Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 711: The secondary immune response is primarily mediated by which immunoglobulin?

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

Explanation: **Explanation:** The secondary immune response (anamnestic response) occurs upon re-exposure to a specific antigen. This response is characterized by a shorter lag phase, a more rapid rise in antibody titers, and higher peak levels compared to the primary response. **IgG** is the predominant immunoglobulin in the secondary response due to the presence of memory B cells that have already undergone class switching and affinity maturation. **Analysis of Options:** * **IgG (Correct):** It is the most abundant antibody in the blood and the primary mediator of long-term immunity and secondary responses. It provides more effective neutralization and opsonization. * **IgM (Incorrect):** This is the first antibody produced during the **primary immune response**. It has a high avidity (pentameric) but lower affinity and does not persist as long as IgG. * **IgA (Incorrect):** This is the primary immunoglobulin found in mucosal secretions (tears, saliva, colostrum) and provides local immunity at mucosal surfaces. * **IgF (Incorrect):** There is no such class of human immunoglobulin. (Note: IgE and IgD are the other two human classes). **High-Yield NEET-PG Pearls:** * **Primary vs. Secondary:** Primary response is slow and IgM-dominant; Secondary response is rapid, intense, and IgG-dominant. * **Placental Transfer:** IgG is the **only** immunoglobulin that crosses the placenta, providing passive immunity to the fetus. * **Avidity vs. Affinity:** IgM has high avidity (10 binding sites), while IgG has high affinity (better "fit" for the antigen). * **Diagnostic Marker:** Detection of IgM indicates a **recent/acute** infection, while IgG indicates a **past** infection or chronic state.

Question 712: Which cells must cooperate with B cells to produce antibodies against certain antigens?

A. Stem cells
B. Plasma cells
C. Macrophages
D. Helper T cells (Correct Answer)

Explanation: **Explanation:** The production of antibodies against **T-dependent (TD) antigens** (typically proteins) requires a complex interaction between B cells and **Helper T cells (CD4+).** 1. **Mechanism of Cooperation:** When a B cell encounters a protein antigen, it internalizes it, processes it, and presents the peptide fragments on its surface via **MHC Class II** molecules. 2. **T-B Interaction:** A specific Helper T cell (Th2 or Tfh subtype) recognizes this MHC-peptide complex through its T-cell receptor (TCR). 3. **Costimulation:** The interaction is solidified by the binding of **CD40** (on B cells) to **CD40L** (on T cells). This "second signal," along with cytokines like IL-4, triggers the B cell to undergo clonal expansion, **isotype switching** (from IgM to IgG/IgA/IgE), and affinity maturation. **Analysis of Incorrect Options:** * **A. Stem cells:** These are undifferentiated progenitors in the bone marrow. While they give rise to B and T cells, they do not participate in the active immune response to specific antigens. * **B. Plasma cells:** These are the end-stage, differentiated forms of B cells that actually secrete antibodies. They are the *result* of B cell activation, not the cells that cooperate to initiate it. * **C. Macrophages:** While macrophages are professional Antigen Presenting Cells (APCs) that activate T cells, the direct "cooperation" required for B-cell antibody class switching specifically involves Helper T cells. **High-Yield Facts for NEET-PG:** * **T-Independent Antigens:** Polysaccharides (e.g., Pneumococcal capsule) can activate B cells directly without T-cell help, but they produce primarily **IgM** and do not induce immunological memory. * **CD40-CD40L Interaction:** Deficiency in this interaction leads to **Hyper-IgM Syndrome**, where patients cannot switch from IgM to other antibody classes. * **Hapten-Carrier Effect:** Haptens are small molecules that are only immunogenic when conjugated to a protein "carrier," which allows for T-cell involvement.

Question 713: Human immunoglobulin A can be described by which of the following statements?

A. It is the predominant immunoglobulin in plasma.
B. It exists in four subclasses, of which IgA2 is predominant.
C. It can prevent attachment of microorganisms to epithelial cell membranes. (Correct Answer)
D. It is prominent early in the immune response and is the major class of antibody in cold agglutinins.

