Immunology Practice Questions

Q: Which of the following is a HLA class III molecule?

TNF-α. **Explanation:** The Human Leukocyte Antigen (HLA) complex is located on the short arm of **Chromosome 6**. It is divided into three classes based on structure and function. **1. Why TNF-α is the Correct Answer:** While HLA Class I and II molecules are primarily involved in antigen presentation, **HLA Class III** molecules are a diverse group of proteins that do not present antigens. Instead, they are involved in the inflammatory response and the complement system. Key Class III molecules include: * **Cytokines:** Tumor Necrosis Factor-alpha (**TNF-α**) and Lymphotoxin (TNF-β). * **Complement components:** C2, C4A, C4B, and Factor B. * **Heat Shock Proteins:** HSP70. **2. Why the Other Options are Incorrect:** * **HLA-G (Option A):** This is a **Non-classical HLA Class I** molecule (Class Ib). It is primarily expressed on cytotrophoblasts at the feto-maternal interface and plays a crucial role in immune tolerance during pregnancy. * **HLA-H (Option B):** This is a **pseudogene** related to Class I. Note: The *HFE* gene (associated with Hemochromatosis) was formerly called HLA-H, but it remains a Class I-like structure. **High-Yield Clinical Pearls for NEET-PG:** * **Class I (A, B, C):** Present endogenous antigens to **CD8+ T cells**. * **Class II (DR, DQ, DP):** Present exogenous antigens to **CD4+ T cells**. * **Class III:** Unique because they are **not** membrane-bound surface markers; they are secreted proteins. * **MHC Restriction:** CD4 cells recognize MHC II; CD8 cells recognize MHC I (Rule of 8: 4×2=8 and 8×1=8).

Q: The Tuberculin test is an example of which type of immunological reaction?

Cell mediated. The Tuberculin test (Mantoux test) is a classic example of a **Type IV Hypersensitivity reaction**, also known as **Delayed-Type Hypersensitivity (DTH)**. ### Why "Cell Mediated" is Correct: Unlike other hypersensitivity reactions, Type IV is **not mediated by antibodies**. Instead, it is mediated by **T-lymphocytes** (specifically Th1 cells). When Purified Protein Derivative (PPD) is injected intradermally, sensitized T-cells recognize the antigen, release cytokines (like IFN-γ), and recruit macrophages. This process takes **48–72 hours** to manifest as induration, hence the term "delayed." ### Why Other Options are Incorrect: * **Option A (Anaphylaxis mediated):** This refers to **Type I Hypersensitivity**, which is IgE-mediated and occurs within minutes (e.g., asthma, urticaria). * **Option C (Antibody mediated):** This refers to **Type II Hypersensitivity**, where IgG or IgM antibodies bind to antigens on cell surfaces (e.g., Autoimmune Hemolytic Anemia). * **Option D (Immune complex mediated):** This refers to **Type III Hypersensitivity**, involving the deposition of antigen-antibody complexes in tissues (e.g., SLE, Arthus reaction). ### High-Yield Clinical Pearls for NEET-PG: * **The "4 Ts" of Type IV Hypersensitivity:** **T**-cells, **T**uberculin test, **T**ransplant rejection (acute/chronic), and Contact **T**oxicants (e.g., Poison ivy, Nickel dermatitis). * **Reading the Test:** Measure the **induration** (palpable raised area), NOT the erythema (redness). * **False Negative:** Can occur in miliary TB, malnutrition, or immunosuppression (Anergy). * **False Positive:** Common in individuals previously vaccinated with **BCG**.

Q: Which of the following statements regarding agammaglobulinemia is FALSE?

Normal B-cell precursors in the bone marrow.. **Explanation:** The question refers to **X-linked Agammaglobulinemia (XLA)**, also known as Bruton’s Agammaglobulinemia. This condition is caused by a mutation in the **Bruton Tyrosine Kinase (BTK) gene**, which is essential for the maturation of B-cells. **Why Option B is the Correct (False) Statement:** In XLA, B-cell development is arrested at the **pre-B cell stage**. Therefore, while there are pro-B cells, there is a **marked deficiency or absence of mature B-cells** and their precursors (like pre-B cells) in the bone marrow and peripheral blood. The statement "Normal B-cell precursors" is incorrect because the maturation pathway is fundamentally disrupted. **Analysis of Other Options:** * **Option A:** Germinal centers are the sites where B-cells proliferate and differentiate. Since B-cells are absent, **germinal centers do not form** in lymph nodes, tonsils, or Peyer’s patches. * **Option C:** The paracortex is a T-cell zone, which remains normal in XLA. However, the **medulla and follicles** (B-cell zones) are depleted. (Note: In the context of this question, the focus is on the absence of B-lineage cells in these specific lymphoid architectures). * **Option D:** The spleen shows a lack of lymphoid follicles (white pulp), leading to an overall reduction in organized lymphoid tissue. **High-Yield NEET-PG Pearls:** * **Inheritance:** X-linked recessive (primarily affects males). * **Clinical Presentation:** Recurrent pyogenic infections (e.g., *S. pneumoniae*, *H. influenzae*) starting after 6 months of age (once maternal IgG wanes). * **Diagnosis:** Absent/low B-cells (CD19+, CD20+) on flow cytometry; all immunoglobulin classes (IgG, IgA, IgM) are severely low. * **Treatment:** Intravenous Immunoglobulin (IVIG) replacement; **Live vaccines are contraindicated.**

Q: What are toxoids?

