Immunology — MCQs
On this page
Which antigen is considered the most immunogenic in the context of typhoid fever?
Which of the following is the predominant immunoglobulin isotype secreted in the human mucosa-associated lymphoid tissue (MALT)?
Immunoglobulin present in mouth local secretions is
Virus-infected cells are killed by
Variable portions of the antibody molecule are -
All of the following statements about NK cells are true except:
Early reactions of the lepromin test are read at:
Which antibody is primarily responsible for opsonization?
The most potent antigen-presenting cell is?
A possible source of the "second signal" to a B-cell bound by specific antigen includes
Immunology Indian Medical PG Practice Questions and MCQs
Question 1041: Which antigen is considered the most immunogenic in the context of typhoid fever?
- A. O antigen
- B. Vi antigen (Correct Answer)
- C. H antigen
- D. M antigen
Explanation: ***Vi antigen*** - The **Vi capsular polysaccharide antigen** is the most immunogenic antigen of *Salmonella Typhi* and is the basis for the modern **Vi polysaccharide vaccine (Typhim Vi)**. - It is highly effective at inducing **protective antibodies** and provides immunity against typhoid fever, making it the preferred antigen for vaccine development. - The Vi antigen protects the bacteria from phagocytosis initially, but once antibodies develop against it, they provide **strong protective immunity** by opsonizing the bacteria. - Vi antigen-based vaccines have shown **70-80% efficacy** in clinical trials, demonstrating its superior immunogenic properties. *O antigen* - The **O antigen** (somatic antigen) is part of the lipopolysaccharide (LPS) layer and is highly immunogenic, forming the basis for the **Widal test** (anti-O antibodies). - While immunogenic, it is less effective than Vi antigen for **long-term protective immunity** and is more useful for diagnostic purposes. - O antibodies appear early in infection and indicate **acute infection**, but they are not as specific as Vi antibodies for typhoid. *H antigen* - The **H antigen** (flagellar antigen) is immunogenic and produces antibodies that can be detected in the Widal test (anti-H antibodies). - However, H antibodies appear **later in infection**, persist longer, and are **less specific** for typhoid fever as they may cross-react with other Salmonella species. - While it contributes to the immune response, it is not the most immunogenic antigen and is not used as the primary component in modern typhoid vaccines. *M antigen* - The **M antigen** is not a recognized antigen of *Salmonella Typhi* and does not play a role in typhoid immunity. - This term is more commonly associated with **M protein** of *Streptococcus pyogenes* and is not relevant to the immunology of typhoid fever.
Question 1042: Which of the following is the predominant immunoglobulin isotype secreted in the human mucosa-associated lymphoid tissue (MALT)?
- A. IgA (Correct Answer)
- B. IgD
- C. IgG
- D. IgE
Explanation: ***IgA*** - **Secretory IgA (sIgA)** is the most abundant immunoglobulin in mucosal secretions, including those found in the MALT, playing a critical role in **mucosal immunity**. - It forms a **dimeric structure** which makes it highly resistant to proteolysis and effective at preventing microbial adherence to epithelial surfaces. *IgD* - **IgD** is primarily found on the surface of naïve B lymphocytes, acting as an **antigen receptor**. - Its secreted form is found in very low concentrations in serum and is not a major component of mucosal immunity. *IgG* - **IgG** is the most abundant immunoglobulin in serum and is crucial for **systemic immunity**, including opsonization and complement activation. - While present in some mucosal tissues, especially during inflammation, it is not the predominant isotype secreted in healthy MALT. *IgE* - **IgE** is primarily associated with **allergic reactions** and defense against parasites, binding to mast cells and basophils. - It is present in very low concentrations in secretions and does not play a dominant role in general mucosal defense.
Question 1043: Immunoglobulin present in mouth local secretions is
- A. IgA (Correct Answer)
- B. IgM
- C. IgG
- D. IgE
Explanation: ***IgA*** - **Secretory IgA (sIgA)** is the predominant immunoglobulin found in external secretions, including saliva, tears, mucus, and breast milk. - It plays a crucial role in **mucosal immunity**, protecting against pathogens at mucosal surfaces. *IgM* - **IgM** is primarily found in the **bloodstream** and serves as the first antibody produced during a primary immune response. - While trace amounts may be present, it is **not the primary immunoglobulin** in mucosal secretions. *IgG* - **IgG** is the most abundant immunoglobulin in **serum** and crosses the placenta, providing passive immunity to the fetus. - Although it can diffuse into tissues, it is **not the main antibody** guarding mucosal surfaces like the oral cavity. *IgE* - **IgE** is involved in **allergic reactions** and defense against parasites, typically found in very low concentrations in serum. - It is **not significant** in mucosal secretions for routine pathogen defense.
