Immunology — MCQs
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Which of the following is a specific feature of acquired immunity?
Haptens are immunogenic when they covalently bind to which type of carrier?
Which human IgG subclass has the highest serum concentration?
Which type of DNase produced by Streptococcus is known to be the most antigenic in humans?
What type of immunity is primarily associated with the administration of transfer factor?
CD3 is a marker for which type of cells?
Which immunoglobulin does Protein A of Staphylococcus aureus bind to?
Phagocytosis of mycobacterium tuberculosis by macrophages is mainly mediated by:
Lattice phenomenon is seen in which of the following?
Which of the following does not stimulate active immunity?
Immunology Indian Medical PG Practice Questions and MCQs
Question 991: Which of the following is a specific feature of acquired immunity?
- A. Immunological memory (Correct Answer)
- B. Affected by genetic makeup
- C. No antigen exposure
- D. Immediate response
Explanation: ***Immunological memory*** - A key characteristic of **acquired immunity** is the ability to "remember" previous encounters with specific pathogens. - This memory leads to a more rapid and robust immune response upon subsequent exposure to the same pathogen. - This is the **defining feature** that distinguishes acquired immunity from innate immunity. *Affected by genetic makeup* - While genetic makeup can influence the *efficiency* of the acquired immune system, it is not a **specific feature** that distinguishes it from innate immunity. - **Both innate and acquired immunity** are affected by genetic factors, determining baseline resistance and immune response capability. *No antigen exposure* - **Acquired immunity** is specifically characterized by its *dependence* on antigen exposure to develop specific responses. - The phrase "no antigen exposure" describes how the **innate immune system** functions, providing immediate, non-specific protection without prior contact with a pathogen. *Immediate response* - **Innate immunity** provides an immediate, non-specific response to pathogens. - **Acquired immunity** takes time to develop (days to weeks) after initial antigen exposure, but provides a faster response upon re-exposure due to immunological memory.
Question 992: Haptens are immunogenic when they covalently bind to which type of carrier?
- A. Lipid carrier
- B. Polysaccharide carrier
- C. Protein carrier (Correct Answer)
- D. None of the above carrier
Explanation: ***Protein carrier*** - Haptens are small molecules that are **antigenic** but not **immunogenic** on their own; they acquire immunogenicity when covalently bound to a larger carrier molecule. - **Proteins** are highly effective carriers because their complex structures and multiple epitopes can induce strong T-cell help, which is crucial for a robust antibody response against the hapten. *Lipid carrier* - While some lipids can be antigenic (e.g., glycolipids), they generally do not serve as effective carriers for haptens to induce a strong adaptive immune response, especially T-cell-dependent responses. - **Lipids** are less likely to be processed and presented by MHC molecules in a way that generates potent helper T-cell activation. *Polysaccharide carrier* - Some polysaccharides can be immunogenic themselves (e.g., bacterial capsular polysaccharides) and can induce T-cell-independent antibody responses. - However, for haptens to become immunogenic and induce a **T-cell-dependent antibody response**, a protein carrier is typically required. *None of the above carrier* - This option is incorrect because haptens do require a carrier to become immunogenic, and specific types of carriers are more effective than others. - The type of carrier chosen significantly impacts the **strength and nature of the immune response** to the hapten.
Question 993: Which human IgG subclass has the highest serum concentration?
- A. IgG1 (Correct Answer)
- B. IgG2
- C. IgG3
- D. IgG4
Explanation: ***IgG1*** - **IgG1** constitutes approximately **60-70%** of the total IgG in human serum, making it the most abundant subclass. - Its high concentration reflects its crucial role in **neutralizing toxins**, agglutinating viruses, and opsonizing bacteria for phagocytosis. *IgG2* - **IgG2** makes up about **20-30%** of total IgG and is primarily important in protecting against **polysaccharide-encapsulated bacteria**. - While significant, its serum concentration is notably lower than that of IgG1. *IgG3* - **IgG3** is the least abundant IgG subclass, accounting for only **5-8%** of total IgG, and is characterized by a shorter half-life. - Despite its low concentration, it is highly efficient in activating the **complement system** due to its flexible hinge region. *IgG4* - **IgG4** comprises approximately **3-6%** of total IgG and is unique for its ability to undergo **Fab arm exchange**, leading to bispecific antibodies. - It does not activate complement and is often associated with allergy and protection against parasitic infections, but its concentration is significantly less than IgG1.
Question 994: Which type of DNase produced by Streptococcus is known to be the most antigenic in humans?
