Immunology — MCQs
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A 2-year-old boy presents with recurrent bacterial infections and Pneumocystis jiroveci pneumonia. Laboratory findings show markedly elevated IgM and low IgG. Which of the following is the most likely cause of his illness?
Which type of hypersensitivity reaction is mediated by immune complexes?
Which of the following components of the immune system is responsible for the secretion of lymphokines?
What are the products formed by the action of papain on an IgG molecule?
A 24-year-old woman presents with a 3-day history of fever and cough with rust-colored sputum. She had a splenectomy 1 year ago. Chest X-ray indicates consolidation of the right lower lobe. Blood cultures are positive for alpha-hemolytic gram-positive diplococci. Immunity to the causative organism is based on what mechanism?
In tuberculosis, immunity is provided by which of the following cell types?
What is the primary application of a hybridoma?
Which immunoglobulin possesses the highest carbohydrate content?
What is the genetic locus of transplant antigens in humans known as?
Which of the following is not an anti-inflammatory cytokine?
Immunology Indian Medical PG Practice Questions and MCQs
Question 141: A 2-year-old boy presents with recurrent bacterial infections and Pneumocystis jiroveci pneumonia. Laboratory findings show markedly elevated IgM and low IgG. Which of the following is the most likely cause of his illness?
- A. Autosomal-dominant mutation in the STAT3 gene
- B. Deficiency in CD40 ligand or CD40 (Correct Answer)
- C. Defect in the NADPH oxidative pathway
- D. Defective cytokine signaling in T-cell precursors
Explanation: ### **Explanation** The clinical presentation of recurrent bacterial infections, *Pneumocystis jiroveci* pneumonia (PJP), and the characteristic laboratory profile (High IgM, Low IgG/IgA/IgE) is diagnostic of **Hyper-IgM Syndrome**. **1. Why the Correct Answer is Right:** Immunoglobulin class-switching (isotype switching) requires a "second signal" interaction between **CD40 ligand (CD154)** on activated T-cells and **CD40** on B-cells. Without this interaction, B-cells cannot switch from producing IgM to IgG, IgA, or IgE. * **CD40L deficiency** (X-linked) is the most common form. * The susceptibility to **PJP** (an opportunistic fungal infection) occurs because CD40-CD40L signaling is also essential for T-cell-mediated activation of macrophages. **2. Why the Other Options are Wrong:** * **Option A (STAT3 mutation):** Causes **Job Syndrome (Hyper-IgE Syndrome)**. It presents with the triad of Eczema, recurrent "cold" Staphylococcal abscesses, and high IgE levels, not high IgM. * **Option C (NADPH oxidase defect):** Causes **Chronic Granulomatous Disease (CGD)**. It presents with infections by catalase-positive organisms (e.g., *S. aureus*, *Aspergillus*) and abnormal Nitroblue Tetrazolium (NBT) or DHR tests. * **Option D (Defective cytokine signaling):** Refers to **SCID** (e.g., IL-2 receptor gamma chain deficiency). While it presents with PJP, it is characterized by the absence of T-cells and low levels of *all* immunoglobulin classes (including IgM). **3. NEET-PG High-Yield Pearls:** * **Inheritance:** Most common form is X-linked Recessive (CD40L deficiency). * **Classic Lab:** ↑ IgM, ↓↓ IgG, ↓↓ IgA, ↓↓ IgE. * **Key Association:** Increased risk of *Cryptosporidium* diarrhea and biliary tract disease. * **Distinction:** Unlike other B-cell defects, Hyper-IgM presents with opportunistic infections (PJP) due to the T-cell signaling defect.
Question 142: Which type of hypersensitivity reaction is mediated by immune complexes?
