Immunology Practice Questions

Q: Interleukin 1 (IL-1) is a potent cytokine. It is best described by which one of the following statements?

It can be produced by natural killer cells. ### Explanation **Interleukin-1 (IL-1)** is a key pro-inflammatory cytokine primarily associated with the innate immune response. **Why Option A is Correct:** While **monocytes and macrophages** are the primary sources of IL-1, it is also produced by a variety of other cells, including **Natural Killer (NK) cells**, B cells, dendritic cells, endothelial cells, and epithelial cells. In the context of NEET-PG, it is crucial to remember that IL-1 production is not exclusive to myeloid cells. **Analysis of Incorrect Options:** * **Option B:** This statement is partially true but less accurate than A. IL-1 acts as a co-stimulator for T-cell activation (enhancing IL-2 production); however, its primary role is as an endogenous pyrogen and a mediator of acute-phase responses. * **Option C:** IL-1 is not multimeric. It exists in two distinct **monomeric** forms, **IL-1α and IL-1β**, which are encoded by separate genes but bind to the same receptor. * **Option D:** This is factually incorrect. Synthesis of IL-1 is significantly **increased and induced** in activated macrophages, especially upon exposure to PAMPs (like LPS) via Toll-like receptors. --- ### High-Yield Clinical Pearls for NEET-PG: * **The "Fever" Cytokine:** IL-1 is a potent **endogenous pyrogen**. It acts on the anterior hypothalamus to increase prostaglandin E2 (PGE2) synthesis, raising the thermoregulatory set point. * **The Inflammasome Connection:** IL-1β is synthesized as an inactive precursor (pro-IL-1β). It requires cleavage by **Caspase-1** within the **NLRP3 inflammasome** complex to become biologically active. * **Acute Phase Response:** Along with IL-6 and TNF-α, IL-1 stimulates the liver to produce acute-phase reactants (e.g., CRP, Fibrinogen). * **Clinical Correlation:** **Anakinra** is a recombinant IL-1 receptor antagonist used in the treatment of Rheumatoid Arthritis and Cryopyrin-Associated Periodic Syndromes (CAPS).

Q: Who discovered phagocytosis?

Metchnikoff. **Explanation:** **1. Correct Answer: A. Metchnikoff** Elie Metchnikoff, a Russian zoologist, discovered **phagocytosis** in 1882 while studying starfish larvae. He observed that specialized cells (phagocytes) could engulf and digest foreign particles. This discovery laid the foundation for **Cellular Immunity**. For his work, he shared the Nobel Prize in 1908 with Paul Ehrlich. **2. Analysis of Incorrect Options:** * **B. Ehrlich:** Paul Ehrlich is known as the "Father of Chemotherapy." He proposed the **Side-Chain Theory** to explain antibody formation and discovered Salvarsan (for syphilis). He is the pioneer of **Humoral Immunity**. * **C. Ruska:** Ernst Ruska designed and built the first **Electron Microscope** in 1931, which revolutionized microbiology by allowing the visualization of viruses and internal cell structures. * **D. Pasteur:** Louis Pasteur is the "Father of Medical Microbiology." His contributions include the **Germ Theory of Disease**, the process of Pasteurization, and the development of vaccines for Anthrax, Cholera, and **Rabies**. **3. NEET-PG High-Yield Pearls:** * **Cellular vs. Humoral:** Remember Metchnikoff = Cellular Immunity; Ehrlich = Humoral Immunity. * **The "Starfish" Experiment:** Metchnikoff’s discovery began by inserting a rose thorn into a starfish larva and observing the cellular reaction. * **Opsonization:** While Metchnikoff discovered the "eating" action, the term "Opsonin" (substances that coat bacteria to make them more "tasty" to phagocytes) was coined by Almroth Wright. * **Phagocytic Cells:** In humans, the primary professional phagocytes are **Neutrophils** (microphages) and **Macrophages**.

Q: Which of the following cell surface markers are associated with T helper cells?

