Immunology Practice Questions

Q: Which of the following interleukins is characteristically produced in a TH1 response?

IL-2. ### Explanation The differentiation of **CD4+ T-helper cells** into specific subsets (Th1, Th2, Th17) is a fundamental concept in immunology. The choice of subset depends on the cytokine environment and determines the type of immune response generated. **Why Option A (IL-2) is Correct:** The **Th1 response** is primarily responsible for **cell-mediated immunity** and defense against intracellular pathogens (e.g., *M. tuberculosis*). Th1 cells characteristically produce **IL-2**, **IFN-γ** (Interferon-gamma), and **TNF-β**. * **IL-2** acts as a potent T-cell growth factor, promoting the clonal expansion of T-cells. * **IFN-γ** activates macrophages and stimulates B-cells to produce IgG (opsonizing antibodies). **Why the Other Options are Incorrect:** Options B, C, and D are characteristic of a **Th2 response**, which mediates **humoral immunity** and defense against helminths/allergens: * **IL-4:** Induces B-cell class switching to **IgE** and promotes further Th2 differentiation. * **IL-5:** Responsible for **eosinophil** activation and recruitment. * **IL-10:** An **anti-inflammatory** cytokine that inhibits Th1 responses by suppressing IL-12 production from macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **Master Switch:** The transcription factor for Th1 is **T-bet**, while for Th2 it is **GATA-3**. * **Inducing Cytokine:** **IL-12** (from macrophages/DCs) is the primary driver for Th1 differentiation. * **Leprosy Link:** A strong **Th1 response** leads to Tuberculoid leprosy (contained), while a dominant **Th2 response** leads to Lepromatous leprosy (disseminated). * **Th17:** Produces **IL-17**; involved in fungal infections and autoimmune diseases.

Q: Which of the following best describes the mechanism(s) of action of CTLA-4?

Competition and inhibition. **Explanation:** **CTLA-4 (Cytotoxic T-Lymphocyte Associated Protein 4)** is a critical checkpoint molecule that functions as a negative regulator of T-cell activation. Its mechanism of action is twofold: 1. **Competition:** CTLA-4 has a significantly higher affinity and avidity for **B7 ligands (CD80/CD86)** on Antigen-Presenting Cells (APCs) than the stimulatory receptor **CD28**. By outcompeting CD28 for binding, it prevents the necessary "Signal 2" (costimulation) required for T-cell activation. 2. **Inhibition:** Upon binding, CTLA-4 delivers inhibitory signals to the T-cell and can actively remove B7 molecules from the APC surface via trans-endocytosis, leading to functional suppression. **Analysis of Incorrect Options:** * **Option A:** Signal 1 (MHC-TCR) and costimulation (CD28-B7) are the requirements for T-cell **activation**, whereas CTLA-4 acts to terminate this process. * **Option C:** CTLA-4 leads to **anergy** or cell cycle arrest, the exact opposite of activation and proliferation. * **Option D:** ZAP-70 and ITAMs are components of the **stimulatory** TCR signaling pathway. CTLA-4 signaling involves phosphatases (like SHP-2) that counteract these stimulatory kinases. **High-Yield Clinical Pearls for NEET-PG:** * **Abatacept:** A fusion protein (CTLA-4 + IgG1 Fc) used in Rheumatoid Arthritis to block costimulation. * **Ipilimumab:** A monoclonal antibody against CTLA-4 used in **Melanoma**; it "removes the brakes" on the immune system to enhance anti-tumor responses. * **Location:** While CD28 is constitutively expressed, CTLA-4 is upregulated *after* T-cell activation (primarily in secondary lymphoid organs).

Q: What is a common consequence of hypogammaglobulinemia?

Chronic recurrent sinusitis. **Explanation:** **Hypogammaglobulinemia** is a state of immune deficiency characterized by low levels of serum antibodies (Immunoglobulins). Antibodies, particularly **IgA and IgG**, are the primary defense against encapsulated bacteria at mucosal surfaces. 1. **Why Option A is Correct:** Antibodies are essential for the opsonization and neutralization of pathogens. In hypogammaglobulinemia (as seen in Common Variable Immunodeficiency or X-linked Agammaglobulinemia), the body cannot effectively clear bacteria from the respiratory tract. This leads to **chronic recurrent sinopulmonary infections**, such as sinusitis, otitis media, and pneumonia, typically caused by *Streptococcus pneumoniae* and *Haemophilus influenzae*. 2. **Why Other Options are Incorrect:** * **B. Epistaxis:** This is usually related to local trauma, coagulopathies, or platelet disorders, not antibody deficiency. * **C. Contractures:** These are permanent shortenings of muscles or joints, often seen in neurological disorders or severe burns, and have no immunological basis. * **D. Eczema:** While eczema is associated with some immunodeficiencies (like **Wiskott-Aldrich Syndrome** or **Hyper-IgE Syndrome/Job Syndrome**), it is not a direct consequence of generalized hypogammaglobulinemia itself. **NEET-PG High-Yield Pearls:** * **X-linked Agammaglobulinemia (Bruton’s):** Characterized by a defect in **BTK (Bruton Tyrosine Kinase)**, leading to a failure of B-cell maturation. Patients present with recurrent infections after 6 months of age (once maternal IgG wanes). * **Common Variable Immunodeficiency (CVID):** Presents later in life (2nd–3rd decade) with low IgG/IgA/IgM and an increased risk of autoimmune diseases and lymphoma. * **Selective IgA Deficiency:** The most common primary immunodeficiency; often asymptomatic but can cause respiratory/GI infections and **anaphylaxis during blood transfusions** due to anti-IgA antibodies.