- A. Type I
- B. Type II
- C. Type III (Correct Answer)
- D. Type IV
Explanation: **Explanation:** **Type III Hypersensitivity** is the correct answer because it is specifically mediated by **immune complexes** (antigen-antibody complexes). These complexes circulate in the blood and deposit in various tissues (like blood vessel walls, synovial membranes, or glomerular basements). Once deposited, they activate the **complement system** (specifically C3a, C4a, and C5a), leading to neutrophil recruitment and subsequent tissue damage through the release of lysosomal enzymes. **Analysis of Incorrect Options:** * **Type I (Immediate):** Mediated by **IgE antibodies** binding to mast cells and basophils, leading to degranulation and release of histamine. (e.g., Anaphylaxis, Asthma). * **Type II (Cytotoxic):** Mediated by **IgG or IgM** antibodies directed against antigens present on **cell surfaces** or connective tissues, leading to cell lysis via the complement system or ADCC. (e.g., Rh incompatibility, Myasthenia Gravis). * **Type IV (Delayed-type):** The only **cell-mediated** hypersensitivity. It does not involve antibodies but is mediated by **T-lymphocytes** (Th1 and CD8+ cells). (e.g., Mantoux test, Contact dermatitis). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (ACID):** **A**naphyalctic (I), **C**ytotoxic (II), **I**mmune-Complex (III), **D**elayed (IV). * **Classic Examples of Type III:** Systemic Lupus Erythematosus (SLE), Post-streptococcal glomerulonephritis (PSGN), Serum Sickness (systemic), and Arthus Reaction (local). * **Key Mediator:** Complement activation (C5a) is the primary driver of neutrophil chemotaxis in Type III reactions.
Question 143: Which of the following components of the immune system is responsible for the secretion of lymphokines?
- A. B lymphocytes
- B. Eosinophils
- C. Macrophages
- D. T lymphocytes (Correct Answer)
Explanation: ### Explanation **1. Why T Lymphocytes are the Correct Answer:** Lymphokines are a subset of cytokines produced specifically by **lymphocytes**, primarily **T lymphocytes (T cells)**. When a T cell encounters an antigen presented by an MHC molecule, it becomes activated and secretes lymphokines (such as IL-2, IL-3, Interferon-gamma, and TNF-beta). These chemical messengers act as "mediators of cellular immunity," signaling other immune cells like macrophages and B cells to migrate to the site of infection and initiate an inflammatory response. **2. Analysis of Incorrect Options:** * **A. B Lymphocytes:** While B cells are lymphocytes, their primary role is **humoral immunity** through the production of **antibodies (immunoglobulins)**. Although they can secrete some cytokines, they are not the classic source of lymphokines. * **B. Eosinophils:** These are granulocytes primarily involved in allergic reactions and defense against **parasitic infections**. They release preformed mediators (like Major Basic Protein) rather than lymphokines. * **C. Macrophages:** Macrophages secrete **monokines** (e.g., IL-1, TNF-alpha). While both lymphokines and monokines are types of cytokines, the term "lymphokine" is strictly reserved for products of lymphocytes. **3. NEET-PG High-Yield Pearls:** * **Cytokine Nomenclature:** Cytokines from lymphocytes = **Lymphokines**; Cytokines from monocytes/macrophages = **Monokines**. * **Th1 vs. Th2:** Th1 cells secrete **IFN-γ and IL-2** (cell-mediated immunity), while Th2 cells secrete **IL-4, IL-5, and IL-13** (humoral/allergic response). * **MIF (Migration Inhibitory Factor):** This was the first lymphokine discovered; it prevents macrophages from leaving the site of an antigen-antibody reaction. * **Key Trigger:** T cell activation and lymphokine release are the basis of **Type IV (Delayed-type) Hypersensitivity** reactions (e.g., Mantoux test).
Question 144: What are the products formed by the action of papain on an IgG molecule?
