Immunology Practice Questions

Q: Which of the following statements regarding Cytotoxic T lymphocytes (CTLs) is correct?

Are important in the control of viral infections. **Explanation:** **Cytotoxic T Lymphocytes (CTLs)**, which are CD8+ T cells, play a pivotal role in cell-mediated immunity by identifying and eliminating cells infected with intracellular pathogens, particularly **viruses**, as well as tumor cells. 1. **Why Option C is Correct:** CTLs recognize viral peptides presented by **MHC Class I** molecules on the surface of any nucleated cell. Once activated, they induce apoptosis of the infected cell via the **Perforin-Granzyme pathway** or **Fas-Fas ligand interaction**, effectively halting viral replication and spread. 2. **Why Other Options are Incorrect:** * **Option A:** CTLs (CD8+) recognize antigens presented on **MHC Class I**, whereas CD4+ T cells recognize antigens on **MHC Class II**. They do not recognize the same antigen-MHC complex. * **Option B:** This is the function of **Helper T cells (CD4+)**, specifically the Th2 subset, which secretes IL-4 and IL-5 to stimulate B cell differentiation into plasma cells. * **Option D:** This describes the **Exogenous Pathway** of antigen processing (MHC II), typical for CD4+ T cells. CTLs recognize antigens processed via the **Endogenous Pathway**, where proteins synthesized within the cytosol (like viral proteins) are degraded by proteasomes. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** CD8+ = MHC I (Rule of 8: 8 × 1 = 8); CD4+ = MHC II (4 × 2 = 8). * **Markers:** CTLs express CD8, CD3, and the T-cell receptor (TCR). * **Granzymes:** Serine proteases that enter the target cell to activate caspases, leading to programmed cell death.

Q: A patient is genetically unable to synthesize J chains. Which immunoglobulins would be affected?

IgM and IgA. **Explanation:** The **J chain (Joining chain)** is a small glycoprotein (approx. 15 kDa) required for the polymerization of specific immunoglobulins. It is synthesized by plasma cells and is essential for stabilizing the multimeric structures of **IgM** and **IgA**. * **IgM:** Typically exists as a **pentamer** in serum. The J chain is required to link the five monomeric units together. * **IgA:** While it exists as a monomer in serum, it forms **dimers** in secretions (Secretory IgA). The J chain is vital for dimerization and facilitates the binding of IgA to the secretory component for transport across mucosal surfaces. Therefore, a genetic inability to synthesize J chains would prevent the formation of pentameric IgM and dimeric IgA, severely compromising mucosal immunity and the primary immune response. **Analysis of Incorrect Options:** * **IgG, IgE, and IgD:** These immunoglobulins always function as **monomers**. They do not possess or require a J chain for their structure or function. * **Options A, B, and D:** These are incorrect because they include IgG or IgE, which are unaffected by J chain deficiency. **NEET-PG High-Yield Pearls:** * **Structure:** IgM is a pentamer (contains J chain); IgA is a dimer (contains J chain) or monomer. * **Valency:** Pentameric IgM has a theoretical valency of 10, but an effective valency of 5 due to steric hindrance. * **Mucosal Immunity:** IgA is the most produced antibody in the body, primarily found at mucosal surfaces (tears, saliva, colostrum, GI tract). * **Complement Activation:** IgM is the most potent activator of the classical complement pathway due to its multimeric structure.

Q: Memory T cells can be identified by using which of the following markers?

CD 45 RO. **Explanation:** The identification of T cell subsets relies on the expression of different isoforms of the **CD45 molecule** (Leukocyte Common Antigen). CD45 is a tyrosine phosphatase essential for T-cell receptor signaling. **Why CD45RO is correct:** Memory T cells (both CD4+ and CD8+) are characterized by the expression of **CD45RO**. When a "naive" T cell encounters an antigen, it undergoes alternative splicing of the CD45 gene, removing the A, B, and C exons. This results in the shortest isoform, CD45RO. This marker indicates that the cell has been previously activated and has transitioned into a long-lived memory state, allowing for a rapid immune response upon re-exposure to the same pathogen. **Analysis of Incorrect Options:** * **CD45RA:** This is the marker for **Naive T cells** (cells that have not yet encountered their specific antigen). It is the longest isoform containing the 'A' peptide. * **CD45RB and CD45RC:** These isoforms are expressed on various subsets of B cells, naive T cells, and NK cells, but they are not specific markers used to identify the memory T cell population in clinical immunology. **High-Yield Facts for NEET-PG:** * **Naive T cells:** CD45RA+ (Think 'A' for "At rest" or "Antigen-inexperienced"). * **Memory T cells:** CD45RO+ (Think 'O' for "Old" or "Once-activated"). * **CD45** is known as the **Leukocyte Common Antigen (LCA)** and is used in immunohistochemistry to differentiate lymphomas from carcinomas. * **Central Memory T cells (Tcm)** also express **CCR7 and L-selectin (CD62L)**, allowing them to home to lymph nodes, whereas **Effector Memory T cells (Tem)** lack these and circulate in peripheral tissues.