Explanation: **Explanation:** **Correct Answer: C. It can prevent attachment of microorganisms to epithelial cell membranes.** IgA is the primary mediator of **mucosal immunity**. In its secretory form (sIgA), it is found in tears, saliva, colostrum, and mucus of the respiratory, GI, and GU tracts. Its primary mechanism of action is **immune exclusion**: it binds to microbial surface antigens (adhesins), thereby preventing the attachment and colonization of pathogens (bacteria, viruses, and parasites) to epithelial surfaces. **Analysis of Incorrect Options:** * **Option A:** **IgG** is the predominant immunoglobulin in plasma (approx. 75–80%), whereas IgA is the second most abundant. However, IgA has the highest total daily production rate in the body due to its presence in secretions. * **Option B:** IgA has only **two subclasses** (IgA1 and IgA2). In serum, IgA1 is predominant; in secretions, the proportion of IgA2 is higher than in serum, but IgA1 still often prevails. * **Option C:** This describes **IgM**. IgM is the first antibody produced in a primary immune response and acts as the major antibody in cold agglutinin disease (Type II hypersensitivity). **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Secretory IgA is a **dimer** connected by a **J-chain** and contains a **Secretory Component** (derived from epithelial cells) which protects it from proteolytic enzymes in the gut. * **Selective IgA Deficiency:** The most common primary immunodeficiency. Patients are often asymptomatic but may present with recurrent sinopulmonary infections or celiac disease. **Crucial:** These patients are at risk of **anaphylaxis** during blood transfusions due to anti-IgA antibodies. * **Neisseria connection:** Pathogenic *Neisseria*, *H. influenzae*, and *S. pneumoniae* produce **IgA1 protease**, which cleaves the hinge region of IgA to bypass mucosal defenses.

Question 714: Which of the following is considered an immune privilege site?

A. Optic nerve
B. Seminiferous tubule (Correct Answer)
C. Area postrema
D. Spinal cord

Explanation: **Explanation:** **Immune privilege** is a physiological adaptation where certain tissues can tolerate the introduction of antigens without eliciting an inflammatory immune response. This mechanism protects vital organs from damage caused by the body's own inflammatory processes. **Why Option B is Correct:** The **Seminiferous tubules** (testis) are classic immune-privileged sites. This privilege is maintained by the **Blood-Testis Barrier (BTB)**, formed by tight junctions between Sertoli cells. This barrier prevents immune cells and antibodies from reaching developing germ cells, which express unique antigens (neo-antigens) that appear only at puberty. Without this protection, the immune system would recognize sperm as "foreign," leading to anti-sperm antibody production and infertility. **Why Other Options are Incorrect:** * **A & D (Optic nerve and Spinal cord):** While the **Brain, Anterior chamber of the eye, and Cornea** are immune-privileged, the optic nerve and spinal cord are part of the central nervous system parenchyma where immune surveillance still occurs, albeit in a restricted manner. They do not possess the same level of sequestered privilege as the interior of the eye or the testis. * **C (Area postrema):** This is one of the **Circumventricular Organs (CVOs)**. Unlike most of the brain, CVOs lack a blood-brain barrier to allow for the sensing of toxins in the blood (triggering vomiting). Therefore, it is highly accessible to the systemic circulation and is not immune-privileged. **High-Yield NEET-PG Pearls:** * **List of Immune Privileged Sites:** Eye (Anterior chamber/Cornea), Testis, Brain, Pregnant Uterus (Placenta), and Hair Follicles. * **Mechanism:** Physical barriers (tight junctions), low expression of MHC Class I molecules, and increased expression of immunosuppressive cytokines (TGF-beta). * **Clinical Correlation:** Trauma to one eye can lead to **Sympathetic Ophthalmia**, where sequestered antigens are released, causing the immune system to attack the healthy "uninjured" eye.

