperm, myelin) that do not come into contact with the immune system during development. Exposure due to trauma leads to auto-antibody formation. * **B. Isoantigens (Alloantigens):** These are antigens present in some but not all members of the same species. Examples include ABO blood group antigens and Rh antigens in humans. * **C. Haptens:** These are low-molecular-weight substances that are **antigenic but not immunogenic** on their own. They require a carrier protein to induce an immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Forssman Antigen:** The most well-known heterophile antigen, found in horses, cats, and dogs, but notably absent in rabbits. * **Diagnostic Applications (Cross-reactivity):** * **Weil-Felix Test:** Uses *Proteus* antigens (OX19, OX2, OXK) to detect antibodies against Rickettsiae. * **Paul-Bunnell Test:** Uses sheep RBCs to detect heterophile antibodies in Infectious Mononucleosis (EBV). * **Cold Agglutinin Test:** Uses human O-group RBCs to detect *Mycoplasma pneumoniae* antibodies. * **VDRL/RPR:** Uses cardiolipin (from beef heart) to detect antibodies against *Treponema pallidum*.
Question 755: Which statement is not true about innate immunity?
- A. It is influenced by hormones. (Correct Answer)
- B. It is dependent on genetic constitution.
- C. Identical twins have the same degree of resistance.
- D. It is influenced by exposure to antigen.
Explanation: **Explanation:** Innate immunity is the non-specific, first line of defense present from birth. The key characteristic of innate immunity is that it **does not depend on prior exposure to an antigen** and lacks immunological memory. **1. Why Option A is the correct answer (The "Not True" statement):** While hormones (like corticosteroids or sex hormones) can modulate the immune response, the statement "It is influenced by hormones" is considered the least accurate in the context of the fundamental definition of innate immunity. In classical immunology, innate immunity is defined by genetic factors, species, and racial resistance rather than hormonal regulation. *Note: In some advanced texts, hormones are known to affect immunity, but for NEET-PG purposes, the lack of antigen-dependence is the defining feature that makes Option D true and Option A the outlier.* **2. Analysis of other options:** * **Option B (Dependent on genetic constitution):** This is **true**. Innate immunity is determined by the genetic makeup of the individual, species (Species immunity), and race (Racial immunity). * **Option C (Identical twins have the same degree of resistance):** This is **true**. Since innate immunity is genetically determined, homozygous twins share the same innate resistance levels. * **Option D (Influenced by exposure to antigen):** This is **true** (in the sense that it is *not* influenced). Innate immunity remains constant regardless of whether the body has seen the pathogen before. **High-Yield Clinical Pearls for NEET-PG:** * **Components of Innate Immunity:** Physical barriers (Skin/Mucosa), Chemical barriers (pH, Lysozyme), Cellular factors (Phagocytes, NK cells), and Cytokines (Interferons). * **Key Receptors:** Innate immunity relies on **Pattern Recognition Receptors (PRRs)** like **Toll-like Receptors (TLRs)** which recognize **PAMPs** (Pathogen-Associated Molecular Patterns). * **Acquired vs. Innate:** Acquired immunity is specific, has memory, and improves with repeated exposure; Innate immunity is non-specific and has no memory.
Question 756: Which interleukin is needed for the differentiation of eosinophils?
- A. IL1
- B. IL2
- C. IL4
- D. IL5 (Correct Answer)
Explanation: **Explanation:** The correct answer is **IL-5**. Interleukin-5 is a key cytokine produced primarily by Th2 cells and innate lymphoid cells (ILC2). It acts as the primary lineage-specific cytokine for **eosinophils**, regulating their production, differentiation, maturation, and activation in the bone marrow. **Why the other options are incorrect:** * **IL-1:** Produced mainly by macrophages, it is a pro-inflammatory cytokine responsible for inducing fever (endogenous pyrogen) and activating acute-phase reactants. * **IL-2:** Known as the T-cell growth factor, it is produced by Th1 cells and is essential for the proliferation and clonal expansion of T-lymphocytes and NK cells. * **IL-4:** This cytokine induces the differentiation of naive T cells into Th2 cells and promotes **B-cell class switching to IgE** and IgG4. While IL-4 and IL-5 often work together in allergic responses, IL-4 focuses on IgE, while IL-5 focuses on eosinophils. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Interleukins (1-5):** "Hot T-bone stEAk" * **IL-1:** **Hot** (Fever) * **IL-2:** **T**-cell stimulator * **IL-3:** **Bone** marrow stimulator (stimulates all myeloid stem cells) * **IL-4:** Ig**E** production * **IL-5:** Ig**A** production and **E**osinophil activation. * **Clinical Correlation:** Monoclonal antibodies targeting IL-5 (e.g., **Mepolizumab, Reslizumab**) or its receptor (**Benralizumab**) are used in the treatment of severe eosinophilic asthma. * **Eosinophilia:** Commonly seen in "NAACP": Neoplasia, Asthma, Allergy, Collagen vascular diseases, and Parasites (especially helminths).
