Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 551: The Oudin procedure is a method of immunodiffusion that utilizes which of the following techniques?

A. Single diffusion in one dimension (Correct Answer)
B. Double diffusion in one dimension
C. Single diffusion in two dimensions
D. Double diffusion in two dimensions

Explanation: ### Explanation **Concept Overview:** Immunodiffusion techniques are categorized based on two factors: the number of reactants moving (Single vs. Double) and the direction of movement (One dimension vs. Two dimensions). **Why Option A is Correct:** The **Oudin procedure** is the classic example of **Single diffusion in one dimension**. * **Single diffusion:** Only one reactant (the antigen) moves, while the other (the antibody) remains incorporated at a fixed concentration within the agar gel. * **One dimension:** The reaction occurs in a narrow glass tube. The antibody-containing agar is layered at the bottom, and the antigen solution is poured on top. The antigen diffuses vertically downward into the agar, forming a band of precipitation where the zone of equivalence is reached. **Analysis of Incorrect Options:** * **Option B (Double diffusion in one dimension):** Known as the **Oakley-Fulthorpe procedure**. Here, both antigen and antibody diffuse toward each other through a central column of plain agar in a tube. * **Option C (Single diffusion in two dimensions):** Known as **Radial Immunodiffusion (Mancini technique)**. Antibody is in the agar plate, and antigen is placed in a well, diffusing outward in all directions to form a precipitin ring. * **Option D (Double diffusion in two dimensions):** Known as the **Ouchterlony technique**. Both antigen and antibody are placed in separate wells in an agar plate and diffuse toward each other. **NEET-PG Clinical Pearls:** * **Mancini Technique (Radial ID):** High-yield for quantifying Immunoglobulins (IgG, IgM, IgA) and complement components (C3, C4). The diameter of the ring is proportional to the antigen concentration. * **Ouchterlony Technique:** Used for comparing antigens (identifying patterns of identity, partial identity, or non-identity) and detecting antibodies in autoimmune diseases (e.g., Anti-nuclear antibodies). * **Elek’s Test:** A specific application of double diffusion in two dimensions used to detect the toxigenicity of *Corynebacterium diphtheriae*.

Question 552: The peptide binding site on Class I MHC molecules is located in which region?

A. The proximal domain of the alpha subunit
B. The distal domain of the alpha subunit (Correct Answer)
C. The proximal domains of the alpha and beta subunits
D. The distal domains of the alpha and beta subunits

Explanation: ### Explanation **1. Why Option B is Correct:** The Class I MHC molecule is a heterodimer consisting of a heavy **$\alpha$ chain** (encoded by HLA-A, B, or C genes) and a non-covalently linked **$\beta_2$-microglobulin**. The $\alpha$ chain is folded into three domains: $\alpha_1, \alpha_2,$ and $\alpha_3$. The peptide-binding groove (cleft) is formed by the interaction of the **$\alpha_1$ and $\alpha_2$ domains**. These are the **distal domains** (farthest from the cell membrane). This groove typically accommodates short peptides of 8–10 amino acids for presentation to CD8+ T cells. **2. Why Other Options are Incorrect:** * **Option A:** The proximal domain of the $\alpha$ subunit is the **$\alpha_3$ domain**. This region is highly conserved and serves as the binding site for the **CD8 coreceptor** of T cells, not for the peptide itself. * **Option C & D:** These options describe the structure of **Class II MHC** molecules. In Class II MHC, the peptide-binding groove is formed by the interaction of two different chains (the distal $\alpha_1$ and $\beta_1$ domains). In Class I MHC, the $\beta$ subunit ($\beta_2$-microglobulin) does not participate in forming the binding cleft. **3. High-Yield Clinical Pearls for NEET-PG:** * **MHC Class I:** Present on all nucleated cells (absent on RBCs). Presents **endogenous** antigens (e.g., viral or tumor proteins) to **CD8+ Cytotoxic T cells**. * **MHC Class II:** Present only on **Antigen-Presenting Cells (APCs)** like dendritic cells, macrophages, and B cells. Presents **exogenous** antigens to **CD4+ Helper T cells**. * **Rule of 8:** MHC I × CD8 = 8; MHC II × CD4 = 8. * **$\beta_2$-microglobulin:** It is encoded on **Chromosome 15**, while the MHC gene complex is on **Chromosome 6**. It is essential for the surface expression of Class I MHC.

