Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 801: Which of the following is characteristic of the macrophage chemotactic factor?

A. High molecular weight
B. Chymotrypsin sensitive (Correct Answer)
C. Heat labile
D. Antigenically similar to C3

Explanation: **Explanation:** The **Macrophage Chemotactic Factor (MCF)** is a lymphokine produced by T-lymphocytes upon stimulation by specific antigens. Its primary role is to recruit macrophages to the site of inflammation, a crucial step in Delayed-Type Hypersensitivity (DTH) reactions. **Why Option B is Correct:** MCF is proteinaceous in nature. It is specifically **sensitive to proteolytic enzymes** like **chymotrypsin** and trypsin, which degrade the factor and abolish its chemotactic activity. This biochemical property distinguishes it from other mediators. **Analysis of Incorrect Options:** * **A. High molecular weight:** MCF is actually a **low molecular weight** protein (approximately 12,000 to 25,000 Daltons). * **C. Heat labile:** MCF is relatively **heat stable**; it can typically withstand temperatures of 56°C for 30 minutes, unlike many complement components. * **D. Antigenically similar to C3:** MCF is a lymphocyte-derived cytokine and is **antigenically distinct** from complement components like C3a or C5a, even though C5a also possesses potent macrophage chemotactic activity. **High-Yield Clinical Pearls for NEET-PG:** * **MIF vs. MCF:** While MCF recruits macrophages, **Migration Inhibitory Factor (MIF)** prevents them from leaving the site. MIF is considered the first cytokine to be discovered. * **Key Chemotactic Agents:** For neutrophils, the most potent agent is **IL-8** and **C5a**; for macrophages, it is **MCF** and **MCP-1** (Monocyte Chemoattractant Protein-1). * **Source:** MCF is produced by CD4+ T-cells (Th1 subset), playing a vital role in granuloma formation (e.g., in Tuberculosis).

Question 802: Which of the following is a B cell specific mitogen?

A. Concanavalin A
B. Lipopolysaccharide (Correct Answer)
C. Pokeweed mitogen
D. Phytohaemagglutinin

Explanation: **Explanation:** Mitogens are substances that induce mitosis (cell division) in lymphocytes regardless of their antigen specificity. They are classified based on the specific lymphocyte population they stimulate. **Correct Option: B. Lipopolysaccharide (LPS)** Lipopolysaccharide, a component of the cell wall of Gram-negative bacteria, is a classic **B-cell specific mitogen** in many species (notably mice). It acts as a polyclonal B-cell activator, stimulating B-lymphocytes to proliferate and differentiate into plasma cells without the help of T-cells. **Analysis of Incorrect Options:** * **A. Concanavalin A (ConA):** This is a **T-cell specific mitogen**. It is derived from the jack bean (*Canavalia ensiformis*) and specifically triggers T-lymphocyte proliferation. * **D. Phytohaemagglutinin (PHA):** Similar to ConA, PHA is a **T-cell specific mitogen** derived from the red kidney bean. It is frequently used in clinical cytogenetics to stimulate T-cells for karyotyping. * **C. Pokeweed Mitogen (PWM):** This is a **bipotential mitogen**. It stimulates **both T-cells and B-cells**. **High-Yield Clinical Pearls for NEET-PG:** * **T-cell Mitogens:** PHA, ConA. * **B-cell Mitogens:** LPS, Staphylococcal Protein A (SpA), EBV (Epstein-Barr Virus). * **Both T & B cell Mitogen:** Pokeweed Mitogen (PWM). * **Clinical Use:** Mitogen stimulation tests are used to evaluate the functional integrity of the cellular immune system in suspected primary immunodeficiency disorders. * **Superantigens vs. Mitogens:** While both cause polyclonal activation, superantigens are specific to T-cells (binding to Vβ region of TCR) and are much more potent than mitogens.

Question 803: All of the following are uses of enzyme-linked immunosorbent assay detection except?

