Immunology Practice Questions

Q: All of the following belong to the group of microbial sensors, EXCEPT?

None of the above. ### Explanation The question asks to identify which of the listed options is **not** a microbial sensor. Microbial sensors, collectively known as **Pattern Recognition Receptors (PRRs)**, are essential components of the innate immune system. They detect conserved molecular structures known as Pathogen-Associated Molecular Patterns (PAMPs). **Why "None of the above" is correct:** All three options (TLRs, NLRs, and MDA-5) are well-established classes of PRRs. Since every listed option functions as a microbial sensor, none of them can be excluded. **Analysis of Options:** * **Toll-like Receptors (TLRs):** These are the most well-characterized PRRs. They are transmembrane proteins located either on the cell surface (e.g., TLR-4 for LPS) or in endosomes (e.g., TLR-3, 7, 8, 9 for nucleic acids). * **NOD-like Receptors (NLRs):** These are cytosolic receptors. **NOD1 and NOD2** sense bacterial peptidoglycans, while other NLRs (like NLRP3) are involved in forming the **inflammasome**, which activates Interleukin-1β. * **MDA-5 (Melanoma Differentiation-Associated gene 5):** This belongs to the **RIG-I-like receptor (RLR)** family. It is a cytosolic sensor that specifically detects viral double-stranded RNA (dsRNA), triggering the production of Type I Interferons. **High-Yield Clinical Pearls for NEET-PG:** * **TLR-4** is the specific sensor for **Lipopolysaccharide (LPS)** of Gram-negative bacteria. * **TLR-3** senses dsRNA; **TLR-5** senses Flagellin; **TLR-9** senses unmethylated CpG DNA. * **Mutations in NOD2** are strongly associated with **Crohn’s Disease**. * **Location matters:** TLRs 1, 2, 4, 5, 6 are on the **plasma membrane**; TLRs 3, 7, 8, 9 are **intracellular (endosomal)**. NLRs and RLRs are always **cytosolic**.

Q: The HLA class-III region genes are important elements in which of the following?

Complement system. The Human Leukocyte Antigen (HLA) complex is located on the short arm of **Chromosome 6**. While Class I and II genes are primarily involved in antigen recognition, the **Class III region** is functionally distinct. ### **Why "Complement System" is Correct** The HLA Class III region does not encode for cell-surface molecules involved in antigen presentation. Instead, it encodes several secreted proteins involved in the innate immune response and inflammation. Specifically, it contains genes for: * **Complement components:** C2, C4 (C4A and C4B), and Factor B of the alternative pathway. * **Cytokines:** Tumor Necrosis Factor (TNF-α and TNF-β). * **Heat Shock Proteins (HSP).** ### **Why Other Options are Incorrect** * **A & D (Transplant rejection / Antigen presentation):** These are functions of **HLA Class I** (HLA-A, B, C) and **Class II** (HLA-DR, DQ, DP). Class I molecules present endogenous antigens to CD8+ T-cells, while Class II molecules present exogenous antigens to CD4+ T-cells. Mismatch in these classes is the primary driver of graft rejection. * **C (Immune surveillance):** This is primarily mediated by Natural Killer (NK) cells and T-lymphocytes through the recognition of HLA Class I molecules. ### **High-Yield Clinical Pearls for NEET-PG** * **Gene Locus:** HLA Class III is located *between* the Class I and Class II loci on Chromosome 6. * **Disease Association:** Deficiencies in C4 (encoded in Class III) are strongly associated with **Systemic Lupus Erythematosus (SLE)**. * **Class I vs. II Structure:** Class I has one heavy chain and a $\beta_2$-microglobulin (Chrom. 15); Class II has two polypeptide chains ($\alpha$ and $\beta$), both encoded within the MHC locus.

Q: A 19-year-old college student who works part-time in a pediatric AIDS clinic develops a rash. If her blood is drawn and tested for specific antibody to the chickenpox virus (varicella-zoster), which of the following antibody classes would you expect to find if she is immune to chickenpox?

