Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 391: The direct Coombs test detects which of the following?

A. Antigens in serum
B. Antibodies on the RBC surface
C. Antigens on the RBC surface
D. Antibodies in serum (Correct Answer)

Explanation: ### Explanation The **Direct Coombs Test (Direct Antiglobulin Test - DAT)** is designed to detect **antibodies (IgG) or complement proteins** that are already bound to the **surface of Red Blood Cells (RBCs)** in vivo. **Why the correct answer is B (Note: Correction to provided key):** There appears to be a discrepancy in the provided key. In medical immunology: * **Direct Coombs Test:** Detects antibodies/complement **on the RBC surface**. * **Indirect Coombs Test:** Detects free, unbound antibodies **in the serum**. If the goal is to identify the mechanism of the *Direct* test, the target is the sensitized RBC. #### Analysis of Options: * **Option B (Correct Mechanism):** In conditions like Autoimmune Hemolytic Anemia (AIHA), antibodies coat the patient's RBCs. Adding "Coombs Reagent" (antihuman globulin) causes these cells to agglutinate, confirming the presence of surface-bound antibodies. * **Option D (Incorrect for Direct):** Detecting antibodies in the serum is the purpose of the **Indirect Coombs Test**, used for prenatal screening (Rh incompatibility) and cross-matching. * **Options A & C (Incorrect):** Coombs tests are designed to detect antibodies (immunoglobulins), not the antigens themselves. #### High-Yield Clinical Pearls for NEET-PG: 1. **Direct Coombs Test Applications:** Hemolytic Disease of the Newborn (HDN), Drug-induced hemolytic anemia, and Transfusion reactions. 2. **Indirect Coombs Test Applications:** Checking maternal blood for anti-Rh antibodies and pre-transfusion compatibility. 3. **Coombs Reagent:** It is **Antihuman Globulin (AHG)**, usually produced by immunizing rabbits against human IgG/complement. 4. **Key Distinction:** Direct = "In the body" (RBCs already coated); Indirect = "In the lab" (Serum tested against reagent RBCs).

Question 392: Memory cells are:

A. Basophils
B. Eosinophils
C. Lymphocytes (Correct Answer)
D. Neutrophils

Explanation: **Explanation:** **Correct Answer: C. Lymphocytes** The cornerstone of the adaptive immune system is its ability to "remember" previous encounters with specific pathogens. This immunological memory is mediated by **Lymphocytes**, specifically **B-lymphocytes** and **T-lymphocytes**. When a naive lymphocyte encounters its specific antigen, it undergoes clonal expansion and differentiates into effector cells (which fight the current infection) and **memory cells**. Memory cells are long-lived and remain in a quiescent state in the lymphoid tissues. Upon re-exposure to the same antigen, these cells mount a faster, more robust, and highly specific secondary immune response. **Why other options are incorrect:** * **A, B, and D (Basophils, Eosinophils, Neutrophils):** These are all types of **Granulocytes** and are primary components of the **Innate Immune System**. Unlike the adaptive system, the innate system is non-specific and lacks immunological memory. * **Neutrophils** are the first responders to acute bacterial inflammation. * **Eosinophils** are primarily involved in parasitic infections and allergic reactions. * **Basophils** release histamine and are involved in Type I hypersensitivity. **High-Yield Clinical Pearls for NEET-PG:** * **B-Memory Cells:** Responsible for the "Anamnestic response," characterized by a shorter lag phase and a rapid rise in **IgG** titers. * **T-Memory Cells:** Can be divided into Central Memory (Tcm) and Effector Memory (Tem) cells based on their homing receptors (e.g., CCR7). * **Vaccination Principle:** The entire concept of active immunization is based on the induction of memory lymphocytes without causing the disease. * **Markers:** CD45RO is a common marker for memory T-cells in humans.

Question 393: What is the functional surface molecule on T-lymphocytes that facilitates antigen recognition by Major Histocompatibility Complex (MHC) molecules?