- A. DNase C
- B. DNase D
- C. DNase A
- D. DNase B (Correct Answer)
Explanation: ***DNase B*** - **DNase B** is the most **antigenic** of the four DNases (A, B, C, D) produced by *Streptococcus pyogenes*. - Antibodies against DNase B are commonly measured as anti-DNase B titers in the diagnosis of **streptococcal infections** and their sequelae, like **acute rheumatic fever** and **post-streptococcal glomerulonephritis**, especially when ASO titers are negative. *DNase A* - While *Streptococcus pyogenes* produces **DNase A**, it is not considered the most antigenically potent or clinically relevant for antibody testing in humans. - Its antigenicity is generally **lower** than that of DNase B in response to streptococcal infection. *DNase C* - **DNase C** is another deoxyribonuclease produced by *Streptococcus pyogenes*. - Similar to DNase A, it does not elicit as strong or as consistent an **antibody response** in human infections compared to DNase B. *DNase D* - **DNase D** is the fourth type of deoxyribonuclease produced by *Streptococcus pyogenes*. - Its **antigenicity** in humans is less pronounced and less diagnostically significant than that of **DNase B**.
Question 995: What type of immunity is primarily associated with the administration of transfer factor?
- A. Natural active immunity
- B. Artificial active immunity
- C. Artificial passive immunity
- D. Adoptive immunity (Correct Answer)
Explanation: ***Adoptive immunity (Correct)*** - **Transfer factor** consists of small, dialyzable molecules extracted from immune T lymphocytes of an immune donor - Its administration transfers **cell-mediated immunity** from donor to recipient, which defines adoptive immunity - This represents transfer of **immune cells or their products** (not antibodies), providing antigen-specific cellular immunity - Also called **adoptive immunotherapy** or adoptive transfer *Artificial passive immunity (Incorrect)* - Involves the **transfer of pre-formed antibodies** (e.g., antitoxins, immunoglobulins, antiserum) from an immune individual or animal - Provides **immediate but temporary humoral protection** - Does NOT transfer cellular immunity - this is the key distinction from adoptive immunity - Examples: Anti-rabies immunoglobulin, anti-tetanus serum *Natural active immunity (Incorrect)* - Occurs when an individual is **naturally exposed to an antigen** (infection) and produces their own antibodies and immune cells - The host's own immune system **actively responds** to develop long-lasting immunity - Example: Immunity after recovering from measles or chickenpox *Artificial active immunity (Incorrect)* - Achieved through **vaccination** with attenuated, inactivated, or subunit antigens - The recipient's body is **actively stimulated** to produce protective immunity - Provides long-lasting protection through immunological memory
Question 996: CD3 is a marker for which type of cells?
- A. B - cells
- B. T - cells (Correct Answer)
- C. NK - cells
- D. Monocytes
Explanation: ***T - cells*** - **CD3** is a complex of proteins that is universally expressed on the surface of all **T lymphocytes** (T cells). - It plays a crucial role in **T cell activation** by transducing signals from the T cell receptor (TCR) to the cell's interior. *B - cells* - **B cells** are characterized by the expression of unique surface markers like **CD19**, **CD20**, and surface **immunoglobulins**, not CD3. - Their primary function is to produce **antibodies** and present antigens. *NK - cells* - **Natural Killer (NK) cells** are a type of lymphocyte that lacks both CD3 and a T cell receptor (TCR), differentiating them from T cells. - They express markers such as **CD16** and **CD56** and are involved in innate immunity, particularly against viral infections and tumor cells. *Monocytes* - **Monocytes** are myeloid cells, not lymphocytes, and are characterized by markers such as **CD14** and **CD68**. - They differentiate into macrophages and dendritic cells, playing a significant role in phagocytosis and antigen presentation.
Question 997: Which immunoglobulin does Protein A of Staphylococcus aureus bind to?
- A. IgA
- B. IgG (Correct Answer)
- C. IgD
- D. IgE
Explanation: ***IgG*** - **Protein A** of *Staphylococcus aureus* binds to the **Fc region** of IgG antibodies. - This binding mechanism helps *S. aureus* in **evading opsonization** and phagocytosis by interfering with the host immune response. *IgA* - IgA is primarily found in **mucosal secretions** and its primary role is in mucosal immunity. - Protein A does not preferentially bind to IgA; instead, some bacteria produce specific **IgA proteases** to cleave and inactivate IgA. *IgD* - IgD is found mostly on the surface of **B lymphocytes** and its main role is in B cell activation. - Protein A does not have a significant binding affinity for IgD. *IgE* - IgE is involved in **allergic reactions** and defense against parasites. - Protein A does not bind to IgE; binding to IgE is typically mediated by specific IgE receptors on mast cells and basophils.