Antigenic and non-toxic. **Explanation:** A **toxoid** is a bacterial exotoxin that has been modified (usually by treatment with formaldehyde, heat, or aging) to eliminate its harmful effects while retaining its ability to stimulate an immune response. **1. Why Option B is Correct:** The fundamental goal of a toxoid is to serve as a vaccine. To do this, it must be **non-toxic** (safe for the patient) but remain **antigenic** (capable of inducing the production of antibodies, specifically antitoxins). The chemical modification alters the "toxicophore" (the part causing disease) but preserves the "immunophore" (the part recognized by the immune system). **2. Why Other Options are Incorrect:** * **Option A:** This describes a **native exotoxin**. If it were still toxic, it would cause disease (e.g., Tetanus) rather than prevent it. * **Option C:** This is biologically impossible for a vaccine candidate; if it is non-antigenic, the body won't react to it, and if it is toxic, it is a poison. * **Option D:** This describes an inert substance that would have no clinical utility in immunization. **3. NEET-PG High-Yield Pearls:** * **Examples:** The most common toxoid vaccines are **Tetanus** and **Diphtheria** (often combined as DPT/DTaP). * **Type of Immunity:** Toxoids induce **active artificial immunity**. * **Mechanism:** They stimulate B-cells to produce **antitoxins** (IgG), which neutralize the toxin during a real infection. * **Adjuvants:** Toxoids are often adsorbed onto aluminum salts (alum) to increase their immunogenicity. * **Stability:** Unlike live vaccines, toxoids are highly stable and resistant to heat and humidity.

Q: Which immunoglobulin is inactivated by heating and elevated in helminthic infections?

IgE. **Explanation:** The correct answer is **IgE**. This immunoglobulin is uniquely characterized by its **heat-lability** and its role in Type I hypersensitivity and parasitic defense. **Why IgE is correct:** * **Heat Sensitivity:** IgE is the most heat-labile immunoglobulin. Heating serum to **56°C for 30 to 60 minutes** inactivates IgE by denaturing the Fc portion of the heavy chain, preventing it from binding to mast cells (the Prausnitz-Küstner reaction becomes negative). * **Helminthic Infections:** IgE levels rise significantly during helminthic infestations. It mediates **Antibody-Dependent Cellular Cytotoxicity (ADCC)** by coating the parasite; eosinophils then bind to the IgE via FcεRI receptors and release major basic protein to kill the helminth. **Why other options are incorrect:** * **IgA:** The primary secretory antibody found in colostrum and mucosal surfaces. It is heat-stable and primarily involved in preventing pathogen attachment to mucosal linings. * **IgG:** The most abundant serum antibody and the only one to cross the placenta. It is heat-stable and involved in secondary immune responses and opsonization. * **IgM:** The largest (pentameric) antibody and the first to appear in primary infection. While it is efficient at complement fixation, it is not inactivated by standard heating at 56°C. **High-Yield NEET-PG Pearls:** * **Reaginic Antibody:** IgE is also known as the reaginic antibody. * **Homocytotropism:** IgE has a high affinity for mast cells and basophils. * **Lowest Serum Concentration:** It is the immunoglobulin present in the lowest concentration in normal serum. * **Structure:** It is a monomer with an extra constant domain (CH4) on its heavy chain.

Q: Patients with C5 through C9 complement deficiencies would be most likely to have which of the following infections?

Meningococcal infection. **Explanation:** The complement system is a vital component of innate immunity. Components **C5 through C9** assemble to form the **Membrane Attack Complex (MAC)**. The MAC creates pores in the lipid bilayer of target cells, leading to osmotic lysis. **Why Meningococcal infection is correct:** *Neisseria* species (specifically *N. meningitidis* and *N. gonorrhoeae*) have thin, gram-negative cell walls that make them uniquely susceptible to complement-mediated lysis. Patients with deficiencies in the terminal complement pathway (C5-C9) cannot form the MAC and are therefore predisposed to recurrent, disseminated infections with *Neisseria* species. This is a classic high-yield association in immunology. **Why other options are incorrect:** * **AIDS:** This is caused by the Human Immunodeficiency Virus (HIV), which primarily targets CD4+ T-cells, leading to a collapse of cell-mediated immunity, not a primary complement deficiency. * **Pneumococcal infection:** *Streptococcus pneumoniae* is an encapsulated bacterium. Defense against it primarily requires **opsonization** (C3b) and splenic clearance. Deficiencies in early complement components (C1, C2, C4) or C3 are more likely to lead to pyogenic infections like pneumonia. * **Giardiasis:** This is a protozoal infection of the gut. Defense depends on **Secretory IgA** and T-cell responses. It is commonly seen in patients with Common Variable Immunodeficiency (CVID) or IgA deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **C1, C2, C4 deficiency:** Associated with Immune Complex diseases like **SLE** (Systemic Lupus Erythematosus). * **C3 deficiency:** Most severe; leads to recurrent infections with all types of pyogenic bacteria. * **C1 Esterase Inhibitor deficiency:** Leads to **Hereditary Angioedema** (due to unregulated bradykinin). * **CH50 Assay:** Used to screen for total complement activity; it will be low/zero in C5-C9 deficiencies.

