Immunology Practice Questions

Q: Nude mice are able to accept xenografts because they lack:

T cells. **Explanation:** The **Nude Mouse** is a laboratory strain characterized by a genetic mutation in the *FOXN1* gene. This mutation results in two primary phenotypic features: the absence of hair (hence "nude") and a **congenital absence of the thymus (thymic aplasia)**. 1. **Why T cells is correct:** The thymus is the primary site for T-cell maturation. Without a functional thymus, nude mice cannot produce mature, functional T lymphocytes. Since **T cells (specifically Cytotoxic T cells)** are the primary mediators of Cell-Mediated Immunity (CMI) and are responsible for the rejection of foreign tissue (allografts and xenografts), their absence allows these mice to accept grafts even from different species (xenografts). 2. **Why other options are incorrect:** * **B-cells:** Nude mice possess a normal population of B-cells. While their antibody response to T-dependent antigens is impaired, the B-cells themselves are present. * **NK cells:** Natural Killer (NK) cell levels are actually **normal or even elevated** in nude mice. They provide a baseline level of innate immunity against tumors and infections. * **LAK cells:** Lymphokine-activated killer cells are derived primarily from NK cells in the presence of IL-2. Since NK cells are present, the potential for LAK cell activity remains. **High-Yield Clinical Pearls for NEET-PG:** * **DiGeorge Syndrome:** The human clinical counterpart to the nude mouse, characterized by thymic hypoplasia and T-cell deficiency. * **Xenograft:** A transplant between different species (e.g., pig to human). * **Role in Research:** Nude mice are extensively used in oncology to grow human tumor explants (xenografts) because their lack of T-cell-mediated rejection prevents them from destroying the human cancer cells.

Q: Which of the following is the major cytokine produced by TH2 helper T cells?

IL-4. **Explanation:** The differentiation of CD4+ T-helper (TH) cells into specific subsets is a high-yield topic in immunology. **TH2 cells** are primarily responsible for the humoral immune response, defense against helminthic parasites, and the mediation of allergic reactions. **Why IL-4 is the correct answer:** **IL-4** is the signature cytokine of the TH2 lineage. It serves two critical functions: 1. **Differentiation:** It acts as the primary inducer for naive T cells to differentiate into TH2 cells (via STAT6 and GATA3 expression). 2. **Effector Function:** It promotes B-cell proliferation and induces **isotype switching to IgE**, which is essential for mast cell activation and combating parasites. TH2 cells also produce IL-5 (eosinophil activation) and IL-13. **Analysis of Incorrect Options:** * **A. IFN-γ:** This is the hallmark cytokine of **TH1 cells**. It activates macrophages and promotes cell-mediated immunity against intracellular pathogens (e.g., *M. tuberculosis*). * **B. IL-17:** This is produced by **TH17 cells**. It plays a key role in recruiting neutrophils and is involved in the defense against extracellular bacteria and fungi, as well as autoimmune diseases. * **D. IL-12:** This is not produced by T cells; it is secreted by **macrophages and dendritic cells** to stimulate the differentiation of naive T cells into the **TH1** subset. **High-Yield Clinical Pearls for NEET-PG:** * **TH1 vs. TH2 Balance:** TH1 (IFN-γ) and TH2 (IL-4) cytokines are mutually inhibitory. IFN-γ inhibits TH2 proliferation, while IL-4 and IL-10 inhibit TH1 differentiation. * **Transcription Factors:** Remember the "Master Regulators": **T-bet** for TH1, **GATA-3** for TH2, and **RORγt** for TH17. * **Leprosy Link:** Tuberculoid leprosy is associated with a strong **TH1** response (contained), while Lepromatous leprosy is associated with a **TH2** response (disseminated).

Q: Which immunoglobulin lacks a hinge region?