- A. Two Fc fragments and one Fab fragment
- B. One Fc fragment and two Fab fragments (Correct Answer)
- C. Two Fc fragments and two Fab fragments
- D. One Fc fragment and one Fab fragment
Explanation: ### Explanation The structure of an Immunoglobulin (IgG) molecule can be analyzed by enzymatic digestion, a high-yield concept for NEET-PG. **1. Why Option B is Correct:** The enzyme **Papain** (derived from papaya) cleaves the IgG molecule **above the hinge region** (on the N-terminal side of the inter-chain disulfide bonds). Because it cuts above the point where the two heavy chains are linked, the molecule splits into three separate pieces: * **Two Fab fragments** (Fragment Antigen Binding): Each contains one entire light chain and the $V_H$ and $C_H1$ domains of the heavy chain. These are monovalent and can bind antigens but cannot precipitate them. * **One Fc fragment** (Fragment Crystallizable): Composed of the remaining $C_H2$ and $C_H3$ domains. This fragment is responsible for effector functions like complement fixation and opsonization. **2. Why Other Options are Incorrect:** * **Option A & C:** These are structurally impossible. An IgG molecule consists of only two heavy chains; therefore, it can only yield one Fc fragment (the joined "tail" of the two heavy chains). * **Option D:** This would imply the molecule was split vertically down the middle, which does not occur with enzymatic digestion. **3. Clinical Pearls & High-Yield Facts:** * **Pepsin Digestion:** Unlike Papain, Pepsin cleaves **below the hinge region**. This results in **one $F(ab')_2$ fragment** (bivalent, can precipitate antigens) and several small peptides (the Fc portion is degraded). * **Mercaptoethanol:** This is a reducing agent that breaks disulfide bonds, splitting IgG into **four separate polypeptide chains** (2 Heavy and 2 Light). * **Memory Aid:** **Pa**pain gives **Pa**rtial fragments (3 pieces); **Pe**psin gives a **P**recious bivalent fragment (1 large $F(ab')_2$).
Question 145: A 24-year-old woman presents with a 3-day history of fever and cough with rust-colored sputum. She had a splenectomy 1 year ago. Chest X-ray indicates consolidation of the right lower lobe. Blood cultures are positive for alpha-hemolytic gram-positive diplococci. Immunity to the causative organism is based on what mechanism?
- A. Alternative complement pathway activation
- B. Antibody to fimbriae
- C. IgA antibodies to C carbohydrate
- D. IgG antibodies to a surface acidic polysaccharide (Correct Answer)
Explanation: ### Explanation **Diagnosis:** The clinical presentation of fever, rust-colored sputum, and lobar consolidation, combined with the laboratory finding of alpha-hemolytic gram-positive diplococci, confirms a diagnosis of **Pneumococcal pneumonia** caused by *Streptococcus pneumoniae*. **Why Option D is Correct:** *Streptococcus pneumoniae* is an encapsulated bacterium. Its primary virulence factor is its **capsular polysaccharide**, which is acidic and prevents phagocytosis by inhibiting complement deposition. In an asplenic patient (like the woman in the scenario), the risk of infection by encapsulated organisms increases significantly because the spleen is the primary site for filtering these bacteria and producing opsonizing antibodies. Protective immunity is mediated by **type-specific IgG antibodies** directed against the capsular polysaccharide. These antibodies act as opsonins, facilitating phagocytosis and clearance by macrophages. **Why Other Options are Incorrect:** * **Option A:** While the alternative pathway provides some innate defense, it is insufficient for clearance without the classical pathway and specific antibodies, especially in asplenic individuals. * **Option B:** Fimbriae (pili) are primarily used for attachment by organisms like *E. coli* or *N. gonorrhoeae*, not the primary target for immunity against *S. pneumoniae*. * **Option C:** The C-carbohydrate (C-substance) is a component of the cell wall that reacts with C-reactive protein (CRP), but antibodies against it are not protective against infection. **High-Yield NEET-PG Pearls:** * **Post-Splenectomy Sepsis:** Most commonly caused by *S. pneumoniae*, *H. influenzae*, and *N. meningitidis*. * **Quellung Reaction:** A biochemical reaction where the capsule swells in the presence of specific antiserum (used for identification). * **Vaccination:** The Pneumococcal polysaccharide vaccine (PPSV23) contains purified capsular polysaccharides, while the conjugate vaccine (PCV13) attaches the polysaccharide to a protein carrier to induce a T-cell dependent response.
Question 146: In tuberculosis, immunity is provided by which of the following cell types?
