Immunology Practice Questions

Q: Which of the following statements regarding NK cells is false?

Antibody-induced proliferation of NK cells occurs.. ### 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.

Q: Same or closely related antigens present in different biological species are known as?

Heterophile antigens. ### 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*.

Q: Which interleukin is needed for the differentiation of eosinophils?

IL5. **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).

Q: Immunity can be natural or acquired. Which of the following best describes acquired immunity?

Maternal transfer of antibodies. **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).

Q: Interleukin-7 is produced by which one of the following?

Stromal cells. ### 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.

Q: What are the features of innate immunity?

Recognizes foreign antigen in blood. **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.

Q: What is the process that increases the ability for phagocytosis of foreign bodies by the body?

Opsonization. ### 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.

Question 1

Which of the following statements regarding NK cells is false?

Accepted Answer

Antibody-induced proliferation of NK cells occurs.

Answer

NK cells are activated by IL-2.

Answer

NK cells express CD3 receptors.

Answer

NK cells are a variant of large lymphocytes.

Question 2

Which of the following are NOT antigen-presenting cells?

Accepted Answer

All of the above

Answer

Astrocytes

Answer

Endothelial cells

Answer

Epithelial cells

Question 3

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?

Accepted Answer

Pro-zone phenomenon

Answer

Zone of equivalence

Answer

Post-zone phenomenon

Answer

Flocculation phenomenon

Question 4

Same or closely related antigens present in different biological species are known as?

Accepted Answer

Heterophile antigens

Answer

Sequestrated antigens

Answer

Isoantigens

Answer

Haptens

Question 5

Which statement is not true about innate immunity?

Accepted Answer

It is influenced by hormones.

Answer

It is dependent on genetic constitution.

Answer

Identical twins have the same degree of resistance.

Answer

It is influenced by exposure to antigen.

Question 6

Which interleukin is needed for the differentiation of eosinophils?

Accepted Answer

IL5

Answer

IL1

Answer

IL2

Answer

IL4

Question 7

Immunity can be natural or acquired. Which of the following best describes acquired immunity?

Accepted Answer

Maternal transfer of antibodies

Answer

Increase in C-reactive protein (CRP)

Answer

Presence of natural killer (NK) cells

Answer

Complement cascade activation

Question 8

Interleukin-7 is produced by which one of the following?

Accepted Answer

Stromal cells

Answer

Macrophage

Answer

B cells

Answer

T cells

Question 9

What are the features of innate immunity?

Accepted Answer

Recognizes foreign antigen in blood

Answer

C-reactive protein

Answer

Complement protein is a part of the innate immune system

Answer

Includes phagocytes and natural killer cells

Question 10

What is the process that increases the ability for phagocytosis of foreign bodies by the body?

Accepted Answer

Opsonization

Answer

Cross reactivity

Answer

Immune Tolerance

Answer

Immune Surveillance

Immunology — MCQs

Immunology — MCQs

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