Immunology — MCQs
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Which of the following statements about the secondary immune response is false?
IgE is secreted by which of the following cells?
CD marker for cytotoxic T-cells:
Which complement component is involved in both the classical and alternative pathways?
Which complement proteins representing both classical and alternative pathways are primarily synthesized in the liver?
Which immunoglobulin provides natural passive immunity to a newborn through the placenta?
Which immunoglobulin is most efficient at fixing C1q in the classical complement pathway?
Viral infection is differentiated from bacterial infection by:
Which of the following is not considered an antigen-presenting cell?
A 62-year-old woman presents with fever, cough, sputum production, and pleuritic chest pain. Chest X-ray reveals a right middle lobe infiltrate, and she is started on antibiotics for the treatment of pneumonia. Her sputum Gram stain is positive for S. pneumoniae. Which of the following immunologic mechanisms is the most specific host defense against pneumococcal infection?
Immunology Indian Medical PG Practice Questions and MCQs
Question 1051: Which of the following statements about the secondary immune response is false?
- A. The lag period is absent or significantly shorter.
- B. There is a negative phase in the response.
- C. Only T-dependent antigens are recognized.
- D. Immune response against a subsequent antigenic challenge is absent. (Correct Answer)
Explanation: ***Immune response against a subsequent antigenic challenge is absent.*** - This statement is **false** because the secondary immune response is characterized by a **much stronger and faster** immune response upon subsequent exposure to the same antigen. - The presence of **memory cells** ensures that the immune system is highly prepared to combat the antigen more efficiently than during the primary response. *The lag period is absent or significantly shorter.* - This statement is **true** for the secondary immune response. The **memory B and T cells** can be rapidly activated, reducing the time needed to mount an effective response. - Unlike primary responses that can take 5-10 days to produce antibodies, secondary responses typically produce antibodies within **1-3 days**. *There is a negative phase in the response.* - This statement is **false** for the secondary immune response. The **negative phase** is characteristic of the **primary immune response**, not the secondary response. - The negative phase in primary response refers to a transient drop in antibody concentration after initial antigen exposure due to antigen-antibody complex formation. However, the **secondary response shows immediate and robust antibody production** without this negative phase due to pre-existing memory cells. - While this statement is technically false, the question asks for THE false statement, and Option D is more obviously and fundamentally false. *Only T-dependent antigens are recognized.* - This statement is **partially false** but has some truth in context. While **T-dependent antigens** generate the most robust secondary responses with strong memory cell formation, the immune system doesn't ONLY recognize T-dependent antigens. - **T-independent antigens** can elicit responses but typically generate weaker, shorter-lived immunity without strong memory formation. The classical, robust secondary immune response with anamnestic features is predominantly associated with T-dependent antigens.
Question 1052: IgE is secreted by which of the following cells?
- A. Mast cell
- B. Basophils
- C. Eosinophils
- D. Plasma cells (Correct Answer)
Explanation: ***Plasma cells*** - Plasma cells are **terminally differentiated B lymphocytes** that are specialized in producing and secreting large quantities of antibodies, including **IgE**. - While other cells like mast cells and basophils have receptors for IgE and play roles in IgE-mediated reactions, they do not synthesize **IgE** themselves. *Mast cell* - Mast cells are key players in allergic reactions and express **FcεRI receptors** that bind to IgE antibodies. - Upon binding the antigen, they degranulate, releasing **histamine** and other mediators, but they do not produce IgE. *Basophils* - Basophils also express **FcεRI receptors** for IgE and are involved in allergic responses, releasing inflammatory mediators. - Similar to mast cells, they do not synthesize IgE, but rather bind pre-formed **IgE** antibodies. *Eosinophils* - Eosinophils are involved in allergic reactions and defense against **parasitic infections**, and their granules contain toxic proteins. - They can be activated by IgE-mediated mechanisms but are not producers of **IgE** antibodies.
Question 1053: CD marker for cytotoxic T-cells:
- A. CD 45
- B. CD 4
- C. CD 8 (Correct Answer)
- D. CD 21
Explanation: ***CD 8*** - CD 8 is a specific **marker for cytotoxic T-cells**, which play a crucial role in directly killing infected or cancerous cells [1]. - Cells expressing CD 8 are essential for **cell-mediated immunity**, particularly in recognizing and eliminating virus-infected cells. *CD 21* - CD 21 is a marker primarily associated with **B-lymphocytes** and acts as a receptor for **C3d**, involved in the activation of B-cells. - It does not play a role in the identification or function of **cytotoxic T-cells**. *CD 4* - CD 4 is a marker for **helper T-cells**, which assist other cells in the immune response but do not directly kill targets [1]. - It is involved in enhancing the immune response rather than cytotoxic functions. *CD 45* - CD 45 is a pan-leukocyte marker found on all **leukocytes** and is not specific to cytotoxic T-cells. - While it indicates the presence of immune cells, it does not differentiate between various T-cell subtypes. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 198-199.