IgG. **Explanation:** The question tests the understanding of immunoglobulin kinetics and the markers of long-term immunity. **Why IgG is the correct answer:** **IgG** is the most abundant immunoglobulin in the serum and is the primary antibody responsible for **long-term immunity** following an infection or vaccination. In the context of Varicella-Zoster Virus (VZV), once the primary infection (chickenpox) resolves, the body maintains memory B cells that produce IgG. If a person is "immune," it implies they have pre-existing protective antibodies (IgG) that can neutralize the virus upon re-exposure. **Why the other options are incorrect:** * **IgA:** This is the primary secretory antibody found in mucous membranes (tears, saliva, colostrum). While it provides local mucosal defense, it is not the standard systemic marker used to define clinical immunity to chickenpox. * **IgM:** This is the first antibody produced during an **acute/primary infection**. Its presence indicates a current or very recent infection, not long-term immunity. * **IgD:** This is primarily found on the surface of B cells as an antigen receptor; it has no significant role in protective immunity or diagnostic serology for VZV. **NEET-PG High-Yield Pearls:** * **IgG:** The only antibody that crosses the placenta (provides passive immunity to the fetus). It has the longest half-life (~23 days). * **IgM:** The largest antibody (pentamer) and the best at complement fixation. It does not cross the placenta. * **VZV Serology:** In a clinical setting, an **IgG ELISA** is the standard test to screen healthcare workers for immunity. If IgG is negative, the individual is susceptible and requires vaccination. * **Tzanck Smear:** Used for rapid diagnosis of VZV/HSV, showing **multinucleated giant cells** with Cowdry type A inclusion bodies.

Q: Which of the following is the most potent antigen-presenting cell?

Dendritic cells. **Explanation:** Antigen-presenting cells (APCs) are specialized cells that capture antigens, process them into peptides, and present them via MHC molecules to T cells. While several cells can perform this function, **Dendritic Cells (DCs)** are considered the most potent and efficient APCs. **Why Dendritic Cells are the correct answer:** Dendritic cells are the only APCs capable of activating **naive T cells**, making them the primary initiators of the adaptive immune response. They possess high levels of MHC Class II molecules and potent co-stimulatory signals (B7-1 and B7-2). Their unique ability to perform **cross-presentation** (presenting exogenous antigens on MHC Class I to CD8+ T cells) further cements their status as the "professional" APC. **Analysis of Incorrect Options:** * **Macrophages (Option C):** While they are professional APCs, their primary role is phagocytosis and killing of microbes. They present antigens to **already activated** effector T cells to enhance their own microbicidal activity. * **B Cells (Option D):** These are professional APCs that present antigens to Helper T cells (CD4+) to receive signals for antibody production. They are efficient at capturing soluble antigens via surface immunoglobulins but are less potent than DCs in initiating primary responses. * **Fibroblasts (Option A):** These are "non-professional" or "atypical" APCs. They can express MHC Class II only under specific inflammatory conditions (e.g., stimulation by IFN-gamma) but lack the constitutive machinery of professional APCs. **NEET-PG High-Yield Pearls:** * **Langerhans cells** are specialized dendritic cells found in the epidermis (contain Birbeck granules). * **Follicular Dendritic Cells (FDCs)** are found in germinal centers; unlike regular DCs, they lack MHC II and trap antigens in the form of immune complexes for B cell memory. * **Professional APCs** include Dendritic cells, Macrophages, and B cells.

Q: T-helper cells (TH2) initiate which type of immunity?

Humoral immunity. ### Explanation The differentiation of CD4+ T-helper (Th) cells into specific subsets determines the nature of the immune response. **Why Humoral Immunity is Correct:** **Th2 cells** are the primary drivers of **Humoral Immunity** (antibody-mediated). Upon activation, Th2 cells secrete cytokines such as **IL-4, IL-5, IL-10, and IL-13**. * **IL-4** induces B-cell proliferation and isotype switching to **IgE**. * **IL-5** activates eosinophils and promotes switching to **IgA**. This pathway is essential for defense against extracellular parasites (helminths) and is central to allergic reactions. **Analysis of Incorrect Options:** * **Option A (Cell-mediated immunity):** This is primarily mediated by **Th1 cells**. Th1 cells secrete **IFN-γ and IL-2**, which activate macrophages and promote intracellular pathogen clearance. * **Option C (Activation of cytotoxic T-cells):** This is also a feature of the Th1 response. IL-2 produced by Th1 cells provides the necessary signal for the proliferation of CD8+ Cytotoxic T-lymphocytes (CTLs), which kill virally infected or tumor cells. **NEET-PG High-Yield Pearls:** * **The "Rule of 4":** IL-4 induces Th2; IL-12 (from macrophages) induces Th1. * **Cross-regulation:** IFN-γ (Th1) inhibits Th2 proliferation, while IL-10 (Th2) inhibits Th1 cytokine production. * **Clinical Correlation:** * **Tuberculoid Leprosy:** Strong **Th1** response (contained infection). * **Lepromatous Leprosy:** Predominant **Th2** response (disseminated infection due to ineffective humoral response against intracellular *M. leprae*). * **Mnemonic:** Th**1** is for **1**nside (intracellular/cell-mediated); Th**2** is for **2** (B-cells/antibodies/humoral).

Q: Which of the following statements best describes a hapten?