- A. CD4+ T cells (Correct Answer)
- B. CD8+ T cells
- C. IgG antibodies
- D. IgM antibodies
Explanation: **Explanation:** **1. Why CD4+ T cells are correct:** *Mycobacterium tuberculosis* is an **obligate intracellular pathogen** that survives and replicates within macrophages. Because the bacteria are sequestered inside cells, humoral immunity (antibodies) is ineffective. Protection relies entirely on **Type IV Hypersensitivity (Cell-Mediated Immunity)**. The process begins when macrophages present mycobacterial antigens to **CD4+ T-helper (Th1) cells**. These Th1 cells secrete **Interferon-gamma (IFN-γ)**, which is the critical cytokine required to activate macrophages, enhance phagolysosome fusion, and stimulate the production of reactive oxygen species to kill the bacilli. This interaction also leads to the formation of granulomas, which contain the infection. **2. Why other options are incorrect:** * **CD8+ T cells:** While they play a minor role by lysing infected cells, they are not the primary mediators of protective immunity in TB compared to the central role of CD4+ cells. * **IgG and IgM antibodies:** These are components of humoral immunity. Since *M. tuberculosis* is intracellular, antibodies cannot reach the pathogen to neutralize it. In TB, antibodies are produced but are **non-protective**. **3. NEET-PG High-Yield Pearls:** * **Cytokine Profile:** The Th1 response (IFN-γ, IL-2, IL-12) is protective; a Th2 response (IL-4, IL-5, IL-10) is associated with disease progression. * **Gold Standard Test:** The **Interferon-Gamma Release Assay (IGRA)** measures the IFN-γ produced by T cells in response to TB antigens. * **Clinical Correlation:** Patients with low CD4+ counts (e.g., HIV/AIDS) are at a significantly higher risk of disseminated and extrapulmonary tuberculosis because they cannot mount an effective cell-mediated response.
Question 147: What is the primary application of a hybridoma?
- A. In situ hybridization
- B. Sequencing DNA
- C. Production of monoclonal antibodies (Correct Answer)
- D. Production of continuous cell lines
Explanation: **Explanation:** The **Hybridoma technology**, pioneered by Kohler and Milstein, is the cornerstone for producing **monoclonal antibodies (mAbs)**. **Why Option C is correct:** A hybridoma is a "hybrid" cell produced by the fusion of two different cells: 1. **B-lymphocytes:** Sourced from an immunized animal (usually a mouse), providing the genetic machinery to produce a specific antibody. 2. **Myeloma cells:** Cancerous B-cells that provide the property of "immortality" (continuous division). The resulting hybridoma cell possesses the ability to produce large quantities of a single, highly specific antibody (monoclonal) indefinitely in culture. **Why other options are incorrect:** * **Option A:** *In situ* hybridization is a laboratory technique used to localize specific DNA or RNA sequences within tissues or cells using labeled probes; it does not involve cell fusion. * **Option B:** DNA sequencing (e.g., Sanger or Next-Generation Sequencing) determines the precise order of nucleotides in a DNA molecule. * **Option C:** While hybridomas are technically continuous cell lines, their **primary medical and diagnostic application** is the specific production of antibodies, not just the creation of cell lines for general study. **High-Yield Clinical Pearls for NEET-PG:** * **Selection Medium:** The **HAT medium** (Hypoxanthine, Aminopterin, Thymidine) is used to select hybridoma cells. Myeloma cells used are deficient in the **HGPRT enzyme**, ensuring only fused hybrid cells survive. * **Clinical Uses:** mAbs are used in diagnostic kits (e.g., ELISA, pregnancy tests) and therapeutics (e.g., Rituximab for Lymphoma, Infliximab for RA). * **Nobel Prize:** Kohler and Milstein received the Nobel Prize in 1984 for this discovery.
Question 148: Which immunoglobulin possesses the highest carbohydrate content?
- A. IgG
- B. IgA
- C. IgD (Correct Answer)
- D. IgM
Explanation: **Explanation:** The carbohydrate content of immunoglobulins varies significantly across different classes, influencing their structural stability and biological functions. **Why IgD is the correct answer:** Among all immunoglobulin classes, **IgD** possesses the highest carbohydrate content, approximately **9–13%**. These carbohydrate chains are primarily attached to the hinge region and the CH2 domain. While the exact physiological function of IgD remains the least understood among antibodies, its high glycosylation is thought to protect the molecule from proteolysis and assist in its role as a surface receptor on B-cells. **Analysis of Incorrect Options:** * **IgG:** Has the lowest carbohydrate content (approx. **3%**). It is the most abundant antibody in serum and the only one to cross the placenta. * **IgA:** Contains approximately **7–10%** carbohydrate content. It is the primary secretory antibody (found in colostrum, saliva, and tears). * **IgM:** Contains approximately **10–12%** carbohydrate content. While very high (often cited as the second highest), it typically falls just below the maximum range of IgD. It is the largest antibody (pentamer) and the first to appear in a primary immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Highest Serum Concentration:** IgG (80%) * **Highest Molecular Weight:** IgM (900,000 Da) * **Highest Sedimentation Coefficient:** IgM (19S) * **Longest Half-life:** IgG (23 days) * **Heat Labile Antibody:** IgE (destroyed at 56°C for 30 mins) * **Memory Tip:** For carbohydrate content, remember **D > M > A > G**.