Question 715: Which of the following does not describe a documented mechanism of tumor-mediated immune evasion?

A. Down regulation of class I MHC
B. Secretion of TGF-b
C. Secretion of decoy molecules
D. Antigenic variation (Correct Answer)

Explanation: **Explanation:** The correct answer is **Antigenic variation**. While common in pathogens (like *Trypanosoma* or Influenza), antigenic variation is not a primary mechanism for tumor evasion. Tumors typically evade the immune system by suppressing the response or hiding their identity, rather than rapidly changing their surface proteins to stay ahead of antibodies. **Why the other options are documented mechanisms:** * **Downregulation of Class I MHC:** Tumors often stop expressing MHC-I molecules. Since CD8+ T-cells require MHC-I to recognize "non-self" tumor antigens, this makes the tumor "invisible" to cytotoxic T-lymphocytes. * **Secretion of TGF-β:** Tumors create an immunosuppressive microenvironment by secreting cytokines like **TGF-β** and **IL-10**. These inhibit T-cell activation and promote the recruitment of Regulatory T-cells (Tregs), which dampen the immune response. * **Secretion of Decoy Molecules:** Some tumors shed their surface antigens as soluble "decoy" molecules. These bind to circulating antibodies or T-cell receptors before they can reach the actual tumor cell, effectively neutralizing the immune attack. **High-Yield Clinical Pearls for NEET-PG:** * **Immune Checkpoints:** Tumors often upregulate **PD-L1**, which binds to PD-1 on T-cells, sending an inhibitory signal that "shuts down" the T-cell (T-cell exhaustion). * **Fas Ligand (FasL):** Some tumors express FasL to induce apoptosis (programmed cell death) in infiltrating T-cells that attempt to attack them. * **NK Cells:** While MHC-I downregulation helps evade T-cells, it theoretically makes tumors vulnerable to **Natural Killer (NK) cells**, which follow the "missing self" hypothesis. However, tumors often evolve additional mechanisms to inhibit NK cell receptors.

Question 716: Which of the following is a non-organ-specific (systemic) autoimmune disease?

A. Myasthenia gravis
B. Systemic lupus erythematosus (Correct Answer)
C. Hashimoto's thyroiditis
D. Pernicious anemia

Explanation: **Explanation:** Autoimmune diseases are broadly classified into two categories: **Organ-specific**, where the immune response is directed against antigens restricted to a single organ, and **Non-organ-specific (Systemic)**, where antibodies react with antigens widespread throughout the body. **Why Systemic Lupus Erythematosus (SLE) is correct:** SLE is the prototype of systemic autoimmune diseases. It is characterized by the production of **Antinuclear Antibodies (ANA)** and antibodies against various self-antigens (like dsDNA and Smith antigen) found in almost all nucleated cells. This leads to immune complex deposition and inflammation across multiple systems, including the skin, joints, kidneys, heart, and central nervous system. **Why the other options are incorrect:** * **Myasthenia Gravis:** Organ-specific; antibodies specifically target **acetylcholine receptors (AChR)** at the neuromuscular junction, affecting skeletal muscle activation. * **Hashimoto’s Thyroiditis:** Organ-specific; the immune attack (via anti-TPO and anti-thyroglobulin antibodies) is localized strictly to the **thyroid gland**, leading to hypothyroidism. * **Pernicious Anemia:** Organ-specific; antibodies target **gastric parietal cells** or **intrinsic factor**, localized to the stomach lining, resulting in Vitamin B12 deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **SLE Marker:** Anti-dsDNA is highly specific and correlates with disease activity (especially lupus nephritis). * **Drug-Induced Lupus:** Associated with **Anti-Histone antibodies** (Common drugs: Hydralazine, Procainamide, Isoniazid). * **Type of Hypersensitivity:** Most systemic autoimmune diseases (like SLE) involve **Type III hypersensitivity** (immune complexes), whereas many organ-specific diseases (like Hashimoto's) involve **Type II or Type IV**.