CD2+ CD3+ CD4+ CD8- TCR+ cell. **Explanation:** The identification of T-lymphocyte subsets is based on the presence of specific **Cluster of Differentiation (CD)** markers. All mature T-cells are characterized by the presence of **CD2** (an adhesion molecule/LFA-2), **CD3** (part of the T-cell receptor complex for signal transduction), and a functional **TCR** (T-cell receptor). **1. Why Option C is Correct:** T helper (Th) cells are defined as **CD4+** cells. They recognize antigens presented by **MHC Class II** molecules. Therefore, a mature T helper cell will express **CD2+, CD3+, CD4+, and TCR+**, while being negative for CD8. **2. Analysis of Incorrect Options:** * **Option A:** This represents a "null cell" or a very early pro-T cell (double negative) found in the thymus before maturation. It lacks the essential markers of a functional T-lymphocyte. * **Option B:** This describes a "double-negative" T-cell. While these exist in small numbers in the peripheral blood (often γδ T-cells), they do not function as classical T helper cells. * **Option D:** This describes a **Cytotoxic T-lymphocyte (CTL)**. These cells are **CD8+** and recognize antigens presented by **MHC Class I** molecules. **Clinical Pearls for NEET-PG:** * **CD3** is the most specific marker for all T-cells. * **CD4:CD8 Ratio:** In a healthy individual, the normal ratio is approximately **2:1**. * **HIV Pathogenesis:** HIV selectively infects CD4+ cells by binding to the CD4 molecule (using gp120), leading to a depletion of T helper cells and a reversal of the CD4:CD8 ratio. * **MHC Restriction Rule:** CD4 x II = 8 and CD8 x I = 8 (The product of the CD marker and the MHC class always equals 8).

Q: Which of the following antigens is found within the nuclei of infected hepatocytes and not usually in the peripheral circulation in Hepatitis B infection?

HBcAg. **Explanation:** The correct answer is **HBcAg (Hepatitis B core Antigen)**. **Why HBcAg is the correct answer:** HBcAg is a particulate antigen that forms the inner nucleocapsid of the Hepatitis B virus. During viral replication, it is sequestered within the **nuclei of infected hepatocytes**. Crucially, HBcAg is enveloped by the surface protein (HBsAg) before being released into the bloodstream. Because it is "hidden" inside the complete virion (Dane particle), **free HBcAg is not detectable in the peripheral circulation**. Its presence can only be demonstrated via immunofluorescence in liver biopsy specimens. **Analysis of Incorrect Options:** * **HBeAg (Option A):** This is a soluble protein secreted by infected cells. It is found in the serum and serves as a marker of high viral replication and infectivity. * **Anti-HBc (Option B):** This is an antibody produced by the host against the core antigen. It is easily detectable in the serum and is a key marker for diagnosing past or current infection (the "Window Period" marker). * **HBsAg (Option D):** This is the surface antigen found on the outer envelope. It is the first marker to appear in the blood during an acute infection and is the primary screening tool. **High-Yield Clinical Pearls for NEET-PG:** * **The "Window Period":** The time between the disappearance of HBsAg and the appearance of Anti-HBs. During this time, **IgM Anti-HBc** is the only reliable serological marker. * **HBeAg vs. Anti-HBe:** HBeAg indicates high infectivity; its disappearance and the appearance of Anti-HBe (seroconversion) indicate a lower risk of transmission. * **HBcAg** is the only HBV antigen that does not circulate freely in the blood.

Q: Which of the following is not a phagocytic cell?