Q: A mother and newborn are exposed to a pathogen while at the hospital for a routine checkup and breastfeeding clinic. This same pathogen had infected the mother about a year previously, and she had successfully recovered from the subsequent illness. Immunity may be innate or acquired. Which of the following best describes acquired immunity with respect to the newborn?

Maternal transfer of antibody. ### Explanation **Correct Option: D. Maternal transfer of antibody** The scenario describes **Naturally Acquired Passive Immunity**. Because the mother was infected a year ago, her immune system developed specific memory B-cells and high titers of **IgG antibodies**. During pregnancy, these IgG antibodies cross the placenta (the only immunoglobulin to do so). Furthermore, since the mother is breastfeeding, she provides **Secretory IgA** to the newborn. These pre-formed antibodies provide immediate, specific protection to the infant against the pathogen they were both exposed to. **Why Incorrect Options are Wrong:** * **A, B, and C (Complement, CRP, and Inflammation):** These are all components of **Innate Immunity**. Innate immunity is non-specific, present from birth, and does not require prior exposure to a pathogen. While they are active in the newborn, they do not represent "acquired" immunity derived from the mother’s previous infection. Specifically, CRP is an acute-phase reactant, and the complement cascade is a biochemical pathway that enhances the ability of antibodies and phagocytes to clear microbes. **High-Yield Clinical Pearls for NEET-PG:** * **Passive Immunity:** Immediate protection but short-lived (no memory). Examples: Placental transfer of IgG, Breast milk (IgA), and administration of Tetanus Immunoglobulin (TIG). * **Active Immunity:** Delayed onset but long-lasting (memory cells). Examples: Natural infection or Vaccination. * **IgG:** The only antibody that crosses the placenta (Neonatal immunity). * **IgA:** The predominant antibody in colostrum and breast milk (Mucosal immunity). * **Mnemonic:** **P**assive = **P**re-formed antibodies; **A**ctive = **A**ntigen exposure.

Q: Which of the following acts as an opsonin?

C3b. **Explanation:** **Opsonization** is the process by which foreign particles (like bacteria) are coated with specific molecules called **opsonins**, making them more "palatable" and easily recognized by phagocytes (neutrophils and macrophages). **Why C3b is the correct answer:** C3b is the most potent opsonin produced by the complement system. When the complement cascade is activated, C3 is cleaved into C3a and C3b. C3b binds covalently to the surface of pathogens. Phagocytes possess specific **CR1 receptors** that bind to C3b, facilitating the attachment and subsequent engulfment of the pathogen. **Analysis of Incorrect Options:** * **C3a and C5a:** These are **Anaphylatoxins**. They trigger mast cell degranulation, leading to histamine release, increased vascular permeability, and smooth muscle contraction. * **C5a and LTB4 (Leukotriene B4):** These are potent **Chemotactic factors**. They act as chemical signals that recruit and attract neutrophils and other inflammatory cells to the site of infection, but they do not coat the pathogen for phagocytosis. **NEET-PG High-Yield Pearls:** 1. **Major Opsonins:** The two most important opsonins in the body are **C3b** (complement-derived) and **IgG** (specifically the Fc portion of IgG1 and IgG3). 2. **Other Opsonins:** Include IgM (indirectly via complement), C4b, Mannose-binding lectin (MBL), and C-reactive protein (CRP). 3. **Mnemonic for Chemotaxis:** "B-A-C-E" (LT**B**4, **A**rachidonic acid metabolites, **C**5a, **E**ndotoxins/Exotoxins). 4. **Deficiency:** Deficiency of C3 leads to recurrent infections with pyogenic bacteria due to impaired opsonization.

Q: Which of the following statements regarding immunoglobulins is true?