A. T4
B. T11
C. T3
D. T8 (Correct Answer)

Explanation: **Explanation:** The question focuses on the interaction between T-lymphocytes and the Major Histocompatibility Complex (MHC). T-lymphocytes recognize antigens only when presented by MHC molecules. This process is facilitated by specific co-receptors: **CD4** (T4) and **CD8** (T8). **Why T8 is the correct answer:** The **T8 (CD8)** molecule is a transmembrane glycoprotein found on cytotoxic T-cells. It acts as a co-receptor that specifically binds to the non-polymorphic region of **MHC Class I** molecules. This binding stabilizes the interaction between the T-cell receptor (TCR) and the antigen-MHC complex, facilitating signal transduction and subsequent immune response. **Analysis of Incorrect Options:** * **A. T4 (CD4):** While T4 also facilitates MHC recognition, it specifically binds to **MHC Class II** molecules on helper T-cells. In many exam contexts, if the question implies MHC Class I or general cytotoxic function, T8 is the specific focus. * **B. T11 (CD2):** This is an adhesion molecule found on T-cells and NK cells. Its primary role is binding to LFA-3 on antigen-presenting cells to stabilize cell-to-cell contact, but it does not directly facilitate MHC-specific antigen recognition. * **C. T3 (CD3):** This is a complex of proteins associated with the T-cell receptor (TCR). While essential for **signal transduction** once an antigen is recognized, it does not "facilitate recognition" by binding to the MHC molecule itself. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction Rule:** Remember the **"Rule of 8"**: CD4 × MHC 2 = 8; CD8 × MHC 1 = 8. * **CD3** is the definitive marker for all T-lymphocytes. * **CD2 (T11)** is used in the "E-rosette" test to identify T-cells (historical diagnostic significance).

Question 394: Recurrent facial, oropharyngeal, or laryngeal edema in a patient is associated with low C4, normal C3, and normal Factor B. The pathogenesis is most likely due to:

A. Immune complex disease
B. C1 esterase inhibitor deficiency (Correct Answer)
C. Hereditary deficiency of C2
D. Classical pathway activation by IgM

Explanation: ### Explanation The clinical presentation of recurrent facial, oropharyngeal, or laryngeal edema without urticaria is characteristic of **Hereditary Angioedema (HAE)**, which is caused by a deficiency or dysfunction of the **C1 esterase inhibitor (C1-INH)**. **Why the correct answer is right:** C1-INH normally inhibits the C1 complex and the kallikrein-kinin system. In its absence, there is uncontrolled activation of C1, leading to the continuous consumption of **C4** and **C2**. This results in **persistently low C4 levels**, even between attacks. Because C1-INH also regulates the conversion of kininogen to **bradykinin**, its deficiency leads to excessive bradykinin, causing increased vascular permeability and edema. **C3 levels remain normal** because the activation does not proceed to the C3 convertase level in a significant way, and **Factor B** is normal because the alternative pathway is not involved. **Why incorrect options are wrong:** * **Option A & D:** Immune complex diseases (e.g., SLE) or IgM-mediated activation involve the consumption of the entire classical pathway. This would typically result in **low C3** in addition to low C4. * **Option C:** While C2 deficiency is a complement disorder, it does not cause angioedema. It usually presents with SLE-like features or recurrent pyogenic infections. **NEET-PG High-Yield Pearls:** * **Screening Test of Choice:** Serum **C4 level** (it is almost always low in HAE). * **Confirmatory Test:** C1-INH antigenic levels or functional assay. * **Key Mediator:** Bradykinin (NOT histamine; hence, HAE does not respond to antihistamines or steroids). * **Management:** Acute attacks are treated with **C1-INH concentrate** or **Icatibant** (bradykinin B2 receptor antagonist). Prophylaxis includes **Danazol** (stanozolol) or Tranexamic acid.