Penicillin is a hapten. **Explanation:** A **hapten** is a small molecule that is **antigenic but not immunogenic**. This means it can react specifically with antibodies once they are formed, but it cannot induce an immune response on its own because of its low molecular weight. To become immunogenic, a hapten must bind to a larger protein molecule called a **carrier**. **Why Option B is correct:** **Penicillin** is a classic clinical example of a hapten. By itself, the penicillin molecule is too small to trigger an immune response. However, in susceptible individuals, penicillin (or its degradation products like penicilloic acid) binds to endogenous serum proteins (carriers). This hapten-carrier complex is recognized as foreign, leading to the production of IgE antibodies and potentially causing Type I hypersensitivity (anaphylaxis). **Analysis of Incorrect Options:** * **Option A:** Haptens cannot activate T cells or B cells independently. They lack the complexity and size required to cross-link receptors or be processed for presentation. * **Option C:** This is incorrect. While haptens cannot *induce* an immune response, they **do react** with pre-formed specific antibodies (Antigenicity is present). * **Option D:** Haptens do not bind directly to MHC molecules. Only processed peptide fragments (from immunogens) are presented via MHC to T-cell receptors. **NEET-PG High-Yield Pearls:** * **Landsteiner’s Experiment:** Karl Landsteiner discovered haptens by showing that small chemical groups (like dinitrobenzene) only caused antibody production when conjugated to a protein. * **Clinical Relevance:** Many drugs (Hydralazine, Procainamide) and toxins (Urushiol from Poison Ivy) act as haptens, leading to drug-induced lupus or contact dermatitis. * **Formula:** Hapten + Carrier = Complete Antigen (Immunogen).

Q: Tube agglutination test is used for the serological diagnosis of which condition?

Enteric fever. **Explanation:** The **Tube Agglutination Test** is a classical serological technique where antibodies in the patient's serum react with particulate antigens (like bacteria) in a test tube, resulting in visible clumping or sedimentation. **Why Enteric Fever is Correct:** The most iconic example of a tube agglutination test is the **Widal Test**, used to diagnose Enteric (Typhoid) fever. It detects antibodies against the *Salmonella Typhi* and *Paratyphi* antigens (O and H). In this test, serial dilutions of the patient's serum are mixed with specific bacterial suspensions in Dreyer’s (for H antigen) or Felix (for O antigen) tubes. A rising titer or a significant single titer (usually >1:80 for O and >1:160 for H) indicates infection. **Why Other Options are Incorrect:** * **Rabies Antigen:** Diagnosis is typically made via **Direct Fluorescent Antibody (DFA)** testing of brain tissue or skin biopsies, or by detecting Negri bodies. Agglutination is not used. * **HIV:** Screening is performed using **ELISA** (an enzyme immunoassay), and confirmation is done via **Western Blot** (detecting specific proteins) or PCR for viral load. * **Syphilis:** Screening uses **flocculation tests** (a variation of precipitation, not tube agglutination) like **VDRL** and **RPR**. Confirmation uses specific treponemal tests like TPHA or FTA-ABS. **High-Yield Clinical Pearls for NEET-PG:** * **Standard Tube Agglutination Tests:** Widal (Enteric fever), Standard Agglutination Test (SAT) for **Brucellosis**, and the Weil-Felix test (for **Rickettsial** infections). * **Prozone Phenomenon:** A false-negative result in tube agglutination due to antibody excess; solved by diluting the serum. * **Widal Timing:** It usually becomes positive only after the **first week** of fever (best results in the 2nd and 3rd weeks).

Q: Which type of antigen is found in the lens?

Sequestered antigens. **Explanation:** The correct answer is **Sequestered antigens**. **1. Why Sequestered Antigens is correct:** Sequestered antigens are "hidden" antigens located in anatomically isolated sites (immunologically privileged sites) that do not come into contact with the immune system during embryonic development. Because the developing immune system never "sees" them, it fails to develop self-tolerance towards them. The **lens of the eye**, sperm (testis), and myelin basic protein (CNS) are classic examples. If these antigens are released into circulation due to trauma or infection, the immune system perceives them as foreign, leading to an autoimmune response (e.g., **Endophthalmitis phacoanaphylactica** after lens injury). **2. Why the other options are incorrect:** * **Cross antigens:** These are antigens found in different species that share similar epitopes. An immune response against one can damage the other (e.g., Molecular mimicry in Rheumatic fever where Streptococcal M-protein crosses with cardiac myosin). * **Heterophile antigens:** A type of cross-reactive antigen found in unrelated species (phylogenetically distant). Example: Forssman antigen or the antigens used in the Weil-Felix test. * **Isoantigens (Alloantigens):** These are antigens present in some but not all members of the same species. Examples include ABO blood group antigens and HLA antigens, which are crucial in blood transfusions and organ transplants. **Clinical Pearls for NEET-PG:** * **Sympathetic Ophthalmia:** A high-yield clinical scenario where trauma to one eye releases sequestered uveal antigens, leading to an autoimmune attack on the contralateral (uninjured) eye. * **Immunological Privilege:** Maintained by physical barriers (like the blood-brain barrier) and local immunosuppressive microenvironments. * **Other Sequestered Sites:** Testis (Sperm), Brain (MBP), and Thyroglobulin (though less strictly sequestered than the lens).