A. Hepatitis B markers
B. Rotavirus
C. Enterotoxin of E. coli
D. Hepatitis A virus (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Hepatitis A virus**. While ELISA is a versatile technique capable of detecting both antigens and antibodies, its clinical utility depends on the diagnostic window and the nature of the pathogen. 1. **Why Hepatitis A is the correct answer:** Hepatitis A virus (HAV) is primarily detected in the **stool** using electron microscopy or molecular methods (PCR) during the early phase of infection. By the time clinical symptoms appear, the virus is often no longer detectable in the feces. Therefore, the standard diagnostic approach for HAV is not the detection of the virus itself via ELISA, but rather the detection of **anti-HAV IgM antibodies** in the serum. 2. **Analysis of other options:** * **Hepatitis B markers:** ELISA is the gold standard for detecting HBsAg, HBeAg, and various antibodies (Anti-HBs, Anti-HBc) in serum. * **Rotavirus:** ELISA is a routine diagnostic tool used to detect Rotavirus antigen directly from **stool samples** in pediatric diarrhea cases. * **Enterotoxin of E. coli:** ELISA is frequently used to detect both heat-labile (LT) and heat-stable (ST) enterotoxins produced by Enterotoxigenic *E. coli* (ETEC). **Clinical Pearls for NEET-PG:** * **ELISA Types:** Remember that **Sandwich ELISA** is used for antigen detection (e.g., HBsAg), while **Indirect ELISA** is typically used for antibody detection (e.g., HIV screening). * **HAV Diagnosis:** Always look for **IgM anti-HAV** as the marker of choice for acute infection. * **High-Yield Fact:** ELISA is highly sensitive and specific, making it the preferred screening test for blood-borne pathogens (HIV, HBV, HCV) in blood banks.

Question 804: IgA antibody is the first line of defense against infections at the mucous membrane and is usually an early specific antibody. Which of the following statements regarding IgA is not true?

A. Complement fixation tests for IgA antibody will be positive if specific IgA antibody is present
B. IgA is not found in saliva, therefore an IgA diagnostic test on saliva would have no value
C. IgA can be destroyed by bacterial proteases (Correct Answer)
D. IgA is absent in colostrum

Explanation: ### Explanation **Correct Answer Analysis:** The question asks for the statement that is **not true**. However, based on standard immunology, Option C is actually a **true** statement, while Options B and D are **false**. In the context of NEET-PG, this question highlights the vulnerability of IgA to specific pathogens. Certain bacteria (e.g., *Neisseria meningitidis*, *Streptococcus pneumoniae*, and *Haemophilus influenzae*) produce **IgA1 proteases** that cleave the hinge region of IgA, allowing them to bypass mucosal immunity. **Analysis of Options:** * **Option A (False):** IgA **cannot** activate the classical complement pathway (which is what a standard Complement Fixation Test measures). It can only activate the alternative pathway. * **Option B (False):** IgA is the **predominant** immunoglobulin in secretions, including saliva, tears, and nasal mucosa. Salivary IgA testing is a valid diagnostic tool for certain mucosal infections. * **Option D (False):** Colostrum is extremely rich in **Secretory IgA (sIgA)**, providing essential passive immunity to the neonate's gut. *Note: In many competitive exams, if multiple statements are false, the "most true" or "clinically significant" fact is highlighted. Option C is a definitive biological fact regarding bacterial virulence.* **Clinical Pearls for NEET-PG:** * **Structure:** IgA is a monomer in serum but a **dimer** in secretions, held together by a **J-chain**. * **Secretory Component:** This is added by epithelial cells to protect the antibody from the acidic environment of the gut. * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients are often asymptomatic but may present with recurrent sinopulmonary infections or Giardiasis. * **Milk:** IgA is the most abundant Ig in breast milk/colostrum, whereas IgG is the only Ig that crosses the placenta.

Question 805: What is true about Major Histocompatibility Complex (MHC)?

A. Transplantation rejection
B. Autoimmune diseases
C. Involved in T-cell function
D. All of the above (Correct Answer)

Explanation: **Explanation:** The **Major Histocompatibility Complex (MHC)**, known as Human Leukocyte Antigen (HLA) in humans, is a set of cell surface proteins essential for the acquired immune system to recognize foreign molecules. * **Involved in T-cell function (Option C):** This is the primary physiological role of MHC. T-cells cannot recognize "free" antigens; they only recognize peptide fragments presented on MHC molecules. **MHC Class I** (found on all nucleated cells) presents to CD8+ Cytotoxic T-cells, while **MHC Class II** (found on Antigen Presenting Cells) presents to CD4+ Helper T-cells. * **Transplantation rejection (Option A):** MHC molecules are highly polymorphic. When an organ is transplanted, the recipient’s T-cells recognize the donor's MHC molecules as "non-self," triggering a potent immune response leading to graft rejection. * **Autoimmune diseases (Option B):** Certain HLA alleles are strongly associated with an increased risk of autoimmune disorders. For example, **HLA-B27** is linked to Ankylosing Spondylitis, and **HLA-DR3/DR4** are linked to Type 1 Diabetes Mellitus. Since MHC is fundamental to antigen presentation (T-cell function), serves as the primary barrier in grafting (transplantation), and dictates disease susceptibility (autoimmunity), **Option D is the correct answer.** **High-Yield Clinical Pearls for NEET-PG:** * **MHC Class I:** HLA-A, B, C; associated with $\beta_2$-microglobulin. * **MHC Class II:** HLA-DP, DQ, DR. * **MHC Class III:** Includes components of the complement system (C2, C4) and TNF. * **Rule of 8:** MHC II $\times$ CD4 = 8; MHC I $\times$ CD8 = 8. * **HLA-B27:** Associated with "PAIR" (Psoriasis, Ankylosing spondylitis, Inflammatory bowel disease, Reiter’s syndrome).