Question 553: What is the basic structure of an antibody?

A. A single peptide chain
B. Two peptide chains
C. Non-sulfur amino acids
D. Two long and two short peptide chains (Correct Answer)

Explanation: **Explanation:** The basic structure of an antibody (Immunoglobulin) is a **Y-shaped** molecule composed of four polypeptide chains: **two identical heavy (long) chains** and **two identical light (short) chains**. These chains are held together by disulfide bonds, forming a monomeric unit. 1. **Why Option D is correct:** Each antibody unit consists of two heavy chains (approx. 50-70 kDa) and two light chains (approx. 25 kDa). The heavy chains determine the **class (Isotype)** of the antibody (IgG, IgM, IgA, IgD, IgE), while the light chains are either **Kappa (κ) or Lambda (λ)**. The "V" portion of the Y-shape contains the variable regions that bind to specific antigens. 2. **Why other options are incorrect:** * **Option A & B:** A single or double chain cannot form the complex functional domains (Fab and Fc) required for antigen binding and biological activity. * **Option C:** This is incorrect because **disulfide bonds** (formed by sulfur-containing amino acid Cysteine) are essential for stabilizing the inter-chain and intra-chain structure of the immunoglobulin. **High-Yield Clinical Pearls for NEET-PG:** * **Fab fragment:** The "Fragment Antigen Binding" portion (contains both H and L chains). * **Fc fragment:** The "Fragment Crystallizable" portion (contains only H chains); it determines the biological properties like placental transfer (IgG) or mast cell binding (IgE). * **Hinge Region:** The flexible segment of the heavy chain that allows the Fab arms to move; it is rich in **proline** and **cysteine**. * **Papain Digestion:** Cleaves the antibody into **two Fab** fragments and **one Fc** fragment. * **Pepsin Digestion:** Cleaves it into one **F(ab')2** fragment and degraded Fc fragments.

Question 554: Which bacteria's antigen cross-reacts with Proteus antigen?

A. Klebsiella
B. Rickettsiae and Klebsiella (Correct Answer)
C. Chlymydiae
D. E. coli

Explanation: The correct answer is **B. Rickettsiae and Klebsiella**. ### **Explanation** This question refers to the phenomenon of **antigenic cross-reactivity**, where antibodies produced against one organism react with the antigens of a different, unrelated organism due to shared epitopes. 1. **Rickettsiae and Proteus (Weil-Felix Reaction):** This is a classic example of heterophile antibody reaction. Certain strains of *Proteus vulgaris* (OX-19, OX-2) and *Proteus mirabilis* (OX-K) share alkali-stable carbohydrate antigens with various species of *Rickettsia*. In the **Weil-Felix test**, a patient's serum is tested for agglutination against these Proteus antigens to diagnose Rickettsial diseases (e.g., Epidemic typhus, Scrub typhus). 2. **Klebsiella and Proteus:** Cross-reactivity has been documented between *Klebsiella pneumoniae* and *Proteus* species. Specifically, studies in patients with **Ankylosing Spondylitis** have shown cross-reactivity involving *Klebsiella* antigens, *Proteus* antigens, and the HLA-B27 molecule (molecular mimicry). ### **Why other options are incorrect:** * **A & D (Klebsiella/E. coli):** While *E. coli* and *Klebsiella* are both Enterobacteriaceae, they do not share the specific diagnostic cross-reactivity with *Proteus* that is characteristic of *Rickettsia*. * **C (Chlamydiae):** Chlamydiae are obligate intracellular bacteria but do not share common antigens with *Proteus*. Diagnosis is usually via NAAT or Giemsa stain. ### **High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Patterns:** * **Scrub Typhus (*O. tsutsugamushi*):** Positive for **OX-K** only. * **Epidemic/Endemic Typhus:** Positive for **OX-19**. * **Rocky Mountain Spotted Fever:** Positive for **OX-19 and OX-2**. * **Q Fever:** Weil-Felix test is **Negative**. * **Molecular Mimicry:** Remember the link between *Klebsiella* and Ankylosing Spondylitis (HLA-B27) as a frequent "match the following" topic.