Q: Microcytotoxicity testing is used for which of the following purposes?

Tissue typing. **Explanation:** **Microcytotoxicity testing** (also known as the **Complement-Dependent Cytotoxicity or CDC assay**) is the gold standard method for **HLA (Human Leukocyte Antigen) typing** and cross-matching before organ transplantation. 1. **Why the correct answer is right:** The test involves adding specific anti-HLA antibodies to a patient’s lymphocytes in a microtiter plate. If the antibodies recognize the HLA antigens on the cell surface, they bind and activate the **complement system**, leading to cell membrane damage (lysis). A vital dye (like trypan blue or eosin) is then added; damaged cells take up the dye, while viable cells remain clear. This allows for precise **Tissue Typing** to ensure donor-recipient compatibility. 2. **Why the incorrect options are wrong:** * **Drug allergy assessment:** This is typically done via skin prick tests, patch tests, or measuring IgE levels (RAST), not by lymphocyte lysis. * **Infection susceptibility:** This depends on innate and adaptive immunity markers (like CD4 counts or immunoglobulin levels), not HLA-specific cytotoxicity. * **Substance toxicity:** General toxicity is assessed via *in vitro* cell viability assays or animal models, which do not rely on antigen-antibody-complement interactions. **High-Yield Clinical Pearls for NEET-PG:** * **Terasaki Plates:** The specialized microtiter plates used for this test. * **Cross-matching:** A "Positive Cross-match" (cell death) is a contraindication for transplantation as it predicts hyperacute rejection. * **HLA Class I** (A, B, C) is tested using T-lymphocytes, while **HLA Class II** (DR, DQ) is tested using B-lymphocytes.

Question 1

All of the following belong to the group of microbial sensors, EXCEPT?

Accepted Answer

None of the above

Answer

Toll-like receptors (TLRs)

Answer

NOD-like receptors (NLRs)

Answer

Melanoma differentiation-associated gene 5 (MDA-5)

Question 2

The HLA class-III region genes are important elements in which of the following?

Accepted Answer

Complement system

Answer

Transplant rejection phenomenon

Answer

Immune surveillance

Answer

Antigen presentation and elimination

Question 3

A 19-year-old college student who works part-time in a pediatric AIDS clinic develops a rash. If her blood is drawn and tested for specific antibody to the chickenpox virus (varicella-zoster), which of the following antibody classes would you expect to find if she is immune to chickenpox?

Accepted Answer

IgG

Answer

IgA

Answer

IgM

Answer

IgD

Question 4

Which of the following is the most potent antigen-presenting cell?

Accepted Answer

Dendritic cells

Answer

Fibroblasts

Answer

Macrophages

Answer

B Cells

Question 5

T-helper cells (TH2) initiate which type of immunity?

Accepted Answer

Humoral immunity

Answer

Cell-mediated immunity

Answer

Immunity by activation of cytotoxic T-cells

Answer

None of the above

Question 6

Which of the following statements best describes a hapten?

Accepted Answer

Penicillin is a hapten

Answer

Haptens activate T cells

Answer

Haptens do not react with specific antibody

Answer

Haptens bind the major histocompatibility complex

Question 7

Tube agglutination test is used for the serological diagnosis of which condition?

Accepted Answer

Enteric fever

Answer

Rabies antigen

Answer

HIV

Answer

Syphilis

Question 8

A 25-year-old female presents with pain and tenosynovitis of the wrists and ankles, and arthralgias of other joints. She notes two prior episodes similar to the present one. She had her menstrual period during the previous week. Physical examination reveals ulcerated lesions overlying the wrists and ankles. These symptoms are likely due to deficiency of which of the following?

Accepted Answer

Complement (C6-C8) components

Answer

C1 esterase inhibitor

Answer

ciliary function

Answer

Endothelial adhesion molecules

Question 9

Which type of antigen is found in the lens?

Accepted Answer

Sequestered antigens

Answer

Cross antigens

Answer

Heterophile antigens

Answer

Isoantigens

Question 10

Microcytotoxicity testing is used for which of the following purposes?

Accepted Answer

Tissue typing

Answer

Drug allergy assessment

Answer

Infection susceptibility determination

Answer

Substance toxicity evaluation

Immunology — MCQs

Immunology — MCQs

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