Question 395: CRP is an acute-phase protein in inflammatory processes. What is the expansion of 'C' in CRP?

A. Concanavalin A
B. Chondroitin sulfate in series with ARP, BRP
C. Capsular polysaccharide of pneumococcus (Correct Answer)
D. Cellular

Explanation: **Explanation:** **C-Reactive Protein (CRP)** is a classic acute-phase reactant synthesized by the liver in response to inflammatory cytokines, primarily **Interleukin-6 (IL-6)**. **Why Option C is Correct:** The protein was first discovered by Tillett and Francis in 1930. They observed that the serum of patients with pneumonia contained a substance that reacted and formed a precipitate with the **'C' polysaccharide** (a phosphocholine-containing carbohydrate) found in the **capsule/cell wall of *Streptococcus pneumoniae***. Thus, the 'C' in CRP stands for the **Capsular polysaccharide of pneumococcus**. **Analysis of Incorrect Options:** * **Option A (Concanavalin A):** This is a lectin (plant protein) derived from the jack bean, used in labs as a lymphocyte mitogen; it has no relation to the naming of CRP. * **Option B (Chondroitin sulfate):** This is a glycosaminoglycan found in cartilage; the "series with ARP, BRP" is a distractor with no medical basis. * **Option D (Cellular):** CRP is a plasma protein (humoral component), not a cellular component. **High-Yield Clinical Pearls for NEET-PG:** * **Function:** CRP acts as an **opsonin**, activating the classical complement pathway by binding to C1q. * **Kinetics:** It rises rapidly (within 6–12 hours) and has a short half-life (19 hours), making it a sensitive indicator of **current** inflammation or infection. * **hs-CRP (high-sensitivity CRP):** Used as a biomarker for chronic low-grade inflammation and is a significant predictor of **Cardiovascular Risk**. * **Diagnostic Value:** It is a non-specific marker; it indicates that inflammation is present but does not identify the specific cause.

Question 396: What type of HLA molecules are primarily involved in graft rejection?

A. MHC Class II
B. MHC Class I (Correct Answer)
C. MHC Class III
D. MHC Class IV

Explanation: ### Explanation **1. Why MHC Class I is the Correct Answer:** Graft rejection is primarily a cell-mediated immune response. **MHC Class I molecules (HLA-A, B, and C)** are expressed on almost all nucleated cells in the body, including the cells of a transplanted organ. During acute cellular rejection, the recipient’s **CD8+ Cytotoxic T cells** recognize these foreign MHC Class I molecules on the graft. This recognition triggers the release of perforins and granzymes, leading to direct lysis of the graft cells. Because Class I molecules are the primary targets for these "killer" T cells, they are the most critical mediators of the rejection process. **2. Why the Incorrect Options are Wrong:** * **MHC Class II (Option A):** These are primarily found on professional Antigen Presenting Cells (APCs) like macrophages and B cells. While they play a role in activating CD4+ Helper T cells (which provide cytokine support), they are not the primary targets of the direct cytotoxic attack that characterizes graft destruction. * **MHC Class III (Option C):** These genes encode for components of the complement system (C2, C4) and certain cytokines (TNF). They are involved in the inflammatory response but do not function as cell-surface recognition markers for T cells in graft rejection. * **MHC Class IV (Option D):** This is a distractor; there is no recognized "MHC Class IV" in human immunology. **3. High-Yield Clinical Pearls for NEET-PG:** * **HLA Linkage:** HLA-B27 is strongly associated with Ankylosing Spondylitis; HLA-DR3/DR4 with Type 1 Diabetes. * **Direct vs. Indirect Pathway:** In the **Direct pathway**, recipient T cells recognize donor MHC on donor APCs (strongest response). In the **Indirect pathway**, recipient APCs process donor MHC and present it to recipient T cells. * **Hyperacute Rejection:** Occurs within minutes due to pre-formed antibodies against ABO or HLA antigens (Type II Hypersensitivity). * **Acute Rejection:** Occurs within days to weeks; primarily mediated by T cells (Type IV Hypersensitivity).