Question 555: Which category of hypersensitivity involves complement activation?

A. Type II
B. Type III
C. Type II and Type III (Correct Answer)
D. Type IV and Type II

Explanation: **Explanation** Hypersensitivity reactions are classified by the Gell and Coombs system based on the underlying immune mechanism. Complement activation is a hallmark of both **Type II** and **Type III** hypersensitivity. * **Type II (Cytotoxic):** Antibodies (IgG or IgM) bind to antigens on the surface of specific cells or tissues. This antigen-antibody complex activates the **classical complement pathway**, leading to the formation of the Membrane Attack Complex (MAC) and cell lysis (e.g., Autoimmune Hemolytic Anemia). * **Type III (Immune-Complex):** Soluble antigens bind to antibodies, forming circulating immune complexes. These complexes deposit in tissues (like blood vessel walls or glomeruli) and trigger **complement activation**. This recruits neutrophils, leading to tissue damage (e.g., SLE, Post-streptococcal Glomerulonephritis). **Why other options are incorrect:** * **Type I:** Mediated by IgE binding to mast cells; involves histamine release, not complement. * **Type IV:** A delayed-type hypersensitivity mediated by T-cells (Th1, Th17, and CD8+), not antibodies or complement. **NEET-PG High-Yield Pearls:** * **Type II** is "Tissue-specific" (Antigen is fixed). * **Type III** is "Systemic" (Antigen is soluble/circulating). * **Complement levels (C3, C4):** Often decreased in Type III reactions (like SLE flares) due to massive consumption during the inflammatory process. * **Mnemonic (ACID):** **A**naphylactic (I), **C**ytotoxic (II), **I**mmune-Complex (III), **D**elayed (IV).

Question 556: Chediak-Higashi syndrome is characterized by which of the following?

A. Defect in phagocytosis (Correct Answer)
B. Neutropenia
C. Agammaglobulinemia
D. IgA deficiency

Explanation: **Explanation:** **Chediak-Higashi Syndrome (CHS)** is a rare autosomal recessive disorder caused by a mutation in the **LYST (Lysosomal Trafficking Regulator) gene**. This mutation leads to a defect in microtubule polymerization, which prevents the fusion of phagosomes with lysosomes. 1. **Why Option A is correct:** The hallmark of CHS is the failure of **phagolysosome formation**. While neutrophils can ingest bacteria, they cannot kill them because the digestive enzymes in lysosomes cannot reach the phagosome. This represents a functional **defect in phagocytosis** (specifically the intracellular killing phase). Microscopically, this is visualized as **giant peroxidase-positive granules** in neutrophils, representing fused, dysfunctional lysosomes. 2. **Why other options are incorrect:** * **B. Neutropenia:** While mild neutropenia can occur due to ineffective granulopoiesis, the primary functional pathology defining the disease is the phagocytic defect. * **C & D. Agammaglobulinemia/IgA deficiency:** CHS is a defect of the innate immune system (phagocytes) and natural killer (NK) cells, not a primary B-cell or antibody deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Tetrad:** Partial oculocutaneous **albinism**, recurrent pyogenic infections (usually *S. aureus*), progressive neurological abnormalities, and mild coagulation defects (bleeding diathesis). * **Diagnosis:** Peripheral blood smear showing **giant lysosomal granules** in neutrophils and melanocytes. * **NK Cell Defect:** Patients also have impaired Natural Killer cell cytotoxicity, increasing susceptibility to viral infections and the "accelerated phase" (hemophagocytic lymphohistiocytosis).

