Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 631: Lysosomes are thought to play an important role in which of the following processes?

A. Class I MHC-restricted antigen presentation
B. Class II MHC-restricted antigen presentation (Correct Answer)
C. Immunoglobulin gene rearrangement
D. T cell receptor alpha chain rearrangement

Explanation: **Explanation:** The processing and presentation of antigens are divided into two distinct pathways based on the origin of the antigen and the type of MHC molecule involved. **Why Option B is correct:** **Class II MHC-restricted antigen presentation** involves the **exogenous pathway**. Extracellular pathogens (like bacteria) are internalized via endocytosis or phagocytosis into vesicles. These vesicles fuse with **lysosomes**, forming phagolysosomes. Within these acidic environments, lysosomal enzymes degrade the protein into smaller peptides. Simultaneously, Class II MHC molecules synthesized in the ER travel to these vesicles. The peptide is loaded onto the MHC II molecule (after the CLIP protein is removed) and transported to the cell surface for presentation to **CD4+ T helper cells**. **Why other options are incorrect:** * **Option A:** Class I MHC presentation involves the **endogenous pathway**. Proteins (viral or tumor antigens) are degraded in the cytoplasm by **proteasomes**, not lysosomes. The peptides are then transported into the ER via TAP transporters. * **Options C & D:** Immunoglobulin and T-cell receptor (TCR) gene rearrangements occur in the **nucleus** of B and T cells, respectively, involving RAG-1 and RAG-2 enzymes. Lysosomes play no role in genetic recombination. **High-Yield Clinical Pearls for NEET-PG:** * **MHC I** = Endogenous antigens + Proteasome + CD8+ T cells ("Rule of 8": 1 × 8 = 8). * **MHC II** = Exogenous antigens + **Lysosome** + CD4+ T cells ("Rule of 8": 2 × 4 = 8). * **Invariant Chain (Ii):** Prevents premature binding of endogenous peptides to MHC II in the ER. * **CLIP:** The remnant of the invariant chain that stays in the MHC II groove until it is exchanged for an exogenous peptide in the lysosome.

Question 632: A single immunoglobulin molecule contains how many light chains and how many heavy chains?

A. 1 light chain, 1 heavy chain
B. 2 heavy chains, 1 light chain
C. 2 light chains, 2 heavy chains (Correct Answer)
D. 2 light chains, 1 heavy chain

Explanation: ### Explanation **Core Concept:** The basic structural unit of an immunoglobulin (antibody) is a **Y-shaped monomer** composed of four polypeptide chains. These consist of **two identical heavy (H) chains** and **two identical light (L) chains**. These chains are held together by covalent disulfide bonds and non-covalent interactions. Each light chain is linked to a heavy chain, and the two heavy chains are linked to each other, forming a symmetrical structure (H₂L₂). **Analysis of Options:** * **Option C (Correct):** As per the structural model of antibodies, a single monomeric unit always contains 2 light and 2 heavy chains. * **Options A, B, and D (Incorrect):** These options describe asymmetrical or incomplete molecules. Immunoglobulins must be symmetrical to provide two identical antigen-binding sites (valency of at least 2). A single chain or an unequal number of chains would not form the functional "Y" structure required for immune complex formation. **NEET-PG High-Yield Pearls:** 1. **Light Chains:** There are two types—**Kappa (κ)** and **Lambda (λ)**. A single antibody molecule will have either two κ or two λ chains, never one of each. In humans, the κ:λ ratio is approximately 2:1. 2. **Heavy Chains:** These determine the **Isotype** (IgG, IgA, IgM, IgD, IgE) based on the type of heavy chain (γ, α, μ, δ, ε). 3. **Fragments:** Papain digestion cleaves the molecule into **two Fab fragments** (antigen-binding) and **one Fc fragment** (crystallizable/effector function). Pepsin digestion yields one **F(ab')₂** fragment. 4. **Polymeric Forms:** While the *basic unit* is H₂L₂, **IgM** exists as a pentamer (10 H and 10 L chains) and **Secretory IgA** usually exists as a dimer, both held together by a **J-chain**.

Question 633: Which immunoglobulin is inactive at high temperatures?