IgE. **Explanation:** The structure of an immunoglobulin molecule consists of two heavy chains and two light chains. In certain classes of antibodies, a flexible **hinge region** (rich in proline and cysteine) exists between the $C_H1$ and $C_H2$ domains of the heavy chains, allowing the Fab arms to move freely to bind antigens. **Why IgE is the Correct Answer:** Immunoglobulins are divided based on their heavy chain structure. **IgE and IgM** do not have a hinge region. Instead of a hinge, they possess an **extra constant domain ($C_H4$)** in their heavy chains. This additional domain provides the structural integrity that the hinge region would otherwise provide. Since IgE is the only option listed among the two that lack a hinge, it is the correct choice. **Analysis of Incorrect Options:** * **IgA, IgG, and IgD:** These three isotopes possess a distinct **hinge region** between the $C_H1$ and $C_H2$ domains. They consist of only three constant domains ($C_H1, C_H2, C_H3$). The hinge region provides these antibodies with segmental flexibility, which is crucial for binding to epitopes spaced at varying distances. **High-Yield Facts for NEET-PG:** * **Mnemonic:** "E and M have an Extra domain" (IgE and IgM have 4 constant domains and no hinge). * **IgE:** Mediates Type I Hypersensitivity (allergic reactions) and provides immunity against helminthic parasites by activating mast cells and basophils. * **IgM:** The largest antibody (pentamer) and the first to appear in a primary immune response. * **IgG:** The only antibody that crosses the placenta and is the most abundant in serum. * **IgA:** The primary secretory antibody found in colostrum, saliva, and tears.

Q: Which of the following features is common to both cytotoxic T cells and NK cells?

Effective against virus-infected cells. **Explanation:** The core similarity between **Cytotoxic T cells (CD8+)** and **Natural Killer (NK) cells** lies in their primary function: identifying and destroying abnormal host cells, particularly those infected by **viruses** or those that have undergone malignant transformation [1]. 1. **Why Option C is correct:** Both cell types utilize the **Perforin-Granzyme pathway** to induce apoptosis in target cells [1]. While their recognition mechanisms differ (CD8+ T cells are MHC-restricted, whereas NK cells are part of the innate system and respond to "missing self" or MHC-I downregulation), their ultimate physiological objective is the clearance of intracellular pathogens, specifically viruses [1]. 2. **Why other options are incorrect:** * **Option A:** Antibody synthesis is the exclusive function of **B-lymphocytes** (specifically plasma cells) [3]. Neither T cells nor NK cells produce immunoglobulins. * **Option B:** While NK cells can participate in Antibody-Dependent Cellular Cytotoxicity (ADCC) via CD16, they do not *require* antibodies for their primary action. Cytotoxic T cells recognize antigens directly via the TCR-MHC I complex [2]. * **Option D:** Recognition of antigens with **HLA Class II** is a feature of **Helper T cells (CD4+)**. Cytotoxic T cells (CD8+) are restricted to **HLA Class I** [2]. NK cells do not require HLA presentation to act; in fact, they often kill cells that lack HLA Class I expression [1]. **High-Yield NEET-PG Pearls:** * **Mnemonic:** Rule of 8 (CD8 × MHC 1 = 8; CD4 × MHC 2 = 8). * **NK Cell Markers:** CD16 (Fc\gammaRIII) and CD56 are the definitive markers for identification. * **Large Granular Lymphocytes (LGLs):** Morphologically, NK cells are identified as LGLs in peripheral blood smears. * **Cytokine Link:** IL-2 and IL-12 are potent activators of NK cell activity [1].

Q: What is the earliest immunoglobulin synthesized by the fetus?

IgM. **Explanation:** The correct answer is **IgM**. **Why IgM is correct:** Immunoglobulin M (IgM) is the first antibody class to be synthesized by the fetus, starting around the **20th week of gestation**. Unlike IgG, IgM is a pentamer with a high molecular weight, which prevents it from crossing the placental barrier. Therefore, any IgM detected in the cord blood or neonatal serum is of fetal origin and serves as a critical diagnostic marker for **intrauterine (congenital) infections** (e.g., TORCH infections). **Why other options are incorrect:** * **IgG:** While IgG is the most abundant immunoglobulin in the fetus, it is **passively acquired** from the mother via the placenta (starting at 12 weeks). The fetus only begins significant endogenous production of IgG after birth. * **IgA:** Secretory IgA is primarily produced after birth and is supplied to the neonate through colostrum and breast milk. Fetal synthesis of IgA is negligible. * **IgE:** This is involved in type I hypersensitivity and parasitic infections; it is produced in trace amounts and is not the earliest to develop. **High-Yield Clinical Pearls for NEET-PG:** * **IgG:** The only immunoglobulin that **crosses the placenta** (provides natural passive immunity). * **IgM:** The first antibody produced in response to an **acute infection** and the first to be synthesized by the fetus. * **IgA:** The most abundant antibody in **body secretions** (tears, saliva, breast milk). * **Order of synthesis:** IgM is followed by IgG and then IgA. * **Diagnostic Significance:** Elevated cord blood IgM indicates a congenital infection because maternal IgM cannot cross the placenta.