Question 397: Autoimmunity can be caused due to all of the following except:

A. Forbidden clones
B. Expression of cryptic antigens
C. Negative selection of T cells in thymus (Correct Answer)
D. Inappropriate expression of MHC proteins

Explanation: **Explanation:** The core concept of this question lies in understanding **Immunological Tolerance**. Autoimmunity occurs when the immune system fails to distinguish "self" from "non-self," leading to an attack on the body's own tissues. **Why "Negative Selection" is the correct answer:** Negative selection is a **protective mechanism**, not a cause of autoimmunity. It occurs in the thymus (for T cells) and bone marrow (for B cells). During this process, developing lymphocytes that react strongly to self-antigens are induced to undergo apoptosis (clonal deletion). This ensures **Central Tolerance**. If negative selection functions correctly, it prevents autoimmunity; its failure, however, would lead to it. **Analysis of Incorrect Options (Causes of Autoimmunity):** * **Forbidden Clones:** According to Burnet’s clonal selection theory, "forbidden clones" are self-reactive lymphocytes that survive despite negative selection. Their persistence and activation lead to autoimmune diseases. * **Expression of Cryptic Antigens:** Some self-antigens are "sequestered" (hidden) in privileged sites (e.g., lens of the eye, sperm, CNS). If these tissues are damaged (trauma/infection), these hidden antigens are released and recognized as foreign by the immune system. * **Inappropriate Expression of MHC Proteins:** Abnormal expression of MHC Class II molecules on non-antigen-presenting cells (e.g., pancreatic beta cells in Type 1 Diabetes) can lead to the presentation of self-antigens to T-helper cells, triggering an immune attack. **NEET-PG High-Yield Pearls:** * **AIRE Gene:** Mutations in the Autoimmune Regulator (AIRE) gene impair negative selection in the thymus, leading to **APECED** syndrome. * **Molecular Mimicry:** A classic cause of autoimmunity where foreign antigens resemble self-antigens (e.g., Group A Strep M-protein mimicking cardiac myosin in Rheumatic Fever). * **Peripheral Tolerance:** Maintained by T-regulatory cells (Tregs) expressing **FoxP3**. Deficiency leads to **IPEX** syndrome.

Question 398: Antibodies are most responsive to which of the following?

A. Recipient's tissue
B. Donor tissue (Correct Answer)
C. Isografts
D. Autografts

Explanation: ### Explanation The core concept behind this question is **Transplant Immunology** and the recognition of "self" versus "non-self" antigens. **Why "Donor Tissue" is Correct:** Antibodies and the adaptive immune system are designed to respond to foreign antigens. In an **allograft** (transplant between genetically different individuals of the same species), the recipient’s immune system recognizes the donor's **Major Histocompatibility Complex (MHC/HLA)** molecules as foreign. This triggers both humoral (antibody-mediated) and cellular immune responses. Antibodies specifically play a critical role in **Hyperacute Rejection** (pre-formed antibodies against donor HLA) and **Acute Antibody-Mediated Rejection**. **Analysis of Incorrect Options:** * **Recipient’s Tissue:** Under normal physiological conditions, the immune system maintains **tolerance** to self-antigens. Antibodies do not respond to the recipient's own tissue unless an autoimmune pathology is present. * **Isografts:** These are grafts between genetically identical individuals (e.g., monozygotic twins). Since the MHC molecules are identical, the recipient’s immune system does not perceive the tissue as foreign, and no significant antibody response is elicited. * **Autografts:** This involves moving tissue from one site to another on the same individual (e.g., a skin graft from the thigh to the arm). There is no genetic disparity, so no immune response occurs. **Clinical Pearls for NEET-PG:** * **Hyperacute Rejection:** Occurs within minutes/hours; mediated by **Pre-formed Type II Cytotoxic Antibodies** (e.g., ABO incompatibility). * **Graft-versus-Host Disease (GVHD):** Occurs when immunocompetent T-cells in the **donor graft** attack the recipient's tissues (common in bone marrow transplants). * **Haplotype:** HLA genes are inherited as a set from each parent; there is a 25% (1 in 4) chance of a perfect HLA match between siblings.

