Immunology Practice Questions

Q: The Postzone phenomenon is seen in which condition?

Antigen excess. The **Postzone phenomenon** occurs during an antigen-antibody reaction when there is a relative **excess of antigen**. This prevents the formation of large, visible lattices (precipitates or agglutinates), leading to a false-negative result despite the presence of antibodies. ### Explanation of Options: * **A. Antigen excess (Correct):** In the postzone, the amount of antigen is so high that each antibody molecule is saturated by individual antigens. This prevents the cross-linking required to form a stable lattice structure. * **B. Antibody excess:** This is known as the **Prozone phenomenon**. Here, every antigenic site is covered by an excess of antibodies, preventing the bridging of antigens and resulting in a false-negative test. This is clinically significant in secondary syphilis (VDRL/RPR) and Brucellosis. * **C. Equivalence zone:** This is the ideal ratio where the concentration of antigen and antibody is optimal for maximum lattice formation and visible precipitation. ### High-Yield Clinical Pearls for NEET-PG: * **Marrack’s Lattice Hypothesis:** Explains that for a visible reaction, both antigen and antibody must be multivalent and present in optimal proportions. * **Clinical Significance:** If a clinician suspects a disease (e.g., Syphilis) but the screening test is negative, the serum should be **diluted** to overcome the Prozone effect. * **Prozone vs. Postzone:** Prozone (Antibody excess) is more common in clinical practice than Postzone (Antigen excess). * **Key mnemonic:** **P**rozone = **P**receding (excess antibody at the start); **P**ostzone = **P**ost-equivalence (excess antigen added).

Q: Toll-like receptors are expressed by which of the following cell types?

All the above. **Explanation:** **Toll-like receptors (TLRs)** are a class of **Pattern Recognition Receptors (PRRs)** that play a critical role in the innate immune system. They recognize highly conserved structural motifs known as **Pathogen-Associated Molecular Patterns (PAMPs)**, such as LPS, flagellin, and viral double-stranded RNA. 1. **Why "All the above" is correct:** While TLRs are most famously associated with professional **sentinel cells** like **Macrophages** and Dendritic cells (which use them to trigger phagocytosis and cytokine release), they are not exclusive to immune cells. * **Epithelial cells** (e.g., in the gut and lungs) express TLRs to act as a first line of defense at mucosal barriers. * **Fibroblasts** express TLRs to sense tissue injury and infection, contributing to the inflammatory response and wound healing. * They are also found on B-cells, neutrophils, and even certain endothelial cells. 2. **Analysis of Options:** * **Option A (Macrophages):** Correct, but incomplete. They express a wide array of TLRs (like TLR4 for LPS) to initiate systemic inflammation. * **Option B (Epithelial cells):** Correct, but incomplete. They use TLRs to maintain barrier integrity and signal the underlying immune system. * **Option C (Fibroblasts):** Correct, but incomplete. They produce chemokines upon TLR activation to recruit leukocytes to the site of infection. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** TLRs are found on the **cell surface** (TLR 1, 2, 4, 5, 6) or in **endosomes** (TLR 3, 7, 8, 9). * **TLR-4:** Specifically recognizes **Lipopolysaccharide (LPS)** of Gram-negative bacteria. * **TLR-3:** Recognizes **dsRNA** (viral). * **TLR-5:** Recognizes **Flagellin**. * **Adapter Protein:** Most TLRs (except TLR3) use the **MyD88** signaling pathway to activate **NF-κB**, leading to cytokine production.

Q: Which of the following cell surface markers is required for lysis of IgG-coated target cells (antibody-dependent, cell-mediated cytotoxicity, or ADCC) by natural killer cells?

CD16. ### Explanation **Correct Option: B (CD16)** Natural Killer (NK) cells mediate **Antibody-Dependent Cell-mediated Cytotoxicity (ADCC)** by recognizing and binding to target cells coated with IgG antibodies. This process is specifically mediated by **CD16**, which is a low-affinity receptor for the **Fc portion of IgG (FcγRIII)**. When CD16 binds to the Fc region of the antibody attached to a target cell (e.g., a virus-infected or tumor cell), it triggers the NK cell to release perforins and granzymes, leading to apoptosis of the target. **Analysis of Incorrect Options:** * **CD3 (Option A):** This is a definitive marker for **T-lymphocytes**. It is part of the T-cell receptor (TCR) complex involved in signal transduction. NK cells are "null cells" (CD3 negative). * **CD19 (Option C):** This is a major surface marker for **B-lymphocytes**, present from the pro-B cell stage until just before differentiation into plasma cells. It plays a role in B-cell activation. * **CD21 (Option D):** Also known as Complement Receptor 2 (CR2), it is found on **B-cells**. It is the receptor for the C3d complement fragment and notably serves as the attachment site for the **Epstein-Barr Virus (EBV)**. **High-Yield Clinical Pearls for NEET-PG:** * **NK Cell Markers:** The most specific markers for identifying NK cells in flow cytometry are **CD56** (adhesion molecule) and **CD16**. * **MHC-I Relationship:** NK cells kill cells that lack **MHC Class I** expression (the "Missing Self" hypothesis), a common tactic used by viruses and tumors to evade Cytotoxic T-cells (CD8+). * **ADCC Mechanism:** Besides NK cells, other cells capable of ADCC include macrophages, neutrophils, and eosinophils (the latter specifically against helminths via IgE).