Question 1044: Virus-infected cells are killed by
- A. Natural killer (NK) cells
- B. Macrophages
- C. Neutrophils
- D. Cytotoxic T lymphocytes (CTLs) (Correct Answer)
Explanation: ***Cytotoxic T lymphocytes (CTLs)*** - **CTLs are the primary adaptive immune cells** specifically designed to recognize and kill virus-infected cells - They recognize **viral antigens presented on MHC class I molecules** on the surface of infected cells - CTLs kill infected cells through **perforin and granzyme-mediated cytotoxicity** and **Fas-FasL pathway** - This is the **classic and most specific answer** for virus-infected cell killing in immunology *Natural killer (NK) cells* - NK cells provide **early innate defense** against viral infections before CTL response develops - They kill virus-infected cells by recognizing **downregulation of MHC class I** ("missing self") - While NK cells do kill virus-infected cells, CTLs are the **specific adaptive immune response** - NK cells act within hours, while CTLs take days to develop but provide targeted immunity *Macrophages* - Macrophages are **phagocytic cells** that primarily engulf pathogens and dead cells - They play a **supporting role** in viral immunity through antigen presentation and cytokine production - They do **NOT directly kill virus-infected cells** as their primary antiviral mechanism - Their main contribution is activating other immune cells, not direct cytotoxicity *Neutrophils* - Neutrophils are the **first responders to bacterial and fungal infections** - They have a **very limited role** in antiviral immunity - Not involved in direct killing of virus-infected cells - May be recruited to sites of inflammation but are not primary antiviral effectors
Question 1045: Variable portions of the antibody molecule are -
- A. C-Terminal
- B. N-Terminal (Correct Answer)
- C. CHO moiety
- D. None of the options
Explanation: ***N-Terminal*** - The **variable regions** of both the heavy and light chains of an antibody molecule are located at the **N-terminal** ends. - These regions contain the **antigen-binding sites** and demonstrate significant sequence diversity, allowing them to recognize a vast array of antigens. *C-Terminal* - The **C-terminal** portions of the antibody chains contain the **constant regions**, which are less variable in sequence and are responsible for structural stability. - These constant regions are involved in **effector functions**, such as binding to Fc receptors and activating complement, rather than antigen recognition. *CHO moiety* - A **CHO (carbohydrate) moiety** refers to glycosylation added to certain antibody molecules, particularly in the Fc region. - While important for antibody function, stability, and effector mechanisms, it is not the part of the antibody that dictates **antigen-specificity** or variability in binding sites. *None of the options* - This option is incorrect because the **N-terminal** region is indeed the variable portion of the antibody molecule crucial for antigen recognition.
Question 1046: All of the following statements about NK cells are true except:
- A. They are derived from large granular cells
- B. They express IgG Fc receptors
- C. They are MHC restricted cytotoxic cells (Correct Answer)
- D. They comprise about 10-15% of human peripheral lymphoid cells
Explanation: ***They are MHC restricted cytotoxic cells*** - NK cells are known for their **cytotoxic activity** against infected or tumor cells, but they operate independently of **MHC restriction** [1]. - They recognize and kill target cells without the need for specific antigen presentation via MHC molecules, differentiating them from T cells [1]. *They are derived from large granular cells* - NK cells are indeed derived from **large granular lymphocytes**, so this statement is true regarding their lineage. - Their role is clear as they are part of the innate immune system but this statement does not contradict the overall knowledge about NK cells. *They express IgG Fc receptors* - This statement is incorrect because NK cells primarily express **CD16** (FcγRIII), but this is not a direct IgG Fc receptor functioning as in other lymphocyte types. - While they can engage with IgG antibodies through **ADCC (Antibody-Dependent Cellular Cytotoxicity)**, they do not directly express traditional IgG Fc receptors. *They comprise about 5% of human peripheral lymphoid cells* - NK cells typically make up about **10-15%** of the total peripheral lymphocyte population, rather than the cited 5%. - This statement underestimates their proportion in the immune system, reducing their perceived importance in immune responses. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 200-201.
Question 1047: Early reactions of the lepromin test are read at:
- A. 48 hours (Correct Answer)
- B. 36 hours
- C. 72 hours
- D. 24 hours
Explanation: ***48 hours*** - The **early reaction** of the lepromin test, known as the **Fernandez reaction**, is read at **48-72 hours** after intradermal injection of lepromin antigen. - This represents a **delayed-type hypersensitivity (Type IV) reaction** mediated by T-cells, not humoral antibodies. - The reaction appears as an indurated papule or nodule at the injection site, with **48 hours** being the standard reading time. - It is **positive in tuberculoid leprosy** and reflects pre-existing cell-mediated immunity to *Mycobacterium leprae*. *24 hours* - At 24 hours, the reaction is typically **too early** to assess the Fernandez reaction adequately. - The peak induration of the delayed-type hypersensitivity reaction occurs at **48-72 hours**, not 24 hours. - Reading at 24 hours may miss or underestimate the early reaction. *36 hours* - While 36 hours falls within the developing phase, it is **not the standard reading time** for the Fernandez reaction. - The reaction continues to develop and is optimally assessed at **48 hours** or later. *72 hours* - 72 hours is also within the acceptable window for reading the **Fernandez reaction** (48-72 hours). - However, **48 hours** is more commonly cited as the standard reading time in most references. - The **late reaction (Mitsuda reaction)** is read much later at **3-4 weeks** and reflects stronger cell-mediated immunity, seen as granuloma formation.