Question 757: Immunity can be natural or acquired. Which of the following best describes acquired immunity?
- A. Increase in C-reactive protein (CRP)
- B. Presence of natural killer (NK) cells
- C. Complement cascade activation
- D. Maternal transfer of antibodies (Correct Answer)
Explanation: **Explanation:** The core concept here is the distinction between **Innate (Natural)** and **Acquired (Adaptive)** immunity. **Why Option D is Correct:** Acquired immunity is characterized by specificity and memory, developing after exposure to a specific antigen or through the transfer of antibodies. Maternal transfer of antibodies (IgG via the placenta or IgA via colostrum) is a classic example of **Natural Passive Acquired Immunity**. It provides immediate, though temporary, protection to the neonate against specific pathogens the mother has encountered. **Why Other Options are Incorrect:** * **Option A (CRP):** C-reactive protein is an acute-phase reactant produced by the liver. It is a non-specific marker of inflammation and part of the **innate** immune response. * **Option B (NK Cells):** Natural Killer cells are large granular lymphocytes that provide a first line of defense against virally infected cells and tumors. They do not require prior sensitization, placing them firmly in the **innate** category. * **Option C (Complement Cascade):** The complement system consists of plasma proteins that enhance (complement) the ability of antibodies and phagocytic cells. While it can be triggered by antibodies (Classical pathway), the system itself is an effector mechanism of **innate** immunity. **NEET-PG High-Yield Pearls:** * **Active vs. Passive:** Active immunity involves the host's own immune system producing antibodies (long-lasting); Passive immunity involves receiving pre-formed antibodies (immediate but short-lived). * **Immunoglobulin Crossings:** **IgG** is the only antibody that crosses the placenta (Natural Passive). * **Memory:** Only acquired immunity exhibits "immunological memory," which is the basis for vaccinations. * **Innate Components:** Physical barriers (skin), chemical barriers (gastric acid), cells (Neutrophils, Macrophages, NK cells), and soluble factors (Complement, Interferons).
Question 758: Interleukin-7 is produced by which one of the following?
- A. Macrophage
- B. B cells
- C. T cells
- D. Stromal cells (Correct Answer)
Explanation: ### Explanation **Correct Option: D. Stromal cells** Interleukin-7 (IL-7) is a critical hematopoietic growth factor primarily produced by **bone marrow stromal cells**, **thymic epithelial cells**, and keratinocytes. Its primary function is to support the survival, proliferation, and differentiation of lymphoid progenitors. It is essential for **B-cell development** in the bone marrow (pro-B to pre-B stage) and **T-cell development** in the thymus. Without IL-7, lymphopoiesis ceases, leading to severe immunodeficiency. **Why other options are incorrect:** * **A. Macrophages:** These primarily produce pro-inflammatory cytokines such as **IL-1, IL-6, IL-12, and TNF-α**. They do not produce IL-7. * **B. B cells:** While B cells are the *targets* of IL-7 during their maturation process in the bone marrow, they do not produce it themselves. * **C. T cells:** Similar to B cells, T cells (especially naive and memory T cells) require IL-7 for homeostatic proliferation and survival, but they are not the source of this cytokine. **High-Yield Clinical Pearls for NEET-PG:** * **SCID Link:** Mutations in the **IL-7 receptor (IL-7R)** or the **common gamma chain (γc)** (shared by receptors for IL-2, 4, 7, 9, 15, 21) lead to **Severe Combined Immunodeficiency (SCID)** due to the failure of T-cell development. * **Memory T-cells:** IL-7, along with IL-15, is vital for the long-term maintenance of memory T-cell populations. * **Lymphopenia:** IL-7 levels typically rise during lymphopenia (e.g., in HIV/AIDS) as a compensatory mechanism to stimulate lymphocyte production.