Q: Regulatory T cells (Tregs) express all of the following, except:

CD8. **Explanation:** Regulatory T cells (Tregs) are a specialized subpopulation of T cells that maintain immune tolerance and prevent autoimmune diseases by suppressing the activation and proliferation of self-reactive lymphocytes. **Why CD8 is the correct answer:** Tregs are primarily a subset of **CD4+ Helper T cells**. Therefore, they do not typically express **CD8**, which is the hallmark marker for Cytotoxic T cells. While rare populations of CD8+ regulatory cells exist, the classic "Natural Tregs" tested in exams are defined by their CD4+ lineage. **Analysis of other options:** * **CD4 (Option A):** Tregs originate from the thymus as a lineage of CD4+ cells. They are defined as CD4+ cells that have escaped negative selection to serve a suppressive role. * **CD25 (Option B):** This is the **alpha chain of the IL-2 receptor**. Tregs constitutively express high levels of CD25, which allows them to consume IL-2 from the environment, effectively "starving" effector T cells of this essential growth factor. * **FOXP3 (Option D):** This is the **master transcriptional regulator** for Treg development and function. It is the most specific intracellular marker for these cells. **High-Yield Clinical Pearls for NEET-PG:** * **IPEX Syndrome:** Mutations in the **FOXP3 gene** lead to "Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked" (IPEX) syndrome, characterized by systemic autoimmunity. * **Mechanism of Action:** Tregs suppress the immune system via inhibitory cytokines like **IL-10** and **TGF-β**, and through **CTLA-4**, which outcompetes CD28 for binding to B7 on APCs. * **Surface Markers Summary:** CD4+, CD25+, FOXP3+, and **CD127 (low/absent)**.

Q: Which of the following cells does not possess cytotoxic activity?

Helper T-cells. **Explanation:** The core concept of this question lies in distinguishing between **effector cells** that directly kill target cells (cytotoxicity) and **regulatory cells** that orchestrate the immune response. **Why Helper T-cells (CD4+) are the correct answer:** Helper T-cells are the "managers" of the immune system. Their primary role is to secrete cytokines (like IFN-γ and IL-2) that activate other immune cells. They **do not possess intrinsic cytotoxic machinery** (such as perforins or granzymes) to directly lyse target cells. Instead, they assist B-cells in antibody production and activate Macrophages and Cytotoxic T-cells. **Analysis of Incorrect Options:** * **NK Cells (Natural Killer Cells):** These are large granular lymphocytes that provide innate immunity. They kill virally infected or tumor cells through the release of perforins and granzymes without prior sensitization. * **Cytotoxic T-cells (CD8+):** These are the primary effectors of adaptive cell-mediated immunity. They recognize MHC-I associated antigens and induce apoptosis in target cells via the perforin-granzyme pathway or Fas-FasL interaction. * **Antibody-Dependent Cells (ADCC):** This refers to cells (primarily NK cells, but also macrophages, neutrophils, and eosinophils) that kill target cells coated with IgG antibodies. The Fc receptors on these effector cells bind to the antibody, triggering the release of cytotoxic granules. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** CD4+ (Helper) cells are MHC-II restricted, while CD8+ (Cytotoxic) cells are MHC-I restricted. * **Markers:** NK cells are identified by **CD56 and CD16**, but they are **CD3 negative**. * **Th1 vs Th2:** Helper T-cells differentiate into Th1 (cell-mediated help) or Th2 (humoral/antibody help) based on the cytokine environment (IL-12 vs IL-4).

Q: Immunoglobulin isotype class switching is determined by which immunoglobulin region?

Constant region of heavy chain. ### Explanation **Why Option B is Correct:** Immunoglobulin (Ig) molecules consist of two heavy (H) chains and two light (L) chains. The **Constant region of the heavy chain ($C_H$)** determines the **isotype** (class) of the antibody (IgM, IgG, IgA, IgE, or IgD). Class switching (Isotype switching) is a biological process where a B cell changes the production of antibody from one class to another (e.g., from IgM to IgG). During this process, the antigen-binding **Variable (V) region** remains the same, but the gene encoding the **Constant (C) region** of the heavy chain is replaced through somatic recombination. This allows the antibody to retain its specificity for the same antigen while changing its effector functions (e.g., crossing the placenta or activating complement). **Why Other Options are Incorrect:** * **Options A & C (Light Chains):** Light chains (Kappa or Lambda) have no role in determining the class of the antibody. They contribute to the antigen-binding site but do not change during class switching. * **Option D (Variable region of heavy chain):** The variable region (V, D, J segments) determines **antigen specificity**. If this region changed during class switching, the B cell would lose its ability to recognize the original invading pathogen. **High-Yield NEET-PG Pearls:** * **Cytokine Influence:** Class switching is driven by cytokines; for example, **IL-4** induces switching to **IgE**, while **TGF-$\beta$** induces switching to **IgA**. * **Key Enzyme:** **Activation-induced cytidine deaminase (AID)** is the essential enzyme required for class switching and somatic hypermutation. * **Clinical Correlation:** A deficiency in AID or CD40L leads to **Hyper-IgM Syndrome**, where B cells cannot switch from IgM to other isotypes. * **Sequence of Events:** Class switching occurs in the **Germinal Centers** of secondary lymphoid organs after antigen stimulation.