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

Explanation: **Explanation:** The correct answer is **IgE**. This immunoglobulin is uniquely characterized by its **heat-lability**. When exposed to a temperature of **56°C for 30 to 60 minutes**, IgE undergoes denaturation and loses its ability to bind to mast cells and basophils (the Prausnitz-Küstner reaction becomes negative). This property is due to the specific structure of its Fc region, which is more sensitive to thermal stress than other antibodies. **Analysis of Options:** * **IgG (Option A):** The most abundant and stable immunoglobulin. It is heat-stable and can withstand 56°C without losing its biological activity or structural integrity. * **IgA (Option B):** Found primarily in secretions (tears, saliva, colostrum). It is relatively stable and does not lose its functional capacity at high temperatures compared to IgE. * **IgM (Option C):** The largest (pentameric) antibody and the first to appear in response to an antigen. While large, it remains heat-stable at standard laboratory inactivation temperatures (56°C). **NEET-PG High-Yield Pearls:** * **Heat Lability:** IgE is the only immunoglobulin inactivated at 56°C in 30 minutes. * **Structure:** IgE has 4 constant domains ($C_H1$ to $C_H4$) and lacks a hinge region, similar to IgM. * **Receptors:** It binds to high-affinity **FcεRI** receptors on mast cells and basophils, mediating Type I Hypersensitivity. * **Clinical Significance:** Elevated in parasitic infections (helminths) and atopic conditions (asthma, eczema). * **Prausnitz-Küstner (PK) Reaction:** A historical test for IgE-mediated hypersensitivity; heating the serum before injection abolishes the PK reaction.

Question 634: Which of the following features is NOT shared between T cells and B cells?

A. Positive selection during development (Correct Answer)
B. Class I MHC expression
C. Antigen-specific receptors
D. All of the above

Explanation: ### Explanation The correct answer is **A. Positive selection during development**. **1. Why Positive Selection is the Correct Answer:** While both T and B cells undergo **negative selection** (deletion of self-reactive cells to ensure self-tolerance), **positive selection** is a process unique to **T cell development** in the thymus. During positive selection, T cells must demonstrate the ability to bind to self-MHC molecules with moderate affinity. If they cannot recognize the host's MHC, they undergo apoptosis. B cells, which develop in the bone marrow, do not require MHC recognition to function; therefore, they do not undergo positive selection. **2. Analysis of Incorrect Options:** * **B. Class I MHC expression:** This is a shared feature. All nucleated cells in the human body express MHC Class I molecules. Since both T and B cells are nucleated lymphocytes, they both express MHC I. * **C. Antigen-specific receptors:** This is a shared feature. Both cell types are part of the adaptive immune system and possess unique surface receptors—the **T-Cell Receptor (TCR)** and the **B-Cell Receptor (BCR/Surface Ig)**—generated through V(D)J recombination to recognize specific antigens. **3. NEET-PG High-Yield Pearls:** * **Site of Maturation:** T cells mature in the **Thymus**; B cells mature in the **Bone Marrow** (or Bursa of Fabricius in birds). * **Selection Process:** * **T cells:** Undergo both Positive selection (Cortex) and Negative selection (Medulla). * **B cells:** Undergo Negative selection only. * **MHC Restriction:** T cells are "MHC restricted" (CD4 to MHC II, CD8 to MHC I), whereas B cells can recognize free, soluble antigens directly. * **Clonal Deletion:** This is the primary mechanism of central tolerance for both cell types.

Question 635: Plasma cells are derived from which of the following immune cells?

A. T cells
B. B cells (Correct Answer)
C. Macrophages
D. Neutrophils

Explanation: **Explanation:** **Correct Answer: B cells** Plasma cells are the final functional stage of **B-cell differentiation**. When a B cell encounters its specific antigen and receives necessary signals (usually from T-helper cells), it undergoes clonal expansion and matures into a plasma cell. The primary function of a plasma cell is to act as an "antibody factory," secreting large quantities of soluble immunoglobulins (IgG, IgA, IgM, IgE, or IgD) into the blood and lymph to neutralize pathogens. **Analysis of Incorrect Options:** * **A. T cells:** These are responsible for cell-mediated immunity. They differentiate into Helper T cells (CD4+), Cytotoxic T cells (CD8+), or Regulatory T cells, but they never produce antibodies or transform into plasma cells. * **C. Macrophages:** These are professional phagocytes derived from **monocytes**. Their role is to engulf debris and act as Antigen-Presenting Cells (APCs), not to produce antibodies. * **D. Neutrophils:** These are granulocytes involved in the acute inflammatory response and phagocytosis of bacteria. They are short-lived and do not differentiate into antibody-secreting cells. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Plasma cells have a characteristic **"Cartwheel" or "Clock-face" nucleus** due to clumps of peripheral chromatin and a prominent **perinuclear halo** (representing the Golgi apparatus). * **Multiple Myeloma:** A plasma cell dyscrasia (malignancy) characterized by the overproduction of monoclonal antibodies (M-protein). * **Russell Bodies:** These are eosinophilic inclusions found in plasma cells representing accumulated immunoglobulins. * **Surface Markers:** While mature B cells express CD19 and CD20, plasma cells typically lose these and express **CD138** (Syndecan-1).

Question 636: Graft versus host reaction is caused by which of the following cell types?