Question 149: What is the genetic locus of transplant antigens in humans known as?
- A. Rhesus (Rh)
- B. Immunoglobulin A and IgM
- C. Human leukocyte antigen (HLA) (Correct Answer)
- D. ABO
Explanation: ### Explanation **Correct Answer: C. Human leukocyte antigen (HLA)** The **Major Histocompatibility Complex (MHC)** in humans is known as the **Human Leukocyte Antigen (HLA)** system. These are specialized surface glycoproteins encoded by a cluster of genes located on the **short arm of Chromosome 6**. In the context of transplantation, HLA molecules act as the primary "self" vs. "non-self" recognition signals. When an organ is transplanted, the recipient’s T-cells recognize foreign HLA antigens on the donor tissue, triggering an immune response that leads to graft rejection. Therefore, HLA typing (matching) is the most critical genetic factor for ensuring transplant compatibility. **Why other options are incorrect:** * **A. Rhesus (Rh) & D. ABO:** These are **blood group systems** found on the surface of erythrocytes. While ABO compatibility is the first step in organ matching (to prevent hyperacute rejection), these are not the primary genetic loci for tissue histocompatibility antigens. * **B. Immunoglobulin A and IgM:** These are classes of antibodies (humoral immunity) involved in mucosal defense and primary immune responses, respectively. They are effector molecules of the immune system, not genetic loci for transplant antigens. **High-Yield 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 only on **Antigen-Presenting Cells (APCs)**; recognized by **CD4+ T-cells**. * **Inheritance:** HLA genes are inherited as a **haplotype** (one set from each parent) in a **codominant** fashion. * **Ankylosing Spondylitis:** Strongly associated with **HLA-B27**. * **Celiac Disease:** Associated with **HLA-DQ2/DQ8**.
Question 150: Which of the following is not an anti-inflammatory cytokine?
- A. IL-13
- B. IL-10
- C. IL-18 (Correct Answer)
- D. TGF-beta
Explanation: **Explanation:** Cytokines are signaling molecules that regulate the immune response and are broadly classified into **Pro-inflammatory** (promote inflammation) and **Anti-inflammatory** (limit inflammation and promote tissue repair). **Why IL-18 is the Correct Answer:** **IL-18** belongs to the IL-1 family and is a potent **pro-inflammatory cytokine**. It is produced by macrophages and dendritic cells. Its primary function is to induce the production of Interferon-gamma (IFN-γ) from T-cells and Natural Killer (NK) cells. Because it stimulates a Th1-mediated immune response and enhances cellular cytotoxicity, it is a driver of inflammation, not a suppressor. **Analysis of Incorrect Options:** * **IL-10:** Often called the "prototypical" anti-inflammatory cytokine. It inhibits the synthesis of pro-inflammatory cytokines (like IL-1, IL-6, and TNF-α) and downregulates MHC Class II expression on macrophages. * **TGF-beta (Transforming Growth Factor-β):** A powerful anti-inflammatory agent that suppresses T-cell proliferation and macrophage activation while promoting tissue repair and fibrosis. * **IL-13:** Similar to IL-4, it is secreted by Th2 cells. It exerts anti-inflammatory effects by inhibiting the production of inflammatory cytokines by monocytes and macrophages. **NEET-PG High-Yield Pearls:** * **The "Big Three" Anti-inflammatory Cytokines:** IL-10, TGF-β, and IL-4/IL-13. * **The "Big Three" Pro-inflammatory Cytokines:** IL-1, IL-6, and TNF-α. * **IL-18 Clinical Link:** It is significantly elevated in conditions like Macrophage Activation Syndrome (MAS) and Adult-onset Still’s Disease. * **IL-10 Source:** Primarily produced by **T-regulatory (Treg) cells**.
Practice by Chapter
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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