Question 1054: Which complement component is involved in both the classical and alternative pathways?
- A. C3 (Correct Answer)
- B. C1
- C. C2
- D. C4
Explanation: ***C3*** - **C3** is a central component of the complement system, acting as a point of convergence for both the **classical** and **alternative pathways**. - Its cleavage product, **C3b**, is crucial for opsonization and the formation of the **C5 convertase**, thereby initiating the **membrane attack complex (MAC)**. *C1* - **C1** is unique to the **classical complement pathway** and is responsible for its activation, typically initiated by antigen-antibody complexes. - It consists of C1q, C1r, and C1s, and its primary role is to cleave **C4** and **C2**. *C2* - **C2** is exclusively involved in the **classical** and **lectin pathways**, where it is cleaved by C1 or **MASP** respectively. - The resulting fragment, **C2a**, combines with C4b to form the **C3 convertase** (C4b2a). *C4* - Similar to C2, **C4** is a component of the **classical** and **lectin pathways**, not the alternative pathway. - It is cleaved by C1 or **MASP** to form **C4a** and **C4b**, with C4b binding to the pathogen surface.
Question 1055: Which complement proteins representing both classical and alternative pathways are primarily synthesized in the liver?
- A. C3, C6, C9
- B. C1, C4, C2, factor B (Correct Answer)
- C. Factor H, factor I, DAF
- D. C1-C9, factor B, factor D, properdin
Explanation: ***C1, C4, C2, factor B*** - This option correctly identifies key complement proteins from **both the classical pathway (C1, C4, C2) and the alternative pathway (factor B)** that are synthesized in the liver. - These proteins represent the liver's critical role in producing components for **multiple complement activation pathways**. - This is the most specific answer highlighting the liver's role in synthesizing proteins from different complement pathways. *C3, C6, C9* - While **C3, C6, and C9** are indeed synthesized in the liver, this option is incomplete. - It omits other major liver-derived complement components from the classical and alternative pathways. - C3 is the most abundant complement protein, but this option doesn't represent both pathways comprehensively. *Factor H, factor I, DAF* - **Factor H** and **Factor I** are regulatory proteins synthesized in the liver, but **DAF (Decay-accelerating factor, CD55)** is a membrane-bound complement regulatory protein. - DAF is expressed on cell surfaces and is not a secreted liver-derived complement protein. - This option focuses on regulatory proteins rather than pathway components. *C1-C9, factor B, factor D, properdin* - While this option is broadly correct (most of these are liver-synthesized), it lists an extensive range of proteins without specifically highlighting the pathway representation. - The question asks for proteins representing both pathways, and this comprehensive list is less focused than the specific classical and alternative pathway components in the correct answer. - This option is too exhaustive for the focused intent of the question.
Question 1056: Which immunoglobulin provides natural passive immunity to a newborn through the placenta?
- A. IgA
- B. IgG (Correct Answer)
- C. IgE
- D. IgD
Explanation: ***IgG*** - **IgG** is the only immunoglobulin that can cross the **placenta**, providing the fetus with **passive immunity** from the mother's antibodies. - This maternal IgG protects the newborn from various infections during the first few months of life until its own immune system matures. *IgA* - **IgA** is primarily found in **mucosal secretions**, such as breast milk, saliva, tears, and gastrointestinal fluids. - While important for newborn immunity via **breastfeeding**, it does not cross the placenta. *IgE* - **IgE** is mainly involved in **allergic reactions** and defense against **parasitic infections**. - It does not cross the placenta to provide passive immunity to the fetus. *IgD* - **IgD** is primarily found on the surface of **B lymphocytes** and is involved in B-cell activation. - Its function is not related to passive fetal immunity through the placenta.
Question 1057: Which immunoglobulin is most efficient at fixing C1q in the classical complement pathway?
- A. IgA
- B. IgG
- C. IgM (Correct Answer)
- D. IgE
Explanation: ***IgM*** - **IgM** is the most efficient immunoglobulin at activating the **classical complement pathway** due to its pentameric structure, which provides multiple binding sites for C1q. - The **Fc portion** of IgM, when bound to an antigen, undergoes a conformational change that exposes binding sites for the **C1q component** of complement. *IgA* - **IgA** primarily functions in **mucosal immunity** and does not efficiently activate the classical complement pathway. - It can weakly activate the **alternative complement pathway** but is not known for fixing C1. *IgG* - **IgG** can activate the classical complement pathway, but it requires two or more IgG molecules to be in close proximity on the cell surface to effectively bind C1q, making it less efficient than IgM. - Its **Fc region** binds C1q only when **antigen-antibody complexes** are formed, but it's not the primary immunoglobulin for initiating C1 binding in solitary form. *IgE* - **IgE** is primarily involved in **allergic reactions** and defense against parasites, binding to Fc receptors on mast cells and basophils. - It does **not activate the complement system** via the classical pathway and therefore does not fix C1.