Q: Administration of the DPT vaccine (diphtheria toxoid, pertussis vaccine, and tetanus toxoid) would stimulate which of the following types of immunity?

Artificial active. ### Explanation The correct answer is **Artificial active immunity**. **1. Why "Artificial Active" is correct:** Immunity is classified based on how it is acquired (Natural vs. Artificial) and how the body responds (Active vs. Passive). * **Active:** The individual’s own immune system is stimulated to produce antibodies and memory cells. In the DPT vaccine, the antigens (toxoids and killed bacteria) trigger an endogenous immune response. * **Artificial:** The exposure to the antigen is intentional and medical, rather than through a clinical or subclinical infection. Therefore, any **vaccination** (like DPT) falls under artificial active immunity. **2. Why the other options are incorrect:** * **Adoptive (A):** This involves the transfer of immune cells (usually lymphocytes) from a donor to a recipient (e.g., bone marrow transplant or certain cancer immunotherapies). * **Artificial Passive (C):** This involves the administration of pre-formed antibodies (immunoglobulins) to provide immediate protection. Examples include Tetanus Immunoglobulin (TIG) or Anti-rabies serum (ARS). * **Natural Active (D):** This occurs when a person is exposed to a live pathogen in the environment and develops the disease (e.g., recovering from a natural Measles or Chickenpox infection). **3. High-Yield Clinical Pearls for NEET-PG:** * **Active Immunity:** Slow onset but long-lasting (due to memory cells). * **Passive Immunity:** Immediate onset but short-acting (no memory cells). * **DPT Components:** Diphtheria and Tetanus are **toxoids** (exotoxins modified to lose toxicity but retain antigenicity), while the Pertussis component in the standard DPT is **killed (whole-cell)** bacteria. * **Combined Immunity:** In cases of a "dirty" wound in an unimmunized person, both Tetanus Toxoid (Active) and TIG (Passive) are given at different sites; this is known as **simultaneous immunization**.

Q: Which cells recognize MHC Class I molecules?

CD8 T cells. **Explanation:** The recognition of Major Histocompatibility Complex (MHC) molecules is governed by the **MHC Restriction Rule**, which ensures that the immune system responds appropriately to different types of antigens. **Why CD8 T cells are correct:** CD8+ T cells (Cytotoxic T lymphocytes) specifically recognize antigens presented on **MHC Class I** molecules. This interaction is stabilized by the CD8 co-receptor, which binds to the α3 domain of the MHC I heavy chain. Since MHC Class I is expressed on almost all nucleated cells, this allows CD8 T cells to monitor and destroy any cell infected by viruses or transformed by cancer (endogenous antigens). **Why other options are incorrect:** * **CD4 T cells:** These cells recognize antigens presented on **MHC Class II** molecules. The CD4 co-receptor binds to the β2 domain of MHC II. This interaction is essential for "helper" functions, such as activating B cells and macrophages. * **Dendritic cells & Macrophages:** These are **Professional Antigen-Presenting Cells (APCs)**. While they *express* both MHC I and MHC II to present antigens to T cells, they are not the cells that "recognize" the MHC molecule in the context of initiating an effector response; rather, they are the presenters. **High-Yield NEET-PG Pearls:** 1. **The Rule of 8:** A simple mnemonic to remember MHC restriction: * 4 (CD4) × 2 (MHC II) = **8** * 8 (CD8) × 1 (MHC I) = **8** 2. **Structure:** MHC I consists of a heavy chain and a **β2-microglobulin**; MHC II consists of α and β chains. 3. **Cross-presentation:** Dendritic cells have the unique ability to present exogenous antigens on MHC I to activate CD8 T cells (priming).

Q: Graves disease is an example of which of the following immunologic processes?