NK cells. **Explanation:** The core concept tested here is the distinction between **phagocytes** (cells that ingest and destroy pathogens) and **cytotoxic lymphocytes** (cells that induce apoptosis in target cells). **1. Why NK cells are the correct answer:** Natural Killer (NK) cells are large granular lymphocytes belonging to the innate immune system. Unlike macrophages or neutrophils, NK cells are **not phagocytic**. Instead, they function through **direct cytotoxicity**. They identify virally infected or tumor cells (often via "missing self" MHC-I recognition) and release **perforins and granzymes** to induce programmed cell death (apoptosis). **2. Why the other options are incorrect:** * **Neutrophils (Option D):** These are the "first responders" of the immune system and are professional phagocytes. They utilize oxygen-dependent (respiratory burst) and oxygen-independent mechanisms to kill ingested microbes. * **Macrophages (Option A):** These are mature forms of monocytes found in tissues. They are highly efficient professional phagocytes and also act as Antigen Presenting Cells (APCs). * **Kupffer cells (Option B):** These are specialized, **fixed macrophages** located in the sinusoids of the liver. As part of the Reticuloendothelial System (RES), their primary role is the phagocytosis of pathogens and debris from the portal circulation. **Clinical Pearls for NEET-PG:** * **Professional Phagocytes:** Neutrophils, Monocytes, Macrophages, and Dendritic cells. * **NK Cell Markers:** CD16 (FcγRIII) and CD56 are characteristic surface markers. * **Mnemonic for Fixed Macrophages:** * Liver: Kupffer cells * CNS: Microglia * Lungs: Alveolar macrophages (Dust cells) * Skin: Langerhans cells * Bone: Osteoclasts

Q: Which antibody has the maximum concentration in gastrointestinal secretions?

IgA. **Explanation:** The correct answer is **IgA**. **Why IgA is correct:** Immunoglobulin A (IgA) is the primary antibody involved in **mucosal immunity**. In the body, it exists in two forms: a monomer in the serum and a **dimer** in secretions. Secretory IgA (sIgA) is specifically adapted to survive in harsh environments like the gastrointestinal (GI) tract. It contains a **J-chain** that holds the dimer together and a **secretory component** (derived from epithelial cells) that protects the antibody from proteolytic enzymes present in GI secretions. It acts by "immune exclusion," preventing the attachment of pathogens to mucosal surfaces. **Why other options are incorrect:** * **IgG:** This is the most abundant antibody in the **serum** (75-80%) and provides systemic immunity. It is the only antibody that crosses the placenta but is not the primary defender in GI secretions. * **IgM:** This is the largest antibody (pentamer) and the first to appear in a primary immune response. While it also contains a J-chain and can be secreted onto mucosal surfaces in IgA-deficient individuals, its concentration is significantly lower than IgA. * **IgD:** Found primarily on the surface of B-cells as a receptor; it has no known major role in mucosal secretions. **NEET-PG High-Yield Pearls:** * **Most abundant Ig overall:** IgG (Serum) vs. **Most produced Ig daily:** IgA (due to the vast surface area of mucous membranes). * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients often present with recurrent GI infections (e.g., *Giardia*) and respiratory infections. * **Breast Milk:** IgA is the predominant antibody in colostrum, providing passive mucosal immunity to the neonate.

Q: A patient presents with generalized edema, sweating, flushing, tachycardia, and fever after a bee sting. What type of hypersensitivity reaction is most likely responsible?

IgE mediated reaction. **Explanation:** The clinical presentation of generalized edema, flushing, tachycardia, and sweating following a bee sting is a classic manifestation of **Anaphylaxis**, which is a **Type I Hypersensitivity Reaction**. **1. Why Option B is Correct:** Type I hypersensitivity is an **IgE-mediated reaction**. Upon re-exposure to an allergen (bee venom), specific IgE antibodies already bound to the surface of **mast cells and basophils** cause cross-linking. This triggers immediate degranulation and the release of potent vasoactive mediators like **histamine**, leukotrienes, and prostaglandins. These mediators cause systemic vasodilation (flushing, tachycardia), increased vascular permeability (edema), and smooth muscle contraction. **2. Why the other options are incorrect:** * **Option A (T cell mediated):** This refers to **Type IV (Delayed) Hypersensitivity**. It involves sensitized T-lymphocytes and typically occurs 48–72 hours after exposure (e.g., Mantoux test or contact dermatitis), not immediately. * **Option C (IgG mediated):** IgG is primarily involved in **Type II** (Cytotoxic, e.g., Autoimmune hemolytic anemia) and **Type III** (Immune-complex, e.g., SLE or Serum Sickness) reactions. While IgG can sometimes play a role in subacute reactions, the rapid systemic collapse seen here is classically IgE-driven. * **Option D (IgA mediated):** IgA is involved in mucosal immunity. It is not a primary mediator of systemic hypersensitivity reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Gell and Coombs Classification:** Remember the mnemonic **ACID** (Type I: **A**naphylactic/Atopic; Type II: **C**ytotoxic; Type III: **I**mmune Complex; Type IV: **D**elayed). * **Drug of Choice:** Intramuscular **Epinephrine (1:1000)** is the first-line treatment for anaphylaxis. * **Diagnostic Marker:** Serum **tryptase** levels can be measured shortly after the event to confirm mast cell degranulation.

