Immunology Practice Questions

Q: What is the primary mechanism by which cytotoxic T lymphocytes (CTLs) recognize target cells in the immune response?

Recognize antigens presented by MHC class I molecules. ***Recognize antigens presented by MHC class I molecules*** - **Cytotoxic T lymphocytes (CTLs)**, or CD8+ T cells, specifically recognize antigens presented by **MHC class I molecules** on the surface of target cells. - This recognition is crucial for identifying and eliminating **virally infected** or **cancerous cells**. *Secrete cytokines that stimulate the differentiation and proliferation of T cells* - While CTLs do secrete some cytokines, their primary role is direct cytotoxicity rather than broadly stimulating T cell differentiation and proliferation. - **Helper T cells (CD4+ T cells)** are primarily responsible for secreting cytokines that orchestrate the immune response and stimulate other immune cells. *Are important in the control of viral infections* - This statement is true, but it describes a *consequence* of CTL function, not their fundamental *role* in antigen recognition. - CTLs eliminate virally infected cells, thereby controlling viral spread, but their initial role is antigen recognition via MHC class I. *Most often recognize antigens presented by MHC class II molecules* - **MHC class II molecules** are primarily recognized by **helper T cells (CD4+ T cells)**, which respond to extracellular antigens processed by professional antigen-presenting cells. - CTLs (CD8+ T cells) are specific for antigens presented by **MHC class I molecules**.

Q: The complement components responsible for bacterial lysis are:

C5-9. ***C5-9*** - The **C5-9 complement complex**, also known as the **membrane attack complex (MAC)**, is directly responsible for the lysis of bacterial cells [1][2]. - This complex forms pores in the **bacterial membrane**, leading to cell death [2]. *C5a* - While C5a is an important **anaphylatoxin** that enhances inflammation, it does not directly cause lysis [2][3]. - C5a primarily functions in **chemotaxis** and activation of immune cells rather than in bacterial membrane disruption [3]. *C3a* - C3a is another **anaphylatoxin** that promotes inflammation and recruitment of immune cells but does not participate in the lytic action against bacteria [2][3]. - Its role is primarily in **modulating the immune response**, not in direct bacterial lysis. *C3b* - C3b plays a key role in **opsonization**, marking bacteria for phagocytosis but it does not directly lyse bacterial cells [2]. - It facilitates the binding of pathogens to immune cells, enhancing **clearance** but does not form the lytic complex like C5-9. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, p. 99. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 99-100. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 163-164.

Q: Which type of lymphocyte is characterized by the following markers: CD3-, CD16+, CD56+, and responds to IL-12?

Natural killer cells. ***Natural killer cells*** - **Natural killer (NK) cells** are characterized by the absence of **CD3** (signifying they are not T cells) and the presence of **CD16** and **CD56**. - They respond to **IL-12**, which promotes their cytotoxicity and interferon-gamma production, crucial for innate immune responses against viruses and tumor cells. *B lymphocytes* - **B lymphocytes** express **CD19**, **CD20**, and **surface immunoglobulin**, but not **CD3**, **CD16**, or **CD56**. - Their primary role is **antibody production**, not direct cytotoxicity, and they respond to different cytokine profiles. *Cytotoxic T lymphocytes* - **Cytotoxic T lymphocytes (CTLs)** are defined by expressing **CD3** and **CD8**, enabling them to recognize and kill target cells in an MHC class I-restricted manner. - They typically do not express **CD16** or **CD56** and are activated by T-cell receptor engagement. *T helper 1 subset* - **T helper 1 (Th1) cells** are a subset of **CD4+ T cells**, meaning they express **CD3** and **CD4**. - They are primarily involved in orchestrating cell-mediated immunity through cytokines like **IFN-γ** and **IL-2**, not direct cytotoxicity via **CD16** or **CD56**.

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: 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: 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: 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: Which type of interferon is predominantly produced by fibroblasts in tissue culture?

Beta. ***Beta*** - **Interferon-beta** (IFN-β) is predominantly produced by **fibroblasts** in response to viral infections. - It is a **Type I interferon** playing a crucial role in the initial innate immune response. *Alpha* - **Interferon-alpha** (IFN-α) is primarily produced by **leukocytes**, particularly **plasmacytoid dendritic cells**. - While also a **Type I interferon**, its cellular origin differs from fibroblasts. *Gamma* - **Interferon-gamma** (IFN-γ) is a **Type II interferon** typically produced by **T lymphocytes** and **natural killer (NK) cells**. - It is involved in adaptive immunity and immune regulation, distinct from the antiviral response of fibroblasts. *None of the options* - This option is incorrect as one of the interferons listed is indeed produced by fibroblasts. - **Interferon-beta** is specifically known for its production by fibroblasts.

Question 1

What is the primary mechanism by which cytotoxic T lymphocytes (CTLs) recognize target cells in the immune response?

Accepted Answer

Recognize antigens presented by MHC class I molecules

Answer

Are important in the control of viral infections

Answer

Secrete cytokines that stimulate the differentiation and proliferation of T cells

Answer

Most often recognize antigens presented by MHC class II molecules

Question 2

The complement components responsible for bacterial lysis are:

Accepted Answer

C5-9

Answer

C3a

Answer

C3b

Answer

C5a

Question 3

Which type of lymphocyte is characterized by the following markers: CD3-, CD16+, CD56+, and responds to IL-12?

Accepted Answer

Natural killer cells

Answer

B lymphocytes

Answer

Cytotoxic T lymphocytes

Answer

T helper 1 subset

Question 4

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 5

Virus-infected cells are killed by

Accepted Answer

Cytotoxic T lymphocytes (CTLs)

Answer

Natural killer (NK) cells

Answer

Macrophages

Answer

Neutrophils

Question 6

Immunoglobulin present in mouth local secretions is

Accepted Answer

IgA

Answer

IgM

Answer

IgG

Answer

IgE

Question 7

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 8

Variable portions of the antibody molecule are -

Accepted Answer

N-Terminal

Answer

C-Terminal

Answer

CHO moiety

Answer

None of the options

Question 9

Which antibody is primarily responsible for opsonization?

Accepted Answer

IgG

Answer

IgA

Answer

IgM

Answer

IgE

Question 10

Which type of interferon is predominantly produced by fibroblasts in tissue culture?

Accepted Answer

Beta

Answer

Alpha

Answer

Gamma

Answer

None of the options

Immunology — MCQs

Immunology — MCQs

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