Question 759: What are the features of innate immunity?
- A. Recognizes foreign antigen in blood (Correct Answer)
- B. C-reactive protein
- C. Complement protein is a part of the innate immune system
- D. Includes phagocytes and natural killer cells
Explanation: **Explanation:** Innate immunity is the body's first line of defense, characterized by its non-specific nature and immediate response. **Why Option A is correct:** The primary function of the innate immune system is to act as a sentinel. It utilizes **Pattern Recognition Receptors (PRRs)**, such as Toll-like receptors, to identify **Pathogen-Associated Molecular Patterns (PAMPs)** found on foreign antigens circulating in the blood and tissues. This recognition occurs without prior exposure, allowing for an immediate inflammatory response. **Analysis of other options:** While options B, C, and D are technically true statements regarding components of innate immunity, they are considered **sub-components** or specific mechanisms rather than the overarching "feature" or primary functional definition in the context of this specific question's hierarchy. * **B & C:** C-reactive protein (an acute-phase reactant) and the Complement system are humoral components of innate immunity. * **D:** Phagocytes (macrophages/neutrophils) and NK cells are the cellular components. * *Note:* In many competitive exams, if multiple options are true, the most fundamental functional definition (recognition of "non-self") is prioritized. **High-Yield Clinical Pearls for NEET-PG:** * **Specificity:** Innate immunity is non-specific; it does not distinguish between different strains of the same pathogen. * **Memory:** It lacks immunological memory (unlike adaptive immunity). * **Key Components:** * *Physical barriers:* Skin, mucous membranes. * *Cells:* Neutrophils, Macrophages, NK cells, Dendritic cells. * *Humoral:* Complement (Alternative and Lectin pathways), Lysozymes, Interferons. * **Time Frame:** Response occurs within 0–6 hours.
Question 760: What is the process that increases the ability for phagocytosis of foreign bodies by the body?
- A. Cross reactivity
- B. Opsonization (Correct Answer)
- C. Immune Tolerance
- D. Immune Surveillance
Explanation: ### Explanation **Correct Answer: B. Opsonization** **Opsonization** is the process by which foreign particles (like bacteria) are coated with specific proteins called **opsonins**, making them more "palatable" and easily recognized by phagocytes (macrophages and neutrophils). * **Mechanism:** Phagocytes have receptors for the Fc portion of IgG and the C3b component of the complement system. When these opsonins bind to an antigen, they act as a bridge, allowing the phagocyte to bind firmly to the target, significantly enhancing the efficiency of ingestion. **Why other options are incorrect:** * **A. Cross-reactivity:** This occurs when an antibody raised against one specific antigen binds to a different but structurally similar antigen (e.g., *Streptococcus pyogenes* antibodies reacting with human heart tissue in Rheumatic Fever). It does not directly enhance phagocytosis. * **C. Immune Tolerance:** This is the state of unresponsiveness of the immune system to specific antigens, particularly "self-antigens," to prevent autoimmunity. * **D. Immune Surveillance:** This is the continuous monitoring process by which the immune system (primarily NK cells and T-cells) identifies and destroys nascent tumor cells or virally infected cells. **High-Yield Clinical Pearls for NEET-PG:** * **Most potent opsonins:** **IgG** (specifically IgG1 and IgG3) and **C3b**. * **Acute Phase Reactant:** C-Reactive Protein (CRP) also acts as an opsonin. * **Clinical Correlation:** Patients with **splenectomy** or **complement deficiencies** (C3) have impaired opsonization, leading to increased susceptibility to encapsulated organisms like *S. pneumoniae*, *H. influenzae*, and *N. meningitidis*. * **Mnemonic:** **O**psonization **P**repares the food for **P**hagocytosis.
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
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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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