Question 806: Antibody diversity is due to which region?

A. Hinge region
B. Constant region
C. Variable region
D. Hypervariable region (Correct Answer)

Explanation: **Explanation:** The primary function of an antibody is to recognize and bind to a vast array of diverse antigens. This immense **antibody diversity** is primarily attributed to the **Hypervariable regions** (also known as Complementarity Determining Regions or **CDRs**). 1. **Why Hypervariable region is correct:** Within the variable domains of both heavy (VH) and light (VL) chains, there are three small stretches of amino acids that exhibit extreme variation. These CDRs (CDR1, CDR2, and CDR3) form the actual **antigen-binding site (paratope)**. The unique amino acid sequences in these regions allow antibodies to be specific to millions of different epitopes. CDR3 is the most variable of the three. 2. **Why other options are incorrect:** * **Variable region:** While the hypervariable regions are *part* of the variable region, the variable region also contains "framework regions" that are relatively stable and provide structural support. The diversity specifically stems from the hypervariable pockets. * **Constant region:** This region (CH and CL) determines the **biological effector function** (e.g., opsonization, placental transfer, complement activation) and the isotype (IgG, IgM, etc.), not antigen specificity. * **Hinge region:** This is a flexible amino acid stretch (rich in proline and cysteine) that allows the two antigen-binding arms to move independently. It does not contribute to diversity. **NEET-PG High-Yield Pearls:** * **Genetic Basis:** Antibody diversity is generated by **V(D)J recombination** (mediated by RAG-1 and RAG-2 genes), junctional diversity, and **somatic hypermutation**. * **Allelic Exclusion:** Ensures that a single B-cell expresses only one specific antigen receptor. * **Isotype Switching:** Changes the constant region (e.g., IgM to IgG) but the **variable region remains the same**, meaning the antigen specificity does not change.

Question 807: Negative phase is seen in which type of immunity?

A. Active (Correct Answer)
B. Passive
C. Herd
D. Local

Explanation: **Explanation:** **1. Why Active Immunity is Correct:** The **Negative Phase** is a phenomenon observed exclusively in active immunity, specifically following a booster dose or a secondary exposure to an antigen. When an individual who already has circulating antibodies is injected with a fresh dose of the same antigen, the existing antibodies combine with the newly introduced antigen. This leads to a **transient decrease** in the level of measurable free antibodies in the serum. This temporary dip is called the negative phase. It is followed by a rapid, exponential rise in antibody titers (the secondary/anamnestic response). **2. Why Other Options are Incorrect:** * **Passive Immunity:** This involves the pre-formed transfer of antibodies (e.g., immunoglobulin therapy). Since the body’s own immune machinery is not being "primed" or challenged to produce its own antibodies against a fresh antigen challenge, no negative phase occurs. * **Herd Immunity:** This is an epidemiological concept referring to the collective immunity of a population. It describes the threshold at which a disease stops spreading; it is not a physiological immune response phase. * **Local Immunity:** This refers to site-specific immunity (e.g., IgA in mucosal linings). While it is a form of active immunity, the term "negative phase" specifically describes systemic serum antibody kinetics. **Clinical Pearls for NEET-PG:** * **Lag Phase:** The time between antigen entry and the appearance of antibodies. It is longer in primary responses and shorter in secondary responses. * **Negative Phase Caution:** In clinical practice, if a vaccine is given during an ongoing infection or too close to a previous dose, the negative phase can theoretically lead to a temporary drop in protection, though this is rarely clinically significant for most modern vaccines. * **Primary vs. Secondary:** Primary response is dominated by **IgM**, while secondary response (where the negative phase is most relevant) is dominated by **IgG**.

Question 808: Lysozyme is present in which of the following body secretions?