Question 399: Which immunoglobulin predominates in the circulation and persists for many years after exposure?

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

Explanation: **Explanation:** **IgG** is the correct answer because it is the most abundant immunoglobulin in the serum, accounting for approximately **75-80% of the total pool**. It is the primary antibody of the **secondary immune response**. Due to its long half-life (approx. 23 days) and the development of memory B cells, IgG persists in the circulation for years, providing long-term immunity after natural infection or vaccination. **Analysis of Incorrect Options:** * **IgA:** Primarily found in secretions (tears, saliva, colostrum) and mucous membranes. It provides local mucosal immunity rather than dominating systemic circulation. * **IgM:** This is the first antibody produced during a **primary immune response**. It is a large pentamer with a short half-life (approx. 5 days) and disappears relatively quickly, indicating an acute or recent infection. * **IgD:** Found mainly on the surface of B cells where it acts as an antigen receptor; it has negligible concentrations in the serum and no known role in long-term systemic immunity. **NEET-PG High-Yield Pearls:** * **IgG:** The only immunoglobulin that can **cross the placenta** (providing passive immunity to the fetus). It is also the best at opsonization. * **IgM:** The most efficient at **complement activation** (classical pathway) and the earliest to appear in phylogeny and ontogeny. * **IgE:** Mediates Type I hypersensitivity reactions and provides defense against helminthic infections. * **Memory Tip:** Remember **"M"** for **M**omentary (Acute/Early) and **"G"** for **G**enerations (Long-term/Chronic).

Question 400: All of the following statements regarding bacteriophages are true EXCEPT:

A. It is a type of bacteria. (Correct Answer)
B. It is involved in the process of transduction.
C. It can confer toxigenicity through lysogenic conversion.
D. It can contribute to drug resistance.

Explanation: ### Explanation **1. Why Option A is the Correct Answer (The False Statement):** Bacteriophages are **viruses**, not bacteria. They are obligate intracellular parasites that specifically infect and replicate within bacterial cells. Structurally, they consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat (capsid). Because they lack cellular machinery and cannot replicate independently, they do not belong to the kingdom Monera (bacteria). **2. Analysis of Other Options:** * **Option B (Transduction):** This is a true statement. Transduction is the process by which a bacteriophage transfers genetic material (DNA) from one bacterium to another. It is a major mechanism of horizontal gene transfer. * **Option C (Lysogenic Conversion):** This is true. In the lysogenic cycle, the phage DNA integrates into the bacterial chromosome (as a prophage). This can confer new properties, such as toxigenicity. For example, *Corynebacterium diphtheriae* only produces the diphtheria toxin when infected by the **Beta-phage**. * **Option D (Drug Resistance):** This is true. Through generalized or specialized transduction, bacteriophages can carry antibiotic resistance genes (R-factors) from a resistant donor bacterium to a sensitive recipient. **3. High-Yield Clinical Pearls for NEET-PG:** * **Phage Typing:** Used for epidemiological surveillance (e.g., typing *Staphylococcus aureus* or *Salmonella Typhi*). * **Examples of Lysogenic Conversion (Mnemonic: ABCD):** * **A:** Group **A** Streptococci (Pyrogenic exotoxin/Erythrogenic toxin) * **B:** **B**otulinum toxin * **C:** **C**holera toxin * **D:** **D**iphtheria toxin * **S:** **S**higa toxin * **Structure:** Most bacteriophages possess a "head and tail" morphology; the tail is used for attachment to specific bacterial receptors.

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Cells and Organs of Immune System

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Innate 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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Immunology — MCQs

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