Q: How many four-peptide subunits does IgM have?

5. **Explanation:** **Correct Answer: C (5)** The basic structural unit of any immunoglobulin (monomer) consists of **four peptide chains**: two identical heavy (H) chains and two identical light (L) chains, linked by disulfide bonds. **IgM** is unique because it primarily exists as a **pentamer** in its secreted form. This means it is composed of **five** of these four-peptide subunits, which are arranged in a circular fashion and held together by disulfide bonds and a specialized polypeptide called the **J chain (Joining chain)**. Therefore, a single IgM molecule contains a total of 20 peptide chains (10 heavy and 10 light). **Analysis of Incorrect Options:** * **Option A (3) & B (4):** No naturally occurring human immunoglobulin exists as a trimer or tetramer in its primary secreted state. * **Option D (6):** While some older literature or specific experimental conditions might mention hexameric IgM (which lacks a J chain), the standard physiological form tested in medical exams is the pentamer. **High-Yield Clinical Pearls for NEET-PG:** * **Valency:** Although IgM is a pentamer (10 antigen-binding sites), its effective valency is often considered **5** due to steric hindrance. * **Molecular Weight:** IgM is the largest antibody ("Millionaire molecule"), making it the most effective at agglutination and complement fixation (via the classical pathway). * **First Responder:** IgM is the first antibody produced in a primary immune response and the first to appear in phylogeny and ontogeny. * **Intravascular Distribution:** Due to its large size, IgM is largely confined to the intravascular compartment.

Q: What is the primary immunoglobulin mediating the secondary immune response?

IgG. **Explanation:** The primary immune response occurs upon the first exposure to an antigen, characterized by a lag phase and the initial production of **IgM**. However, the **secondary immune response** (anamnestic response) occurs upon re-exposure to the same antigen. This response is faster, more intense, and longer-lasting due to the activation of memory B cells. **IgG** is the predominant immunoglobulin in this phase because of class switching and affinity maturation, providing high-affinity protection. **Analysis of Options:** * **IgG (Correct):** It is the most abundant antibody in serum and the primary mediator of the secondary response. It is the only antibody that crosses the placenta. * **IgM (Incorrect):** This is the first antibody produced during the **primary immune response**. Its presence indicates an acute or recent infection. * **IgA (Incorrect):** This is the primary secretory immunoglobulin, found in colostrum, saliva, and tears. It provides mucosal immunity but is not the hallmark of the systemic secondary response. * **IgE (Incorrect):** This antibody is primarily involved in Type I hypersensitivity (allergic) reactions and provides defense against helminthic parasitic infections. **High-Yield NEET-PG Pearls:** * **Memory:** The secondary response is mediated by Memory B cells. * **Avidity vs. Affinity:** While IgM has higher valency (pentamer), IgG produced in the secondary response has higher **affinity** due to somatic hypermutation. * **Diagnostic Marker:** High IgM levels suggest **acute infection**, while high IgG levels suggest **past infection or chronic state**. * **Half-life:** IgG has the longest half-life (approx. 23 days) among all immunoglobulins.

Q: Which theory is most commonly accepted for antibody production?