The half-life of IgG is 23 days.. This question tests the fundamental structural and functional properties of immunoglobulins, a high-yield topic for NEET-PG. **Correct Option Explanation:** **Option B** is correct. **IgG** has the longest half-life of all immunoglobulins, averaging **23 days** (except for the IgG3 subclass, which is ~7 days). This longevity is due to its interaction with the **neonatal Fc receptor (FcRn)**, which protects IgG from lysosomal degradation by recycling it back into the circulation. **Analysis of Incorrect Options:** * **A. IgA crosses the placenta:** Incorrect. **IgG** is the **only** immunoglobulin class capable of crossing the placenta, providing passive immunity to the fetus. * **C. IgD is heat stable:** Incorrect. IgD is highly **heat-labile** and susceptible to proteolytic enzymes. It primarily serves as a surface receptor on B-cells. * **D. IgE has the highest carbohydrate content:** Incorrect. **IgD** has the highest carbohydrate content (~13%). IgE also has high carbohydrate content (~12%), but IgD is the standard answer for the "highest" in this category. **High-Yield Clinical Pearls for NEET-PG:** * **IgG:** Most abundant in serum (80%); responsible for the secondary immune response. * **IgM:** Largest (Pentamer); first to appear in primary response; best at complement fixation. * **IgA:** Found in secretions (tears, saliva, colostrum) as a dimer with a **J-chain** and **secretory component**. * **IgE:** Mediates Type I hypersensitivity and provides defense against helminthic infections. * **Order of serum concentration:** G > A > M > D > E (**GAMDE**).

Q: Which of the following is NOT a peripheral lymphoid organ?

Thymus. ### Explanation The lymphoid system is divided into two functional categories: **Primary (Central)** and **Secondary (Peripheral)** lymphoid organs. **Why Thymus is the Correct Answer:** The **Thymus** (along with Bone Marrow) is a **Primary Lymphoid Organ**. These are the sites where lymphocytes are produced and undergo antigen-independent maturation. In the thymus, T-cell precursors from the bone marrow differentiate into mature, immunocompetent T-cells. Since the question asks for an organ that is *NOT* peripheral, the Thymus is the correct choice. **Why the other options are incorrect:** Secondary (Peripheral) lymphoid organs are sites where mature lymphocytes interact with antigens and undergo clonal expansion. * **A. Lymph nodes:** These filter lymph and are the primary sites for initiating immune responses to tissue-borne antigens. * **B. Spleen:** This is the largest peripheral lymphoid organ; it filters blood and responds to blood-borne antigens. * **C. Mucosa-associated lymphoid tissue (MALT):** This includes Peyer’s patches, tonsils, and appendix. They protect mucosal surfaces (respiratory, GI, and urogenital tracts). **High-Yield Clinical Pearls for NEET-PG:** * **Primary Lymphoid Organs:** Bone Marrow (B-cell maturation) and Thymus (T-cell maturation). * **Hassall’s Corpuscles:** These are characteristic histological features found in the medulla of the **Thymus**. * **DiGeorge Syndrome:** A congenital defect (3rd and 4th pharyngeal pouches) leading to thymic hypoplasia and T-cell deficiency. * **MALT:** Specifically, **Peyer’s patches** in the ileum are characterized by M-cells that sample intestinal antigens. * **Major site of antibody production:** The Spleen (specifically the white pulp) and Lymph nodes.

Q: The hybridoma technique is used to obtain which of the following?

Specific antibody. ### Explanation The **Hybridoma Technique**, pioneered by Georges Köhler and César Milstein in 1975, is the gold-standard method for producing **Monoclonal Antibodies (mAbs)**. These are highly specific antibodies derived from a single clone of B-cells, ensuring they all recognize the same epitope on an antigen. **Why Option C is Correct:** The technique involves fusing short-lived, antibody-producing **B-lymphocytes** (usually from a mouse spleen) with immortal **Myeloma cells** (cancerous B-cells). The resulting "Hybridoma" cell inherits two key properties: 1. **Immortality** from the myeloma cell (allowing continuous growth in culture). 2. **Specificity** from the B-lymphocyte (allowing the production of a single, specific antibody). **Why Other Options are Incorrect:** * **A. Specific Antigen:** Antigens are the targets that trigger the immune response; they are used to immunize the animal before the fusion process, not produced by it. * **B. Complement:** These are heat-labile plasma proteins synthesized primarily by the liver and macrophages, not by hybridoma cells. * **D. Interleukins:** These are cytokines produced by various leukocytes (like T-cells and macrophages) to modulate the immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Selection Medium:** **HAT Medium** (Hypoxanthine, Aminopterin, Thymidine) is used to select only the fused hybrid cells. Unfused myeloma cells die because Aminopterin blocks the *de novo* pathway, and they lack the **HGPRT enzyme** for the salvage pathway. * **Applications:** Monoclonal antibodies are used in **ELISA**, **Western Blotting**, and as therapeutic agents (e.g., **Rituximab** against CD20, **Infliximab** against TNF-α). * **Nobel Prize:** Köhler and Milstein received the Nobel Prize in 1984 for this discovery.

