Immunology Practice Questions

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

Class II MHC-restricted antigen presentation. **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.

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

2 light chains, 2 heavy chains. ### 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**.

Q: Which immunoglobulin is inactive at high temperatures?

IgE. **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.

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

Positive selection during development. ### 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.

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

B cells. **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).

Q: Where does antigen-antibody binding occur?

At the surface. **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.

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

Casoni's test. **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.

Q: Transfusion reactions are due to which type of hypersensitivity?

Antibody-mediated. **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).

Q: Which of the following is a central lymphoid organ?

Bone marrow. **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.

Question 1

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

Accepted Answer

Class II MHC-restricted antigen presentation

Answer

Class I MHC-restricted antigen presentation

Answer

Immunoglobulin gene rearrangement

Answer

T cell receptor alpha chain rearrangement

Question 2

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

Accepted Answer

2 light chains, 2 heavy chains

Answer

1 light chain, 1 heavy chain

Answer

2 heavy chains, 1 light chain

Answer

2 light chains, 1 heavy chain

Question 3

Which immunoglobulin is inactive at high temperatures?

Accepted Answer

IgE

Answer

IgG

Answer

IgA

Answer

IgM

Question 4

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

Accepted Answer

Positive selection during development

Answer

Class I MHC expression

Answer

Antigen-specific receptors

Answer

All of the above

Question 5

Plasma cells are derived from which of the following immune cells?

Accepted Answer

B cells

Answer

T cells

Answer

Macrophages

Answer

Neutrophils

Question 6

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

Accepted Answer

T-lymphocytes

Answer

B-Lymphocytes

Answer

Monocytes

Answer

Leukocytes

Question 7

Where does antigen-antibody binding occur?

Accepted Answer

At the surface

Answer

At the center

Answer

Inside the molecule

Answer

Anywhere in the structure

Question 8

Which of the following is an example of Type I hypersensitivity?

Accepted Answer

Casoni's test

Answer

Lepromin test

Answer

Tuberculin test

Answer

Arthus reaction

Question 9

Transfusion reactions are due to which type of hypersensitivity?

Accepted Answer

Antibody-mediated

Answer

Immediate

Answer

Immune complex-mediated

Answer

Delayed-type

Question 10

Which of the following is a central lymphoid organ?

Accepted Answer

Bone marrow

Answer

Lymph node

Answer

Spleen

Answer

All of the above

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?