Immunology Practice Questions

Q: Which antigen is considered the most immunogenic in the context of typhoid fever?

Vi antigen. ***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.

Q: Which of the following is the predominant immunoglobulin isotype secreted in the human mucosa-associated lymphoid tissue (MALT)?

IgA. ***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.

Q: Virus-infected cells are killed by

Cytotoxic T lymphocytes (CTLs). ***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

Q: Immunoglobulin present in mouth local secretions is

IgA. ***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.

Q: Variable portions of the antibody molecule are -

N-Terminal. ***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.

Q: All of the following statements about NK cells are true except:

They are MHC restricted cytotoxic cells. ***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.

Q: Early reactions of the lepromin test are read at:

48 hours. ***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.

Q: Which antibody is primarily responsible for opsonization?

IgG. ***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.

Q: The most potent antigen-presenting cell is?

Dendritic cells. ***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.

Q: A possible source of the "second signal" to a B-cell bound by specific antigen includes

Antigen-specific T-cells. ***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.

Question 1

Which antigen is considered the most immunogenic in the context of typhoid fever?

Accepted Answer

Vi antigen

Answer

O antigen

Answer

H antigen

Answer

M antigen

Question 2

Which of the following is the predominant immunoglobulin isotype secreted in the human mucosa-associated lymphoid tissue (MALT)?

Accepted Answer

IgA

Answer

IgD

Answer

IgG

Answer

IgE

Question 3

Virus-infected cells are killed by

Accepted Answer

Cytotoxic T lymphocytes (CTLs)

Answer

Natural killer (NK) cells

Answer

Macrophages

Answer

Neutrophils

Question 4

Immunoglobulin present in mouth local secretions is

Accepted Answer

IgA

Answer

IgM

Answer

IgG

Answer

IgE

Question 5

Variable portions of the antibody molecule are -

Accepted Answer

N-Terminal

Answer

C-Terminal

Answer

CHO moiety

Answer

None of the options

Question 6

All of the following statements about NK cells are true except:

Accepted Answer

They are MHC restricted cytotoxic cells

Answer

They are derived from large granular cells

Answer

They express IgG Fc receptors

Answer

They comprise about 10-15% of human peripheral lymphoid cells

Question 7

Early reactions of the lepromin test are read at:

Accepted Answer

48 hours

Answer

36 hours

Answer

72 hours

Answer

24 hours

Question 8

Which antibody is primarily responsible for opsonization?

Accepted Answer

IgG

Answer

IgA

Answer

IgM

Answer

IgE

Question 9

The most potent antigen-presenting cell is?

Accepted Answer

Dendritic cells

Answer

B cells

Answer

T cells

Answer

NK cells

Question 10

A possible source of the "second signal" to a B-cell bound by specific antigen includes

Accepted Answer

Antigen-specific T-cells

Answer

Non-specific endotoxin (LPS)

Answer

Differentiated plasma cells

Answer

Epstein-Barr virus (EBV) infection

Immunology — MCQs

Immunology — MCQs

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