Q: Which of the following reactions is NOT produced by heterophile antigens?

Widal reaction. ### Explanation **Concept of Heterophile Antigens** Heterophile antigens are closely related antigens present in completely different species (phylogenetically unrelated). An antibody produced against one of these antigens can cross-react with the antigen of another species. This principle is used in various diagnostic serological tests. **Why the Widal Reaction is the Correct Answer** The **Widal reaction** is a **specific** agglutination test used for the diagnosis of Enteric fever (Typhoid). It detects antibodies against the specific O (somatic) and H (flagellar) antigens of *Salmonella typhi* and *S. paratyphi*. Since it involves specific homologous antigen-antibody interactions rather than cross-reactive antigens from different species, it is **not** a heterophile reaction. **Analysis of Incorrect Options** * **Forssman Antigen-Antibody Reaction:** This is the classic example of a heterophile system. Forssman antigens are found in the tissues of horses, guinea pigs, and cats; antibodies against them cross-react with various other species. * **Weil-Felix Reaction:** A heterophile agglutination test where antibodies produced during **Rickettsial infections** cross-react with the alkali-stable polysaccharide antigens of certain **Proteus strains** (OX19, OX2, and OXK). * **Paul-Bunnel Reaction:** Used to diagnose **Infectious Mononucleosis** (EBV). It detects heterophile antibodies in the patient's serum that have the unique property of agglutinating **sheep erythrocytes**. **High-Yield Clinical Pearls for NEET-PG** * **Paul-Bunnel Test:** Specific for EBV; uses sheep RBCs. * **Weil-Felix Test:** Uses *Proteus* antigens to diagnose Rickettsial diseases (except Q fever). * **Cold Agglutination Test:** Another heterophile test where antibodies in *Mycoplasma pneumoniae* patients agglutinate human O-group RBCs at 4°C. * **Widal Test:** A "TUBE" agglutination test; significant titers are usually >1:80 for O and >1:160 for H.

Question 1

Which of the following interleukins is characteristically produced in a TH1 response?

Accepted Answer

IL-2

Answer

IL-4

Answer

IL-5

Answer

IL-10

Question 2

Which of the following best describes the mechanism(s) of action of CTLA-4?

Accepted Answer

Competition and inhibition

Answer

Signal 1 and costimulation

Answer

Activation and proliferation

Answer

ZAP-70 and ITAMs

Question 3

What is a common consequence of hypogammaglobulinemia?

Accepted Answer

Chronic recurrent sinusitis

Answer

Epistaxis

Answer

Contractures

Answer

Eczema

Question 4

A mother and newborn are exposed to a pathogen while at the hospital for a routine checkup and breastfeeding clinic. This same pathogen had infected the mother about a year previously, and she had successfully recovered from the subsequent illness. Immunity may be innate or acquired. Which of the following best describes acquired immunity with respect to the newborn?

Accepted Answer

Maternal transfer of antibody

Answer

Complement cascade

Answer

Increase in C-reactive protein (CRP)

Answer

Inflammatory response

Question 5

Which of the following acts as an opsonin?

Accepted Answer

C3b

Answer

C3a

Answer

C5a

Answer

LTB4

Question 6

Which of the following statements regarding immunoglobulins is true?

Accepted Answer

The half-life of IgG is 23 days.

Answer

IgA crosses the placenta.

Answer

IgD is heat stable.

Answer

IgE has the highest carbohydrate content.

Question 7

Immunoglobulin bound on the surface of bacteria mediates phagocytosis by:

Accepted Answer

C3b and Fc

Answer

Receptor mediated endocytosis

Answer

Oxidase action

Answer

Lysosomal burst

Question 8

Which of the following is NOT a peripheral lymphoid organ?

Accepted Answer

Thymus

Answer

Lymph nodes

Answer

Spleen

Answer

Mucosa-associated lymphoid tissue

Question 9

The hybridoma technique is used to obtain which of the following?

Accepted Answer

Specific antibody

Answer

Specific antigen

Answer

Complement

Answer

Interleukins

Question 10

Which of the following reactions is NOT produced by heterophile antigens?

Accepted Answer

Widal reaction

Answer

Forssman antigen-antibody reaction

Answer

Weil-Felix reaction

Answer

Paul-Bunnel reaction

Immunology — MCQs

Immunology — MCQs

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