Question 557: Recurrent Neisseria infections are predisposed by which of the following?

A. Late complement component deficiency (Correct Answer)
B. Properdin deficiency
C. Early complement component deficiency
D. C1 esterase deficiency

Explanation: ### Explanation **Correct Answer: A. Late complement component deficiency** **The Medical Concept:** The complement system is a crucial arm of innate immunity. The **late complement components (C5, C6, C7, C8, and C9)** assemble to form the **Membrane Attack Complex (MAC)**. The MAC is essential for the lysis of Gram-negative bacteria, particularly *Neisseria* species (*N. meningitidis* and *N. gonorrhoeae*), which have thin peptidoglycan layers. Individuals with deficiencies in these components cannot form the MAC, leaving them uniquely susceptible to recurrent, disseminated Neisserial infections. **Analysis of Incorrect Options:** * **B. Properdin deficiency:** Properdin stabilizes the alternative pathway C3 convertase. While its deficiency can increase susceptibility to *Neisseria*, it is much rarer than late component deficiencies and is X-linked. Late component deficiency remains the classic, most high-yield association. * **C. Early complement component deficiency (C1, C4, C2):** These are primarily involved in the classical pathway and the clearance of immune complexes. Deficiencies here typically predispose patients to **Systemic Lupus Erythematosus (SLE)** and pyogenic infections (e.g., *S. pneumoniae*), rather than specific *Neisseria* susceptibility. * **D. C1 esterase deficiency:** This leads to **Hereditary Angioedema** due to the overproduction of bradykinin. It is not associated with an increased risk of bacterial infections. **High-Yield Clinical Pearls for NEET-PG:** * **C3 deficiency:** The most severe complement deficiency; predisposes to recurrent pyogenic infections and Type III hypersensitivity reactions. * **CH50 Assay:** Used to screen for classical pathway and MAC deficiencies. * **Mnemonic:** "Late is for Lysis" — C5-C9 are needed for lysis of *Neisseria*. * **Vaccination:** Patients with terminal complement deficiencies must receive the meningococcal vaccine.

Question 558: Where are B cells processed?

A. Bone marrow (Correct Answer)
B. Liver
C. Spleen
D. Lymph nodes

Explanation: **Explanation:** The development of B cells occurs in two distinct phases: **antigen-independent** (maturation) and **antigen-dependent** (activation). **Why Bone Marrow is correct:** In humans, the **Bone Marrow** is the primary lymphoid organ responsible for the generation and maturation of B cells. Here, hematopoietic stem cells undergo gene rearrangement to produce a functional B-cell receptor (BCR). This process is known as "processing" or "central maturation." Once B cells express IgM on their surface (immature B cells), they leave the bone marrow to seed peripheral lymphoid organs. (Note: In birds, this occurs in the *Bursa of Fabricius*, which is the origin of the letter "B" in B cells). **Why other options are incorrect:** * **Liver:** This is the primary site of B-cell production during **fetal life** (extramedullary hematopoiesis). However, in adults, this function shifts entirely to the bone marrow. * **Spleen & Lymph Nodes:** These are **secondary lymphoid organs**. They are sites where mature B cells encounter antigens, undergo activation, and differentiate into plasma cells. They do not "process" or create the B cells initially. **High-Yield Clinical Pearls for NEET-PG:** * **T-cell Processing:** Occurs in the **Thymus**. * **Negative Selection:** The process in the bone marrow where B cells that react strongly to "self-antigens" are eliminated or undergo **receptor editing** to prevent autoimmunity. * **Bruton’s Agammaglobulinemia:** A clinical condition where B-cell maturation is arrested at the pre-B stage due to a mutation in the *Btk* gene, leading to a lack of mature B cells and antibodies.