Clonal selection theory. **Explanation:** The **Clonal Selection Theory**, proposed by **Frank Macfarlane Burnet** in 1957, is the most widely accepted model for antibody production and immune response. **1. Why Clonal Selection Theory is Correct:** This theory states that the body contains a vast repertoire of pre-existing B-lymphocytes, each carrying a unique surface receptor for a specific antigen. When an antigen enters the body, it "selects" the specific lymphocyte that matches its shape. This selected cell is then activated to proliferate (clone itself) and differentiate into: * **Plasma cells:** Which secrete large quantities of antibodies specific to that antigen. * **Memory cells:** Which provide long-term immunity. **2. Why Other Options are Incorrect:** * **Direct Template Theory (Pauling):** Suggested that antigens act as a physical mold or "template" around which a generic antibody protein folds. This was proven wrong because protein folding is determined by amino acid sequences, not external templates. * **Indirect Template Theory:** Proposed that the antigen modifies the DNA or protein-synthesis machinery to produce specific antibodies. This contradicts the central dogma of molecular biology. * **Natural Selection Theory (Jerne):** Suggested that all possible antibodies already circulate in the blood, and the antigen simply binds to one and transports it to a cell for replication. While it introduced the idea of pre-existing diversity, it lacked the cellular mechanism of clonal expansion. **High-Yield Clinical Pearls for NEET-PG:** * **Key Scientist:** Burnet (Nobel Prize winner). * **Self-Tolerance:** Clonal selection explains how the body avoids autoimmunity; clones that react to "self-antigens" are deleted during embryonic development (**Clonal Deletion**). * **Monoclonal Antibodies:** The concept of "one cell, one antibody" is the basis for producing monoclonal antibodies (Hybridoma technology).

Question 1

Interleukin 1 (IL-1) is a potent cytokine. It is best described by which one of the following statements?

Accepted Answer

It can be produced by natural killer cells

Answer

It exerts its effects on T and B cells as a costimulator

Answer

It is multimeric and consists of more than one protein

Answer

Synthesis of IL-1 is inhibited in activated macrophages

Question 2

Who discovered phagocytosis?

Accepted Answer

Metchnikoff

Answer

Ehrlich

Answer

Ruska

Answer

Pasteur

Question 3

Which of the following cell surface markers are associated with T helper cells?

Accepted Answer

CD2+ CD3+ CD4+ CD8- TCR+ cell

Answer

CD2- CD3- CD4- CD8- TCR- cell

Answer

CD2+ CD3+ CD4- CD8- TCR+ cell

Answer

CD2+ CD3+ CD4- CD8+ TCR+ cell

Question 4

Which of the following antigens is found within the nuclei of infected hepatocytes and not usually in the peripheral circulation in Hepatitis B infection?

Accepted Answer

HBcAg

Answer

HBeAg

Answer

Anti-HBc

Answer

HBsAg

Question 5

Which of the following is not a phagocytic cell?

Accepted Answer

NK cells

Answer

Macrophages

Answer

Kupffer cells

Answer

Neutrophils

Question 6

Which antibody has the maximum concentration in gastrointestinal secretions?

Accepted Answer

IgA

Answer

IgG

Answer

IgM

Answer

IgD

Question 7

A patient presents with generalized edema, sweating, flushing, tachycardia, and fever after a bee sting. What type of hypersensitivity reaction is most likely responsible?

Accepted Answer

IgE mediated reaction

Answer

T cell mediated cytotoxicity

Answer

IgG mediated reaction

Answer

IgA mediated hypersensitivity reaction

Question 8

How many four-peptide subunits does IgM have?

Accepted Answer

5

Answer

3

Answer

4

Answer

6

Question 9

What is the primary immunoglobulin mediating the secondary immune response?

Accepted Answer

IgG

Answer

IgA

Answer

IgE

Answer

IgM

Question 10

Which theory is most commonly accepted for antibody production?

Accepted Answer

Clonal selection theory

Answer

Direct template theory

Answer

Indirect template theory

Answer

Natural selection theory

Immunology — MCQs

Immunology — MCQs

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