Question 717: Which of the following principally mediates delayed type hypersensitivity?

A. Cytotoxic T cells, TH1 cells, and macrophages (Correct Answer)
B. IgE, basophils, and mast cells
C. IgG and complement
D. IgG, IgM, neutrophils, and macrophages

Explanation: ### Explanation **Delayed-Type Hypersensitivity (DTH)**, also known as **Type IV Hypersensitivity**, is the only hypersensitivity reaction that is **cell-mediated** rather than antibody-mediated. **Why Option A is Correct:** The process occurs in two phases: 1. **Sensitization Phase:** CD4+ T cells (specifically **TH1 cells**) recognize antigens presented by APCs and undergo clonal expansion. 2. **Effector Phase:** Upon re-exposure, TH1 cells release cytokines (IFN-γ, TNF-β, and IL-2). **IFN-γ** is the most critical cytokine as it activates **macrophages**, enhancing their phagocytic and microbicidal activity. In some cases (like graft rejection or contact dermatitis), **CD8+ Cytotoxic T cells** directly destroy target cells. Thus, TH1 cells, macrophages, and Cytotoxic T cells are the primary mediators. **Why Other Options are Incorrect:** * **Option B:** Describes **Type I (Immediate) Hypersensitivity**, mediated by IgE binding to mast cells and basophils (e.g., Anaphylaxis, Asthma). * **Option C:** Describes **Type II (Cytotoxic) Hypersensitivity**, where IgG/IgM antibodies bind to cell surface antigens, leading to complement activation (e.g., Rh incompatibility, Myasthenia Gravis). * **Option D:** Describes **Type III (Immune-Complex) Hypersensitivity**, involving circulating Ag-Ab complexes that deposit in tissues, attracting neutrophils (e.g., SLE, Post-streptococcal glomerulonephritis). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Type IV is "**D**elayed" (starts after 24–48 hours). * **Classic Examples:** Mantoux Test (Tuberculin reaction), Contact Dermatitis (Poison ivy, Nickel), Lepromin test, and Graft rejection. * **Granuloma Formation:** Chronic Type IV reactions lead to granulomas (seen in Tuberculosis and Sarcoidosis) due to persistent macrophage activation by TH1 cells.

Question 718: Which of the following does not secrete interleukin 1 alpha?

A. Lymphocyte (Correct Answer)
B. Monocyte
C. Macrophage
D. Neutrophil

Explanation: **Explanation:** Interleukin-1 (IL-1) is a key pro-inflammatory cytokine that exists in two forms: **IL-1α** and **IL-1β**. The primary source of IL-1 is the **mononuclear phagocytic system**. **Why Lymphocytes are the correct answer:** Lymphocytes (T-cells and B-cells) are primarily responsible for secreting cytokines like IL-2, IL-4, and Interferon-gamma. While they are the *targets* of IL-1 (which acts as a costimulator for T-cell activation), they do not synthesize or secrete IL-1α. IL-1 is an "innate" cytokine, whereas lymphocytes are mediators of "adaptive" immunity. **Analysis of incorrect options:** * **Monocytes & Macrophages:** These are the **principal sources** of IL-1. Upon activation by bacterial endotoxins (LPS) or TNF, they produce large amounts of IL-1 to initiate the inflammatory cascade. * **Neutrophils:** These are professional phagocytes of the innate immune system and are capable of secreting IL-1α during acute inflammatory responses. **High-Yield NEET-PG Pearls:** * **IL-1α vs. IL-1β:** IL-1α usually remains cell-associated (membrane-bound), acting as an autocrine or paracrine messenger, while IL-1β is the secreted form found in circulation. * **The "Endogenous Pyrogen":** IL-1 acts on the anterior hypothalamus to increase prostaglandin E2 (PGE2) production, resulting in **fever**. * **Acute Phase Response:** IL-1 stimulates the liver to produce acute-phase proteins (e.g., CRP) and induces "leukocytosis" by stimulating the bone marrow. * **Synergy:** IL-1 and TNF-α often work together to mediate systemic inflammatory response syndrome (SIRS).