A. B-Lymphocytes
B. T-lymphocytes (Correct Answer)
C. Monocytes
D. Leukocytes

Explanation: **Explanation:** **Graft-versus-Host Disease (GVHD)** occurs when immunocompetent cells in the donor tissue (the graft) recognize the recipient (the host) as foreign and initiate an immune attack. **Why T-lymphocytes are correct:** The primary mediators of GVHD are **mature donor T-lymphocytes**. When a graft (typically bone marrow or hematopoietic stem cells) containing these T-cells is transplanted into an immunocompromised host, the donor T-cells recognize the host’s Major Histocompatibility Complex (MHC) antigens as non-self. This triggers a Type IV (delayed-type) hypersensitivity reaction where cytotoxic T-cells (CD8+) directly damage host tissues and helper T-cells (CD4+) release cytokines that amplify the inflammatory response. **Why other options are incorrect:** * **B-Lymphocytes:** While they produce antibodies, they are not the primary initiators of GVHD. Their role is more prominent in hyperacute organ rejection. * **Monocytes:** These are phagocytic cells that act as antigen-presenting cells but do not possess the specific recognition receptors required to initiate the systemic graft-versus-host response. * **Leukocytes:** This is a general term for all white blood cells. While T-cells are a subset of leukocytes, the question asks for the specific cell type responsible; "T-lymphocytes" is the more precise and accurate answer. **High-Yield Facts for NEET-PG:** * **Requirements for GVHD (Billingham’s Criteria):** 1. Graft must contain immunocompetent cells (T-cells). 2. Host must possess antigens lacking in the donor. 3. Host must be immunologically crippled (unable to reject the graft). * **Common Sites:** Skin (rash), Liver (jaundice/elevated enzymes), and GI tract (diarrhea). * **Prevention:** Depletion of T-cells from the donor graft and the use of immunosuppressants like Cyclosporine or Methotrexate. * **Irradiation:** Blood products are irradiated before transfusion in susceptible patients to prevent **Transfusion-Associated GVHD** by inactivating donor T-cells.

Question 637: Where does antigen-antibody binding occur?

A. At the surface (Correct Answer)
B. At the center
C. Inside the molecule
D. Anywhere in the structure

Explanation: **Explanation:** The binding between an antigen and an antibody is a highly specific molecular interaction that occurs exclusively **at the surface** of the molecules involved. 1. **Why the correct answer is right:** Antigen-antibody reactions are governed by the interaction between the **Epitope** (antigenic determinant) on the antigen and the **Paratope** (antigen-binding site) on the antibody. These sites are located on the outermost structural surfaces to allow for physical contact and the formation of non-covalent bonds (such as Van der Waals forces, electrostatic forces, and hydrogen bonds). Since these molecules are often large proteins or polysaccharides, the internal residues are structurally shielded and inaccessible for binding. 2. **Why the incorrect options are wrong:** * **At the center / Inside the molecule:** The core of an antigen or antibody usually consists of hydrophobic amino acids that maintain the structural integrity (scaffold) of the protein. These areas are not exposed to the aqueous environment where the reaction occurs and lack the spatial accessibility required for a paratope to dock. * **Anywhere in the structure:** Binding is not random. It is restricted to specific "hotspots" (epitopes) on the surface that are complementary in shape and charge to the antibody's CDRs (Complementary Determining Regions). **High-Yield NEET-PG Pearls:** * **Valency:** The number of epitopes on an antigen surface is its valency. * **Affinity:** The strength of a single Ag-Ab bond at one surface site. * **Avidity:** The overall combined strength of all binding sites (e.g., IgM has high avidity due to 10 binding sites). * **Haptens:** Small molecules that are antigenic but not immunogenic unless attached to a surface carrier protein.

Question 638: Which of the following is an example of Type I hypersensitivity?

A. Lepromin test
B. Tuberculin test
C. Casoni's test (Correct Answer)
D. Arthus reaction

Explanation: **Explanation:** **Type I Hypersensitivity** (Immediate/Anaphylactic) is mediated by **IgE antibodies** binding to mast cells and basophils, leading to the release of vasoactive amines like histamine upon re-exposure to an antigen. **Why Casoni’s Test is correct:** Casoni’s test is an immediate hypersensitivity skin test used for the diagnosis of **Hydatid disease** (*Echinococcus granulosus*). When a small amount of sterile hydatid fluid is injected intradermally, a wheal-and-flare response occurs within 20 minutes in sensitized individuals. This rapid reaction is a classic clinical example of a Type I hypersensitivity response. **Analysis of Incorrect Options:** * **A & B. Lepromin and Tuberculin tests:** These are examples of **Type IV (Delayed-type) hypersensitivity**. They are mediated by T-cells (not antibodies) and take 48–72 hours to manifest as induration at the injection site. * **D. Arthus reaction:** This is a localized **Type III hypersensitivity** reaction. It involves the formation of antigen-antibody (immune) complexes that deposit in local blood vessels, leading to complement activation and inflammatory tissue damage. **High-Yield Clinical Pearls for NEET-PG:** * **Type I mnemonic:** "Atopy/Anaphylaxis" (IgE). Examples: Asthma, Urticaria, Allergic rhinitis, and Casoni’s test. * **Type II:** Cytotoxic (IgG/IgM). Examples: Rh incompatibility, Myasthenia Gravis, Goodpasture syndrome. * **Type III:** Immune-Complex. Examples: SLE, Post-streptococcal glomerulonephritis (PSGN), Serum sickness. * **Type IV:** Delayed. Examples: Contact dermatitis, Graft rejection, Mantoux test. * **Note:** Casoni’s test is now largely replaced by serology (ELISA) and imaging due to low specificity and risk of sensitization.