Q: MHC class II antigens are located on which of the following cells?

Dendritic cells. ### Explanation **MHC Class II** molecules are essential for the exogenous pathway of antigen presentation. They are constitutively expressed only on **Professional Antigen-Presenting Cells (pAPCs)**. **Why Dendritic Cells (Option B) is Correct:** Dendritic cells are the most potent professional APCs. Their primary role is to capture exogenous antigens, process them into peptides, and present them via **MHC Class II** molecules to **CD4+ T-helper cells**. Other professional APCs include B-cells and Macrophages. **Why Other Options are Incorrect:** * **Platelets (Option A) & RBCs (Option C):** MHC molecules are generally expressed on nucleated cells. Since RBCs and platelets are non-nucleated, they lack MHC expression. Specifically, RBCs lack MHC Class I, which is why they are not targets for cytotoxic T-cells (important in malaria and blood transfusions). * **T-cells (Option D):** Resting T-cells express **MHC Class I** (found on all nucleated cells) but do *not* express MHC Class II. However, human T-cells can express MHC Class II only upon **activation**. In the context of standard medical exams, T-cells are categorized as recipients of MHC presentation, not the primary carriers of MHC II. --- ### High-Yield Clinical Pearls for NEET-PG: * **MHC Restriction:** MHC Class I presents to CD8+ (Cytotoxic) T-cells, while MHC Class II presents to CD4+ (Helper) T-cells (Rule of 8: 1×8=8; 2×4=8). * **Genetic Loci:** In humans, MHC is known as HLA (Human Leukocyte Antigen). MHC Class I is encoded by HLA-A, B, and C; MHC Class II is encoded by **HLA-DP, DQ, and DR**. * **Structure:** MHC II is a heterodimer consisting of an **α chain and a β chain**, both of which are polymorphic and span the cell membrane. * **Invariant Chain (Ii):** This protein prevents endogenous peptides from binding to MHC II in the endoplasmic reticulum, ensuring it only binds exogenous antigens in the endosome.

Q: Which of the following bodily secretions contains IgA?

Tears and saliva. **Explanation:** **Immunoglobulin A (IgA)** is the primary antibody class associated with **mucosal immunity**. Specifically, **Secretory IgA (sIgA)** is the predominant isotype found in external secretions that bathe mucosal surfaces. 1. **Why Option A is correct:** IgA is synthesized by plasma cells in the lamina propria and transported across epithelial cells into secretions. It is found in high concentrations in **tears, saliva, colostrum (first milk), nasal mucus, and gastrointestinal/genitourinary secretions**. Its primary role is "immune exclusion"—preventing the attachment of bacteria and viruses to mucosal surfaces. 2. **Why other options are incorrect:** * **Cerebrospinal fluid (CSF):** Under normal physiological conditions, CSF contains very low levels of proteins and immunoglobulins due to the blood-brain barrier. * **Hairs:** Hair consists of keratinized dead cells and does not contain active immunoglobulins. * **Vaginal fluid:** While vaginal fluid *does* contain some IgA, it is predominantly characterized by **IgG** (derived from systemic circulation). In the context of standard medical examinations, tears and saliva are the classic, high-yield examples of IgA-rich fluids. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** IgA exists as a **monomer** in serum but as a **dimer** in secretions. * **The J-Chain:** Holds the two IgA monomers together. * **Secretory Component:** A polypeptide produced by epithelial cells that protects sIgA from digestion by proteolytic enzymes in the gut/saliva. * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients often present with recurrent sinopulmonary and GI infections. * **Milk:** IgA provides passive natural immunity to the neonate via breast milk.

Q: What is the distinct amino acid sequence at the antigen combining site called?