Question 998: Phagocytosis of mycobacterium tuberculosis by macrophages is mainly mediated by:
- A. Interleukin 6
- B. Interleukin 3
- C. Interleukin 12
- D. Interferon Gamma (Correct Answer)
Explanation: ***Interferon Gamma*** - **Interferon gamma (IFN-γ)** is the most critical cytokine for **macrophage activation** in tuberculosis, enabling effective phagocytosis and intracellular killing of **Mycobacterium tuberculosis**. - IFN-γ (produced by **Th1 cells** and **NK cells**) primes macrophages by: - Enhancing **phagosome-lysosome fusion** - Increasing expression of **Fc receptors** and **complement receptors** for better opsonization - Stimulating production of **reactive oxygen species (ROS)** and **nitric oxide (NO)** - Upregulating **MHC class II** for improved antigen presentation - Without IFN-γ, macrophages cannot effectively control intracellular mycobacterial growth (as seen in **IFN-γ or IL-12 receptor deficiencies** leading to disseminated mycobacterial infections). *Interleukin 6* - **IL-6** is a pro-inflammatory cytokine involved in **acute-phase responses**, fever induction, and B-cell differentiation. - While it contributes to systemic inflammatory responses in TB, it does not directly activate macrophages for mycobacterial phagocytosis and killing. *Interleukin 3* - **IL-3** is a **hematopoietic growth factor** that promotes proliferation and differentiation of myeloid and lymphoid progenitor cells in bone marrow. - It plays no direct role in the effector functions of mature macrophages against *M. tuberculosis*. *Interleukin 12* - **IL-12** (produced by macrophages and dendritic cells) is essential for initiating **Th1 immunity** by promoting differentiation of naive CD4+ T cells into **Th1 cells** that produce IFN-γ. - IL-12 acts **upstream** of IFN-γ in the immune cascade but does not directly mediate macrophage phagocytic function. - The **IL-12/IFN-γ axis** is critical for TB immunity, but IFN-γ is the direct macrophage activator.
Question 999: Lattice phenomenon is seen in which of the following?
- A. Complement fixation test
- B. Precipitation test (Correct Answer)
- C. None of the options
- D. Neutralization reaction
Explanation: ***Precipitation test*** - The **lattice phenomenon** describes the formation of an interconnected network of antigen-antibody complexes, which is essential for visible precipitation to occur. - This phenomenon dictates that optimal precipitation requires a specific **antigen-to-antibody ratio**; an excess of either can lead to false-negative results due to soluble immune complexes (prozone or postzone effects). *Complement fixation test* - This test relies on the **binding of complement** to antigen-antibody complexes, leading to the lysis of indicator red blood cells if complement is not fixed. - It is a **two-stage test** that measures the consumption of complement, not the direct observation of a lattice. *None of the options* - This option is incorrect because the **precipitation test** clearly demonstrates the lattice phenomenon. - The formation of a visible precipitate is a direct result of antigen-antibody lattice formation. *Neutralization reaction* - Neutralization involves antibodies binding to toxins or viruses, **blocking their biological activity** rather than forming a visible precipitate. - It is a **functional assay** that measures the ability of antibodies to inhibit harmful effects, not the formation of large immune complexes.
Question 1000: Which of the following does not stimulate active immunity?
- A. Clinical infection
- B. Vaccination
- C. Transplacental antibody transfer in newborn (Correct Answer)
- D. Subclinical infection
Explanation: ***Transplacental antibody transfer in newborn*** - This is a form of **passive immunity**, where pre-formed **antibodies from the mother** are transferred to the newborn, providing immediate but temporary protection. - It does not involve the newborn's own immune system generating an immune response or creating **memory cells**. *Subclinical infection* - Even without overt symptoms, a **natural infection** exposes the immune system to pathogens, triggering an active immune response and generating **memory cells**. - This leads to **long-term immunity** against future exposures to the same pathogen. *Clinical infection* - A **symptomatic natural infection** involves the immune system actively responding to the pathogen, producing antibodies and **memory cells**. - This process is the basis of **naturally acquired active immunity** and provides durable protection. *Vaccination* - Vaccines contain weakened or inactive forms of pathogens, or their components, which stimulate the immune system to produce **antibodies** and **memory cells** without causing disease. - This is an example of **artificially acquired active immunity**, providing long-lasting protection.
Practice by Chapter
Cells and Organs of Immune System
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Innate Immunity
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Adaptive Immunity
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Antigens and Antibodies
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Major Histocompatibility Complex
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Complement System
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Cytokines and Chemokines
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Hypersensitivity Reactions
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Autoimmunity and Autoimmune Diseases
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Immunodeficiency Disorders
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Transplantation Immunology
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Tumor Immunology
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