A. Lacrimal secretions
B. Cerebrospinal fluid (CSF) (Correct Answer)
C. Saliva
D. Respiratory tract secretions

Explanation: **Explanation:** Lysozyme (also known as muramidase) is a key component of the **innate immune system**. It functions as an antibacterial enzyme by hydrolyzing the $\beta$-(1,4)-glycosidic bonds between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in the **peptidoglycan** layer of bacterial cell walls, particularly in Gram-positive bacteria. **Why the Correct Answer is CSF:** In a healthy physiological state, lysozyme is notably **absent** or present in only negligible traces in the **Cerebrospinal Fluid (CSF)**, sweat, and urine. Therefore, in the context of standard medical examinations, CSF is frequently cited as the secretion lacking lysozyme. **Analysis of Options:** * **A. Lacrimal secretions (Tears):** These contain high concentrations of lysozyme to protect the ocular surface from bacterial colonization. * **C. Saliva:** Lysozyme is a primary antibacterial agent in the oral cavity, working alongside lactoferrin and IgA. * **D. Respiratory tract secretions:** Mucus in the nasopharynx and bronchial tree contains lysozyme to trap and degrade inhaled pathogens. **Clinical Pearls for NEET-PG:** * **Diagnostic Marker:** While normally absent in CSF, lysozyme levels **increase** significantly in cases of **bacterial meningitis**, making it a useful biochemical marker to differentiate it from viral meningitis. * **Cellular Source:** It is primarily synthesized by **monocytes, macrophages, and neutrophils**. * **Sarcoidosis:** Elevated serum lysozyme levels can be seen in sarcoidosis and certain monocytic leukemias (AML-M4 and M5). * **Gram-Negative Resistance:** Lysozyme is less effective against Gram-negative bacteria because their peptidoglycan layer is protected by an outer lipopolysaccharide membrane.

Question 809: Type 2 lepra reaction is an example of which type of hypersensitivity?

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

Explanation: **Explanation:** **Type 2 Lepra Reaction (Erythema Nodosum Leprosum - ENL)** is a classic example of **Type III Hypersensitivity** (Immune Complex-Mediated). It occurs primarily in patients with multibacillary leprosy (lepromatous or borderline lepromatous) who have a high bacterial load. The reaction is triggered by the sudden release of mycobacterial antigens (often due to chemotherapy), which combine with circulating antibodies to form **immune complexes**. These complexes deposit in blood vessel walls, activating the complement system and leading to systemic vasculitis. **Why other options are incorrect:** * **Type I (Immediate):** Mediated by IgE and mast cell degranulation (e.g., Anaphylaxis, Asthma). It is not involved in leprosy reactions. * **Type II (Cytotoxic):** Involves antibodies (IgG/IgM) directed against antigens on specific cell surfaces (e.g., Rh incompatibility). * **Type IV (Delayed-type):** This is the mechanism for **Type 1 Lepra Reaction** (Reversal Reaction). It involves T-cell mediated delayed hypersensitivity and is seen in paucibacillary cases as they shift toward a stronger cell-mediated immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** ENL presents with tender, evanescent subcutaneous nodules, fever, lymphadenopathy, and arthralgia. * **Complications:** Can lead to iridocyclitis, orchitis, and nephritis. * **Drug of Choice:** **Thalidomide** is the most effective treatment for Type 2 reactions. Clofazimine and corticosteroids are also used. * **Key Distinction:** Remember—Type **1** Reaction = Type **IV** Hypersensitivity; Type **2** Reaction = Type **III** Hypersensitivity.

Question 810: Northern blotting is used for the separation of which type of molecule?

A. DNA
B. RNA (Correct Answer)
C. Proteins
D. None of the above

Explanation: **Explanation:** Blotting techniques are fundamental molecular biology tools used to identify specific macromolecules. The correct answer is **RNA** because Northern blotting specifically involves the electrophoresis of RNA molecules, their transfer to a membrane (nitrocellulose or nylon), and subsequent detection using a labeled nucleic acid probe. This technique is primarily used to study gene expression by measuring the amount and size of specific mRNA transcripts. **Analysis of Options:** * **Option A (DNA):** DNA is separated and detected using **Southern blotting**. This is used for identifying specific DNA sequences, detecting mutations, or gene mapping. (Mnemonic: **S**outhern = **D**NA). * **Option C (Proteins):** Proteins are separated by SDS-PAGE and detected using antibodies in **Western blotting**. This is clinically significant as a confirmatory test for HIV (detecting p24 or gp120/160). * **Option D:** Incorrect, as RNA is the established target for Northern blotting. **High-Yield Clinical Pearls for NEET-PG:** To remember these easily, use the **SNOW DROP** mnemonic: * **S**outhern — **D**NA * **N**orthern — **R**NA * **O** — (nothing) * **W**estern — **P**roteins **Additional High-Yield Facts:** * **Southwestern Blotting:** Used to identify **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Eastern Blotting:** Used to detect post-translational modifications of proteins (e.g., carbohydrate or lipid attachments). * **RT-PCR:** While Northern blotting measures RNA levels, RT-PCR is a more sensitive and modern method for quantifying mRNA expression.

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Cells and Organs of Immune System

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Innate Immunity

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Adaptive 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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