Idiotype. ### Explanation The correct answer is **Idiotype**. **1. Why Idiotype is correct:** An **Idiotype** refers to the unique set of antigenic determinants (idiotopes) located within the **Variable (V) regions** of an antibody or T-cell receptor. These are determined by the specific **amino acid sequence** at the antigen-binding site. Since each clone of B-cells produces antibodies with a unique V-region to fit a specific antigen, the idiotype serves as a signature for that specific clone. **2. Why other options are incorrect:** * **Allotype:** Refers to antigenic variations in the **Constant (C) region** of antibodies that differ between individuals of the same species due to allelic inheritance (e.g., Gm markers in IgG). * **Epitope:** Also known as the antigenic determinant, this is the specific part of the **antigen** molecule to which an antibody binds. * **Paratope:** This is the actual **physical pocket** or site on the antibody (formed by the Hypervariable regions) that binds to the epitope. While closely related to idiotype, the "sequence" itself that defines the specificity is the idiotype. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isotype:** Determines the class/subclass of the antibody (IgG, IgM, etc.) based on heavy chain constant regions; same in all individuals of a species. * **Hypervariable Regions (HVRs):** Also called **Complementarity Determining Regions (CDRs)**. There are 3 CDRs in the light chain and 3 in the heavy chain. These form the idiotype. * **Monoclonal Gammopathy:** In conditions like Multiple Myeloma, all myeloma cells produce antibodies with the **same idiotype**, appearing as an 'M-band' on electrophoresis.

Q: Which of the following statements is FALSE regarding Natural Killer (NK) cells?

NK cell activity is MHC restricted.. **Explanation:** **Why Option D is the correct (False) statement:** Natural Killer (NK) cells are a component of the **innate immune system**. Unlike T-cells, which require the presentation of antigens via Major Histocompatibility Complex (MHC) molecules to recognize targets, NK cells are **MHC-unrestricted**. In fact, NK cells operate on the "Missing Self" hypothesis: they preferentially kill cells that have *downregulated* or lost their MHC Class I expression (a common evasion tactic used by viruses and tumors). **Analysis of other options:** * **Option A:** Interferons (IFN-α and IFN-β) and Interleukin-12 are potent activators of NK cells, enhancing their cytotoxic capabilities during an immune response. * **Option B:** NK cells are the first line of defense against viral infections. They provide immediate non-specific cytotoxicity before the adaptive immune system (CD8+ T-cells) can mount a specific response. * **Option C:** NK cells play a crucial role in "immunosurveillance." They identify and destroy malignant cells that display stress-induced ligands or abnormal surface proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Origin:** NK cells are large granular lymphocytes derived from the lymphoid progenitor but do not express T-cell receptors (TCR) or CD3. * **Markers:** The definitive surface markers for NK cells are **CD56** and **CD16** (FcγRIII). CD16 allows them to perform **Antibody-Dependent Cellular Cytotoxicity (ADCC)**. * **Mechanism of Killing:** They induce apoptosis in target cells using **perforins** (to create pores) and **granzymes** (to activate caspases). * **Receptors:** They possess Killer Immunoglobulin-like Receptors (KIRs). Inhibitory KIRs bind to MHC I on healthy cells to prevent "self" destruction.

Question 1

The Postzone phenomenon is seen in which condition?

Accepted Answer

Antigen excess

Answer

Antibody excess

Answer

Equivalence zone

Answer

None of the above

Question 2

Toll-like receptors are expressed by which of the following cell types?

Accepted Answer

All the above

Answer

Macrophages

Answer

Epithelial cells

Answer

Fibroblasts

Question 3

What is the primary function of Toll-like receptors (TLRs)?

Accepted Answer

Recognition of microbial products to initiate acute inflammation.

Answer

Activation of oncogenes by viruses and playing a role in cancer.

Answer

Regulation of glucose homeostasis.

Answer

Facilitating the entry of retroviruses into immune cells.

Question 4

Which of the following cell surface markers is required for lysis of IgG-coated target cells (antibody-dependent, cell-mediated cytotoxicity, or ADCC) by natural killer cells?

Accepted Answer

CD16

Answer

CD3

Answer

CD19

Answer

CD21

Question 5

MHC class II antigens are located on which of the following cells?

Accepted Answer

Dendritic cells

Answer

Platelets

Answer

RBCs

Answer

T-cells

Question 6

Which of the following bodily secretions contains IgA?

Accepted Answer

Tears and saliva

Answer

Cerebrospinal fluid

Answer

Hairs

Answer

Vaginal fluid

Question 7

Cell fusion is an innovative technique for preparing specific antibodies. Which of the following techniques is NOT used to induce cell fusion?

Accepted Answer

Reducing the viscosity of the membrane

Answer

Adhering some inactivated viruses to the surface membrane

Answer

Adding polyethylene glycol

Answer

Applying a mild electric shock

Question 8

Which of the following is a component of innate immunity?

Accepted Answer

NK cells

Answer

T lymphocytes

Answer

Complement proteins

Answer

B lymphocytes

Question 9

What is the distinct amino acid sequence at the antigen combining site called?

Accepted Answer

Idiotype

Answer

Allotype

Answer

Epitope

Answer

Paratope

Question 10

Which of the following statements is FALSE regarding Natural Killer (NK) cells?

Accepted Answer

NK cell activity is MHC restricted.

Answer

NK cell activity is augmented by interferons.

Answer

NK cells provide the initial response in viral infections.

Answer

NK cells are active against tumor cells.

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?