Question 559: What is the other name for sex-linked graft rejection?

A. Eichwald Silmser effect (Correct Answer)
B. Schultz-Dale phenomenon
C. Shwartzman reaction
D. Theobald Smith phenomenon

Explanation: ### Explanation **Correct Option: A. Eichwald Silmser effect** The **Eichwald-Silmser effect** refers to the rejection of male skin grafts by female recipients of the same highly inbred (isogenic) strain. This occurs because males possess a Y chromosome that encodes for specific minor histocompatibility antigens, known as **H-Y antigens**. Since females lack the Y chromosome, their immune systems recognize these H-Y antigens as "foreign," leading to graft rejection. Conversely, male recipients do not reject female grafts because both sexes possess X chromosomes. **Analysis of Incorrect Options:** * **B. Schultz-Dale phenomenon:** This is an *in vitro* method used to demonstrate immediate (Type I) hypersensitivity. It involves the contraction of isolated smooth muscle (e.g., guinea pig ileum) when exposed to a specific antigen to which the animal was previously sensitized. * **C. Shwartzman reaction:** This is a phenomenon of non-specific tissue necrosis. It is not an immune response but rather a localized or systemic inflammatory reaction (often involving DIC) triggered by two sequential injections of bacterial endotoxins (LPS). * **D. Theobald Smith phenomenon:** This is an archaic term for **experimental anaphylaxis**. It describes the fatal reaction seen in guinea pigs when they are injected with a second, "challenging" dose of an antigen (like horse serum) after an initial sensitizing dose. **High-Yield Pearls for NEET-PG:** * **H-Y Antigen:** A minor histocompatibility antigen; it is a protein encoded by the *KDM5D* gene on the Y chromosome. * **Laws of Transplantation:** Grafts between identical twins (isografts) are usually accepted, but the Eichwald-Silmser effect is the notable exception to this rule based on sex. * **Hyperacute Rejection:** Occurs within minutes due to pre-formed antibodies (Type II hypersensitivity). * **Acute Rejection:** Occurs within days to weeks, primarily mediated by T-cells (Type IV hypersensitivity).

Question 560: Which of the following antibodies acts as an opsonin?

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

Explanation: **Explanation:** **1. Why IgG is correct:** Opsonization is the process by which a pathogen is marked for ingestion and destruction by a phagocyte. **IgG** is the only class of antibody that acts as an opsonin. This occurs because the **Fab portion** of the IgG molecule binds to the specific antigen on the pathogen, while the **Fc portion** binds to the **Fcγ receptors (FcγR)** expressed on the surface of phagocytic cells (neutrophils and macrophages). This "bridge" significantly enhances the efficiency of phagocytosis. Specifically, **IgG1 and IgG3** are the most potent opsonins. **2. Why the other options are incorrect:** * **IgM:** While IgM is the most efficient antibody for **complement activation** (via the classical pathway), it does not act as a direct opsonin because phagocytes lack specific receptors for the Fc portion of IgM. * **IgE:** This antibody is primarily involved in **Type I Hypersensitivity** reactions and defense against helminthic parasites by binding to mast cells and basophils. * **IgA:** Found predominantly in secretions (mucosal immunity), IgA prevents the attachment of pathogens to mucosal surfaces (agglutination) but does not function as a classical opsonin for systemic phagocytosis. **Clinical Pearls for NEET-PG:** * **Major Opsonins:** The two most important opsonins in the body are **IgG** and the complement fragment **C3b**. * **IgG Characteristics:** It is the most abundant antibody in serum, the only one to cross the **placenta**, and is responsible for the secondary (anamnestic) immune response. * **Mnemonic:** "Ig**G** **G**rooms the bacteria for eating."

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Immunology — MCQs

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