Question 1058: Viral infection is differentiated from bacterial infection by:
- A. Interferon production (Correct Answer)
- B. Toxin production
- C. Lymphocytosis
- D. Neutrophilia
Explanation: ***Interferon production*** - **Interferons** are cytokines primarily produced by host cells in response to **viral infections**, playing a crucial role in antiviral defense. - Interferon production is a **hallmark of viral infection** and the primary mechanism differentiating viral from bacterial pathogenesis. - While bacteria can induce some interferon responses, their primary immune evasion and pathogenesis mechanisms do **not center around interferon production** in the same way as viruses. *Toxin production* - **Bacterial toxins** (exotoxins, endotoxins) are far more common and represent a primary mechanism of bacterial pathogenicity. - Most viruses do not produce classical toxins; they damage cells primarily through direct cytopathic effects and immune-mediated mechanisms. *Neutrophilia* - An increase in **neutrophils** (neutrophilia) is a classic hallmark of acute **bacterial infections**, as neutrophils are the primary phagocytes that fight bacteria. - Viral infections typically lead to a normal or decreased neutrophil count, often accompanied by a relative increase in lymphocytes. *Lymphocytosis* - A relative **lymphocytosis** (increased lymphocyte count) is often indicative of **viral infection**, as part of the adaptive immune response. - Both viral and bacterial infections activate lymphocytes, but viral infections characteristically show predominant lymphocyte response, while bacterial infections more commonly cause **neutrophilic leukocytosis**.
Question 1059: Which of the following is not considered an antigen-presenting cell?
- A. Macrophages
- B. Langerhans cells
- C. Thymocytes (Correct Answer)
- D. B lymphocytes
Explanation: ***Thymocytes*** - Thymocytes are **developing T cells** found in the thymus and do not function as antigen-presenting cells (APCs) [1]. - Unlike APCs, thymocytes are primarily involved in the **maturation** and selection of T lymphocytes. *Langerhans cells* - Langerhans cells are a type of **dendritic cell** found in the skin and are effective antigen-presenting cells to T cells [1]. - They play a crucial role in **immune surveillance** and response to skin infections. *Macrophages* - Macrophages are well-known antigen-presenting cells that engulf pathogens and present antigens to T cells [1]. - They are also involved in **phagocytosis** and secrete various cytokines to modulate immune responses. *M-cells* - M-cells (microfold cells) are specialized epithelial cells that transport antigens from the intestinal lumen to underlying immune cells. - Although not traditional APCs, they play a role in immune surveillance and stimulating **mucosal immunity**. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 200, 207-208.
Question 1060: A 62-year-old woman presents with fever, cough, sputum production, and pleuritic chest pain. Chest X-ray reveals a right middle lobe infiltrate, and she is started on antibiotics for the treatment of pneumonia. Her sputum Gram stain is positive for S. pneumoniae. Which of the following immunologic mechanisms is the most specific host defense against pneumococcal infection?
- A. IgG antibody directed against capsular antigens (Correct Answer)
- B. complement system activity
- C. macrophage activity in the alveoli
- D. functional splenic activity
Explanation: ***IgG antibody directed against capsular antigens*** - **IgG antibodies** against the capsular polysaccharides of *S. pneumoniae* are crucial for **opsonization** and subsequent **phagocytosis**, providing specific and effective immunity. - This antibody response is essential for **humoral immunity** and forms the basis of pneumococcal vaccines. *complement system activity* - The **complement system** plays a role in immunity against *S. pneumoniae* by promoting **opsonization** and bacterial lysis, especially through the classical and alternative pathways. - However, while important, it is not as specific as IgG antibodies directed against the **capsular antigens** because it can be activated by various pathogens and mechanisms. *macrophage activity in the alveoli* - **Alveolar macrophages** are crucial initial responders in the lungs, phagocytosing bacteria and initiating inflammation. - While they are important in the **early innate immune response**, their effectiveness against encapsulated bacteria like *S. pneumoniae* is significantly enhanced by opsonization through antibodies and complement. *functional splenic activity* - The **spleen** is vital for clearing encapsulated bacteria, including *S. pneumoniae*, from the bloodstream, particularly in individuals with **asplenia**. - Its role is primarily in filtering blood and housing phagocytic cells, but it relies on an effective **antibody-mediated mechanism** for optimal clearance.
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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