Question 1048: Which antibody is primarily responsible for opsonization?
- A. IgA
- B. IgG (Correct Answer)
- C. IgM
- D. IgE
Explanation: ***Correct: IgG*** - **IgG** is the most abundant antibody in serum and plays a crucial role in **opsonization**, coating pathogens to enhance phagocytosis by macrophages and neutrophils. - Its Fc region binds to **Fc receptors** on phagocytic cells, facilitating the engulfment and destruction of the opsonized microbe. - **IgG1 and IgG3 subclasses** are particularly effective at opsonization due to their high-affinity Fc receptors. *Incorrect: IgA* - **IgA** is primarily found in **mucosal secretions** (e.g., saliva, tears, breast milk) where it provides local immunity, mainly by neutralizing toxins and pathogens at epithelial surfaces. - While it can sometimes participate in immune exclusion, its role in **opsonization** in the bloodstream is minor compared to IgG. *Incorrect: IgM* - **IgM** is the first antibody produced during a primary immune response and exists as a pentamer in serum, making it highly effective at **complement activation**. - Its large size and multiple binding sites make it less efficient for direct opsonization compared to IgG, though complement activation can indirectly lead to opsonization via **C3b**. *Incorrect: IgE* - **IgE** is primarily involved in **allergic reactions** and defense against **parasitic infections**. - It binds to **Fc receptors** on mast cells and basophils, triggering the release of histamine and other mediators in response to allergens or parasites.
Question 1049: The most potent antigen-presenting cell is?
- A. B cells
- B. T cells
- C. NK cells
- D. Dendritic cells (Correct Answer)
Explanation: ***Dendritic cells*** - Dendritic cells are the **most potent antigen-presenting cells** [1], crucial for T-cell activation and initiating immune responses. - They capture, process, and present antigens to T cells, leading to a **robust adaptive immune response** [1]. *T cells* - T cells are primarily **effector cells** in the immune system, not main antigen presenters. - While they can present antigens through MHC molecules, they are less efficient compared to dendritic cells. *NK cells* - Natural Killer (NK) cells primarily target **virus-infected** and **tumor** cells, functioning in innate immunity. - They do not serve as antigen-presenting cells and do not activate T cells as effectively as dendritic cells. *B cells* - B cells can present antigens and produce **antibodies**, but they are not as potent as dendritic cells in initiating T-cell responses. - Their main role is in **humoral immunity**, while dendritic cells excel in activating naïve T cells. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 199-200, 204-208.
Question 1050: A possible source of the "second signal" to a B-cell bound by specific antigen includes
- A. Antigen-specific T-cells (Correct Answer)
- B. Non-specific endotoxin (LPS)
- C. Differentiated plasma cells
- D. Epstein-Barr virus (EBV) infection
Explanation: ***Antigen-specific T-cells*** - T-cells, specifically **helper T-cells (Th2 cells)**, provide co-stimulatory signals (the "second signal") to antigen-bound B-cells via interactions such as **CD40L on the T-cell binding to CD40 on the B-cell**. - This interaction, along with cytokine release from the T-cell, is crucial for B-cell **activation, proliferation, differentiation**, and class switching. *Epstein-Barr virus (EBV) infection* - EBV can activate B-cells **polyclonally** and promote their proliferation and differentiation, bypassing the need for T-cell help or an initial antigen-specific signal. - However, this is an antigen-independent activation mechanism, not a typical "second signal" for a naturally antigen-bound B-cell. *Non-specific endotoxin (LPS)* - **Lipopolysaccharide (LPS)** is a potent **mitogen** for B-cells, meaning it can cause their proliferation and activation without specific antigen recognition. - While it can initiate B-cell responses, it functions as a **T-cell-independent activator**, not providing the co-stimulatory "second signal" required for T-cell dependent antigens. *Differentiated plasma cells* - **Plasma cells** are the terminally differentiated effector cells of the B-cell lineage that primarily produce and secrete **antibodies**. - They are not involved in providing the "second signal" for the initial activation of naive B-cells; rather, they are the end product of B-cell activation.
Practice by Chapter
Cells and Organs of Immune System
Practice Questions
Innate Immunity
Practice Questions
Adaptive Immunity
Practice Questions
Antigens and Antibodies
Practice Questions
Major Histocompatibility Complex
Practice Questions
Complement System
Practice Questions
Cytokines and Chemokines
Practice Questions
Hypersensitivity Reactions
Practice Questions
Autoimmunity and Autoimmune Diseases
Practice Questions
Immunodeficiency Disorders
Practice Questions
Transplantation Immunology
Practice Questions
Tumor Immunology
Practice Questions
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free