Question 639: Transfusion reactions are due to which type of hypersensitivity?

A. Immediate
B. Immune complex-mediated
C. Antibody-mediated (Correct Answer)
D. Delayed-type

Explanation: **Explanation:** Transfusion reactions (specifically Acute Hemolytic Transfusion Reactions) are classic examples of **Type II Hypersensitivity**, also known as **Antibody-mediated** cytotoxicity. 1. **Why Option C is Correct:** In Type II hypersensitivity, pre-formed antibodies (IgM or IgG) in the recipient’s plasma bind to specific antigens on the surface of the donor’s red blood cells (e.g., ABO incompatibility). This antigen-antibody binding activates the **complement system** (classical pathway) and leads to MAC formation, resulting in direct osmotic lysis of RBCs (intravascular hemolysis) or opsonization and phagocytosis by splenic macrophages (extravascular hemolysis). 2. **Why other options are incorrect:** * **Option A (Immediate/Type I):** Mediated by IgE and mast cell degranulation (e.g., Anaphylaxis, Urticaria). While allergic transfusion reactions exist, the standard "transfusion reaction" refers to hemolytic types. * **Option B (Immune complex-mediated/Type III):** Involves soluble antigen-antibody complexes depositing in tissues (e.g., SLE, Serum Sickness, Arthus reaction). * **Option D (Delayed-type/Type IV):** Cell-mediated immunity involving T-lymphocytes, not antibodies (e.g., Mantoux test, Contact dermatitis). **High-Yield Clinical Pearls for NEET-PG:** * **Type II Hypersensitivity Examples:** Myasthenia Gravis, Goodpasture Syndrome, Rheumatic Fever, Erythroblastosis Fetalis, and Pemphigus Vulgaris. * **Mnemonic for Hypersensitivity (ACID):** **A**naphyalctic (I), **C**ytotoxic/Antibody (II), **I**mmune-Complex (III), **D**elayed (IV). * **Febrile Non-Hemolytic Transfusion Reaction (FNHTR):** The most common transfusion reaction, caused by cytokines released from donor leukocytes (not Type II).

Question 640: Which of the following is a central lymphoid organ?

A. Bone marrow (Correct Answer)
B. Lymph node
C. Spleen
D. All of the above

Explanation: **Explanation:** Lymphoid organs are categorized into two types based on their function in the development and maturation of lymphocytes: **Primary (Central)** and **Secondary (Peripheral)** lymphoid organs. **1. Why Bone Marrow is correct:** Primary lymphoid organs are the sites where lymphocytes are generated and undergo antigen-independent maturation. In humans, these include the **Bone Marrow** and the **Thymus**. * **Bone Marrow:** The site of origin for all immune cells and the site of maturation for B-lymphocytes. * **Thymus:** The site where T-lymphocyte progenitors (from the bone marrow) migrate to mature and undergo selection. **2. Why other options are incorrect:** * **Lymph Nodes (Option B) and Spleen (Option C):** These are **Secondary Lymphoid Organs**. Their role is not to produce lymphocytes, but to provide a structured environment where mature lymphocytes can trap antigens and initiate an immune response. Other secondary organs include MALT (Mucosa-Associated Lymphoid Tissue) such as Peyer’s patches and tonsils. **NEET-PG High-Yield Pearls:** * **B-cell maturation:** Occurs in the **B**one marrow (B for Bone marrow). * **T-cell maturation:** Occurs in the **T**hymus (T for Thymus). * **Bursa of Fabricius:** In birds, this is the primary lymphoid organ for B-cells (the equivalent of bone marrow in humans). * **Involution:** The Thymus undergoes atrophy after puberty, whereas the bone marrow remains active throughout life. * **Major site of antibody production:** While maturation happens in the bone marrow, the **Spleen** and **Lymph nodes** are the major sites of active antibody synthesis during an infection.

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Complement System

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Cytokines and Chemokines

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Hypersensitivity Reactions

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Immunology — MCQs

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