Question 719: Which of the following is NOT a secondary lymphoid organ?

A. Spleen
B. Lymph node
C. Bone marrow (Correct Answer)
D. MALT

Explanation: **Explanation:** Lymphoid organs are classified into two categories based on their function in lymphocyte development: **Primary** and **Secondary**. **1. Why Bone Marrow is the Correct Answer:** Bone marrow is a **Primary (Central) Lymphoid Organ**. These are the sites where lymphocytes are produced and undergo antigen-independent maturation. In humans, B-cells mature in the bone marrow, while T-cells migrate to the Thymus to mature. Since the question asks for which is *NOT* a secondary organ, Bone Marrow is the correct choice. **2. Analysis of Incorrect Options (Secondary Lymphoid Organs):** Secondary (Peripheral) lymphoid organs are sites where mature lymphocytes reside, encounter antigens, and initiate an immune response. * **Spleen (Option A):** Filters blood-borne pathogens and is the site of immune responses to systemic infections. * **Lymph Nodes (Option B):** Filter lymph and are the primary sites where B and T cells encounter antigens drained from tissues. * **MALT (Option D):** Mucosa-Associated Lymphoid Tissue (including Peyer’s patches, tonsils, and appendix) protects mucosal surfaces. **Clinical Pearls for NEET-PG:** * **Primary Lymphoid Organs:** Bone Marrow and Thymus (Site of *Maturation*). * **Secondary Lymphoid Organs:** Spleen, Lymph nodes, MALT, Tonsils (Site of *Proliferation and Antigen Interaction*). * **Thymic Involution:** The thymus reaches maximum size at puberty and then undergoes atrophy (replaced by fat), a process often tested in pathology. * **Bursa of Fabricius:** The primary lymphoid organ for B-cell maturation in birds (the "B" in B-cell originally stood for Bursa).

Question 720: Which of the following is NOT a rapid serological diagnostic test?

A. Latex agglutination
B. Spectrophotometry (Correct Answer)
C. Gel electrophoresis
D. Radioimmunoassay

Explanation: **Explanation:** The core of this question lies in distinguishing between **serological diagnostic tests** (which detect antigen-antibody interactions) and **analytical laboratory techniques**. **Why Spectrophotometry is the correct answer:** Spectrophotometry is a physical method used to measure the intensity of light absorbed by a chemical substance at a specific wavelength. While it is used as a *detection tool* within certain assays (like reading the final color change in an ELISA), it is not a serological test in itself. It does not involve the fundamental principle of immunology: the specific binding of an antigen to an antibody. **Analysis of incorrect options:** * **Latex Agglutination:** A classic rapid serological test where antibodies (or antigens) are coated onto latex beads. When the corresponding antigen (or antibody) is present in the patient's serum, visible clumping occurs within minutes (e.g., ASO titer, CRP, RA factor). * **Gel Electrophoresis:** In immunology, this is used in **Immunoelectrophoresis** or **Western Blotting**. It separates proteins based on size/charge before they are reacted with antibodies. It is a definitive serological diagnostic tool (e.g., confirmatory test for HIV). * **Radioimmunoassay (RIA):** A highly sensitive serological technique that uses radiolabeled antigens or antibodies to detect minute concentrations of substances (e.g., HBsAg, hormones) in the serum. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Serological Tests:** ELISA, RIA, Immunofluorescence (no visible precipitation/agglutination). * **Secondary Serological Tests:** Precipitation, Agglutination, Complement Fixation Test (visible reaction). * **Prozone Phenomenon:** False negative results in agglutination tests due to antibody excess; solved by serial dilution of serum. * **Coombs Test:** A specialized agglutination test used to detect non-agglutinating (incomplete) antibodies in Rh incompatibility.

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