Q: What is the first line of defense against tumors and viruses?

NK cell. **Explanation:** **Natural Killer (NK) cells** are the correct answer because they represent the **first line of defense** (innate immunity) against virally infected cells and tumor cells. Unlike T cells, NK cells do not require prior sensitization or MHC-restricted antigen presentation. They function via the "missing self" hypothesis: they identify and kill cells that have downregulated **MHC Class I** molecules—a common strategy used by viruses and tumors to evade detection by cytotoxic T lymphocytes (CTLs). **Analysis of Incorrect Options:** * **B. T cells:** These are part of the adaptive immune system. While CD8+ T cells are potent killers of tumors and viruses, they require antigen processing, presentation via MHC I, and several days to proliferate, making them a **secondary** rather than a first-line response. * **C. Histiocytes:** These are tissue-resident macrophages (e.g., Kupffer cells, Microglia). While they participate in phagocytosis and antigen presentation, they are not the primary effector cells for direct tumor lysis. * **D. Macrophages:** These act as professional phagocytes and antigen-presenting cells (APCs). Although they secrete TNF-alpha and can phagocytose debris, they lack the specific "natural" cytotoxicity mechanism that defines the NK cell's immediate response to malignancy. **NEET-PG High-Yield Pearls:** * **Markers:** NK cells are identified by **CD56** and **CD16** (FcγRIII), but they are **CD3 negative**. * **Mechanism:** They induce apoptosis using **perforins and granzymes**. * **Cytokine Activation:** Their activity is significantly enhanced by **IL-2 and IL-12**. * **Clinical Correlation:** Deficiency in NK cell function leads to recurrent viral infections (especially Herpesviridae) and increased susceptibility to various malignancies.

Question 1

Which of the following statements regarding Cytotoxic T lymphocytes (CTLs) is correct?

Accepted Answer

Are important in the control of viral infections

Answer

Recognize the same antigens as CD4 T cells

Answer

Secrete cytokines that stimulate the differentiation and proliferation of B cells

Answer

Most often recognize antigens that have been phagocytosed, degraded, and presented on the surface of an APC

Question 2

A patient is genetically unable to synthesize J chains. Which immunoglobulins would be affected?

Accepted Answer

IgM and IgA

Answer

IgG and IgA

Answer

IgM and IgE

Answer

IgG and IgM

Question 3

Memory T cells can be identified by using which of the following markers?

Accepted Answer

CD 45 RO

Answer

CD 45 RA

Answer

CD 45 RB

Answer

CD 45 RC

Question 4

Which of the following statements best describes a key feature of innate immunity?

Accepted Answer

It includes interaction between pattern recognition receptors on phagocytes and pathogen-associated molecular patterns

Answer

It improves with repeated exposure to a given antigen

Answer

It requires antigen processing for activation

Answer

It is not associated with primary immune deficiency

Question 5

Which type of hypersensitivity reaction can occur when a single dose of antigen acts as both the sensitizing and shocking dose?

Accepted Answer

Serum sickness

Answer

Anaphylaxis

Answer

Arthus reaction

Answer

Contact dermatitis

Question 6

Which part of bacteria is most antigenic?

Accepted Answer

Lipids

Answer

Protein coat

Answer

Lipopolysaccharide

Answer

Nucleic acid

Question 7

Regulatory T cells (Tregs) express all of the following, except:

Accepted Answer

CD8

Answer

CD4

Answer

CD25

Answer

FOXP3

Question 8

Which of the following cells does not possess cytotoxic activity?

Accepted Answer

Helper T-cells

Answer

NK cells

Answer

Cytotoxic T-cells

Answer

Antibody-dependent cells

Question 9

Immunoglobulin isotype class switching is determined by which immunoglobulin region?

Accepted Answer

Constant region of heavy chain

Answer

Constant region of light chain

Answer

Variable region of light chain

Answer

Variable region of heavy chain

Question 10

What is the first line of defense against tumors and viruses?

Accepted Answer

NK cell

Answer

T cell

Answer

Histiocyte

Answer

Macrophage

Immunology — MCQs

Immunology — MCQs

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