Q: What is the primary function of Toll-like Receptors?

Activation of the immune system. **Explanation:** **Toll-like Receptors (TLRs)** are a class of **Pattern Recognition Receptors (PRRs)** that play a fundamental role in the **innate immune system**. They are strategically located on the surface or in the endosomes of sentinel cells like macrophages and dendritic cells. 1. **Why Option A is correct:** TLRs function by recognizing highly conserved microbial structures known as **Pathogen-Associated Molecular Patterns (PAMPs)**—such as Lipopolysaccharide (LPS) on Gram-negative bacteria (TLR-4) or viral dsRNA (TLR-3). Upon binding, TLRs trigger intracellular signaling cascades (primarily via the **NF-κB pathway**), leading to the production of pro-inflammatory cytokines, interferons, and the upregulation of co-stimulatory molecules. This process effectively **activates the immune system**, bridging innate and adaptive immunity. 2. **Why other options are incorrect:** * **Vasodilation (B):** This is a physiological response mediated by chemical mediators like histamine, nitric oxide, or bradykinin, not a direct function of TLRs. * **Regulation of calcium channels (C):** This is primarily the role of voltage-gated or ligand-gated ion channels in excitable tissues (nerves/muscles). * **Second messenger signaling (D):** While TLRs *utilize* signaling pathways, this is a broad cellular mechanism. Their *primary biological function* is immune activation. **High-Yield Facts for NEET-PG:** * **TLR-4:** Recognizes **LPS** (Endotoxin) of Gram-negative bacteria. * **TLR-3:** Recognizes **double-stranded RNA** (viruses). * **TLR-5:** Recognizes **Flagellin**. * **TLR-7 & 8:** Recognize single-stranded RNA. * **TLR-9:** Recognizes unmethylated **CpG DNA**. * **Adapter Molecule:** Most TLRs (except TLR-3) use **MyD88** for downstream signaling.

Question 1

Which test assesses phagocytic function?

Accepted Answer

Reduction of NBT (Nitroblue tetrazolium test)

Answer

Proliferative response to mitogen

Answer

Serum immunoglobulin assay

Answer

Skin test with purified protein derivative

Question 2

What is the difference between natural killer (NK) cells and cytotoxic T-lymphocytes (CTLs)?

Accepted Answer

Cytotoxic T-lymphocytes lyse IgG-coated target cells.

Answer

Interferon decreases natural killer activity.

Answer

NK cells express CD4.

Answer

NK cells contain azurophilic granules.

Question 3

Which of the following is a HLA class III molecule?

Accepted Answer

TNF-α

Answer

HLA-G

Answer

HLA-H

Answer

None of the above

Question 4

The Tuberculin test is an example of which type of immunological reaction?

Accepted Answer

Cell mediated

Answer

Anaphylaxis mediated

Answer

Antibody mediated

Answer

Immune complex mediated

Question 5

Which of the following statements regarding agammaglobulinemia is FALSE?

Accepted Answer

Normal B-cell precursors in the bone marrow.

Answer

Absence of germinal centers in lymph nodes.

Answer

Normal B-cell precursors in the paracortex and medulla.

Answer

Decreased red pulp in the spleen.

Question 6

What are toxoids?

Accepted Answer

Antigenic and non-toxic

Answer

Antigenic and toxic

Answer

Non-antigenic and toxic

Answer

Non-antigenic and non-toxic

Question 7

Which immunoglobulin is inactivated by heating and elevated in helminthic infections?

Accepted Answer

IgE

Answer

IgA

Answer

IgG

Answer

IgM

Question 8

Regarding Schick's test, which of the following statements is false?

Accepted Answer

A positive test means that the person is immune to diphtheria.

Answer

An erythematous reaction in both arms indicates hypersensitivity.

Answer

Diphtheria antitoxin is given intradermally.

Answer

The test is done to find out the immune status against diphtheria.

Question 9

Patients with C5 through C9 complement deficiencies would be most likely to have which of the following infections?

Accepted Answer

Meningococcal infection

Answer

AIDS

Answer

Pneumococcal infection

Answer

Giardiasis

Question 10

What is the primary function of Toll-like Receptors?

Accepted Answer

Activation of the immune system

Answer

Vasodilation

Answer

Regulation of calcium channels

Answer

Second messenger signaling

Immunology — MCQs

Immunology — MCQs

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