Autoimmune disease associated with HLA gene DR3. **Explanation:** **Graves’ Disease** is an organ-specific autoimmune disorder characterized by hyperthyroidism due to the production of **Thyroid Stimulating Immunoglobulins (TSI)**. These are Type II Hypersensitivity antibodies that bind to and activate the TSH receptor, mimicking the action of TSH. 1. **Why Option B is Correct:** Susceptibility to Graves’ disease is strongly linked to the **HLA-DR3** gene (and HLA-B8). The Human Leukocyte Antigen (HLA) system plays a crucial role in antigen presentation; specific alleles like DR3 are thought to inefficiently clear self-reactive T-cells or present thyroid-derived peptides in a way that triggers an autoimmune response. 2. **Analysis of Incorrect Options:** * **Option A (HLA-B27):** This is classically associated with **Seronegative Spondyloarthropathies**, remembered by the mnemonic **PAIR**: **P**soriatic arthritis, **A**nkylosing spondylitis (strongest association), **I**nflammatory bowel disease, and **R**eactive arthritis. * **Option C (HLA-DR2):** This allele is associated with Multiple Sclerosis, Hay fever, Goodpasture syndrome, and SLE (along with DR3). It is not primarily linked to Graves'. * **Option D (HLA-DR4):** This is the high-yield association for **Rheumatoid Arthritis** ("Four-thritis") and Type 1 Diabetes Mellitus (which is also linked to DR3). **High-Yield Clinical Pearls for NEET-PG:** * **HLA-DR3 Associations:** Graves’ disease, SLE, Myasthenia Gravis, and Type 1 Diabetes Mellitus. * **HLA-DR4 Associations:** Rheumatoid Arthritis, Pemphigus Vulgaris, and Type 1 Diabetes Mellitus. * **Graves’ Triad:** Hyperthyroidism (Goiter), Exophthalmos (Proptosis), and Pretibial Myxedema. * **Pathogenesis:** It is a **Type II (Cytotoxic) Hypersensitivity** reaction, specifically the "Stimulatory" subtype.

Question 1

Nude mice are able to accept xenografts because they lack:

Accepted Answer

T cells

Answer

B-cells

Answer

NK cells

Answer

LAK cells

Question 2

Which of the following is the major cytokine produced by TH2 helper T cells?

Accepted Answer

IL-4

Answer

IFN-γ

Answer

IL-17

Answer

IL-12

Question 3

Which immunoglobulin lacks a hinge region?

Accepted Answer

IgE

Answer

IgA

Answer

IgG

Answer

IgD

Question 4

Which of the following molecules are expressed by a TH1 T-cell?

Accepted Answer

Interleukin-2, Interleukin-3, Interferon-gamma and Tumor Necrosis Factor-alpha

Answer

Interleukin-2, Interleukin-3, Interleukin-4 and Tumor Necrosis Factor-alpha

Answer

Interleukin-2, Interleukin-3, Interferon-gamma and Endotoxin

Answer

Interleukin-2, Interleukin-3, Interleukin-10 and Endotoxin

Question 5

Which of the following features is common to both cytotoxic T cells and NK cells?

Accepted Answer

Effective against virus-infected cells

Answer

Synthesize antibodies

Answer

Require antibodies to be present for action

Answer

Recognize antigen in association with HLA class II markers

Question 6

What is the earliest immunoglobulin synthesized by the fetus?

Accepted Answer

IgM

Answer

IgA

Answer

IgG

Answer

IgE

Question 7

Anaphylaxis refers to which of the following?

Accepted Answer

Severe reaction following the injection of protein solutions in a sensitized individual

Answer

Severe reaction following primary injection of protein solutions

Answer

State of immunity developed by repeated injections of any foreign substance

Answer

Severe reaction resulting from sensitivity to common allergens

Question 8

Administration of the DPT vaccine (diphtheria toxoid, pertussis vaccine, and tetanus toxoid) would stimulate which of the following types of immunity?

Accepted Answer

Artificial active

Answer

Adoptive

Answer

Artificial passive

Answer

Natural active

Question 9

Which cells recognize MHC Class I molecules?

Accepted Answer

CD8 T cells

Answer

CD4 T cells

Answer

Dendritic cells

Answer

Macrophages

Question 10

Graves disease is an example of which of the following immunologic processes?

Accepted Answer

Autoimmune disease associated with HLA gene DR3

Answer

Autoimmune disease associated with HLA gene B27

Answer

Immune deficiency associated with HLA gene DR2

Answer

Immune deficiency associated with HLA gene DR4

Immunology — MCQs

Immunology — MCQs

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