Immunology — MCQs
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An 80-year-old woman, a retirement home resident, has multiple bouts of pneumonia caused by Streptococcus pneumoniae. In an attempt to prevent such infections, polyvalent vaccines directed at multiple serotypes of the organism have been administered but have not elicited long-acting immunity. Which of the following is the probable explanation for this phenomenon?
All of the following forces are involved in antigen-antibody reactions, except:
Congenital passive immunity is INADEQUATE in -
Which type of antibody is primarily involved in the secondary immune response?
Which interleukin is specifically secreted by Th17 cells?
Immunology Indian Medical PG Practice Questions and MCQs
Question 1061: An 80-year-old woman, a retirement home resident, has multiple bouts of pneumonia caused by Streptococcus pneumoniae. In an attempt to prevent such infections, polyvalent vaccines directed at multiple serotypes of the organism have been administered but have not elicited long-acting immunity. Which of the following is the probable explanation for this phenomenon?
- A. The bacterial capsule binds C3b, facilitating activation of the alternative complement pathway, inducing complement-mediated lysis, and preventing immunization.
- B. The capsular polysaccharides of S. pneumoniae have limited hapten potential.
- C. S. pneumoniae evades host immune response by forming capsular coatings composed of host proteins and recognized as "self" antigens.
- D. Memory T lymphocytes respond poorly to polysaccharide antigens. (Correct Answer)
Explanation: ***Correct: Memory T lymphocytes respond poorly to polysaccharide antigens.*** - T cells are activated by **peptide antigens** presented by MHC molecules; they do not recognize **polysaccharide antigens** directly. - Vaccines composed of purified polysaccharide antigens (like in the polyvalent *S. pneumoniae* vaccine) primarily stimulate a **T-cell-independent B-cell response**, which typically results in a weaker immune response, poor memory, and limited class switching, especially in older individuals. - This is why **conjugate vaccines** (polysaccharide linked to protein carriers) were developed—they convert the T-independent antigen into a T-dependent one, generating better memory responses. *Incorrect: S. pneumoniae evades host immune response by forming capsular coatings composed of host proteins and recognized as "self" antigens.* - The capsule of *S. pneumoniae* is composed of **polysaccharides**, not host proteins. - It evades the immune system by being poorly immunogenic and preventing phagocytosis, but not by mimicking "self" antigens. *Incorrect: The bacterial capsule binds C3b, facilitating activation of the alternative complement pathway, inducing complement-mediated lysis, and preventing immunization.* - The **capsule** of *S. pneumoniae* actually **inhibits C3b binding** and prevents activation of the alternative complement pathway, thereby *resisting* complement-mediated lysis and opsonization. - This resistance is a mechanism of immune evasion, not prevention of immunization. *Incorrect: The capsular polysaccharides of S. pneumoniae have limited hapten potential.* - While polysaccharide antigens can be considered haptens in a sense if they require a carrier protein to become fully immunogenic, the primary issue is their inability to activate T cells. - The limitation in hapten potential isn't the most direct or impactful explanation for the lack of long-lasting immunity compared to the T-cell dependence of memory responses.
Question 1062: All of the following forces are involved in antigen-antibody reactions, except:
- A. Electrostatic bond
- B. Hydrogen bond
- C. Covalent bond (Correct Answer)
- D. Van der Waals forces
Explanation: ***Covalent bond*** - **Covalent bonds** are strong, irreversible bonds that involve the sharing of electrons between atoms. - Antigen-antibody interactions are predominantly **non-covalent** and reversible, allowing for dynamic binding and release. *Vander Waal's forces* - **Van der Waals forces** are weak attractive forces that arise from temporary fluctuations in electron distribution, creating transient dipoles. - They are crucial in antigen-antibody binding, especially when the molecules are in **close proximity**, contributing to overall affinity. *Electrostatic bond* - **Electrostatic (ionic) bonds** occur between oppositely charged groups on the antigen and antibody surfaces. - These interactions are significant for **initial recognition** and overall binding stability, particularly at appropriate pH levels. *Hydrogen bond* - **Hydrogen bonds** form between a hydrogen atom covalently linked to an electronegative atom (like oxygen or nitrogen) and another electronegative atom. - They play a vital role in the **specificity and strength** of antigen-antibody interactions by providing numerous weak, directional contacts.
Question 1063: Congenital passive immunity is INADEQUATE in -
- A. Measles
- B. Mumps
- C. RSV (Respiratory Syncytial Virus)
- D. Pertussis (Correct Answer)
Explanation: ***Pertussis*** - **Congenital passive immunity** against *Bordetella pertussis* is **most inadequate** among the listed infections. - **Minimal transplacental transfer** of protective IgG antibodies occurs, and maternal antibodies decline rapidly in infants. - Newborns have **virtually no protection** from maternal antibodies, making them highly susceptible to severe whooping cough. - This is why **early vaccination at 6 weeks** is critical, unlike measles which can wait until 9-12 months. *Measles* - Maternal antibodies provide **excellent passive immunity** protecting infants for **6-9 months**. - This robust protection is why measles vaccination is delayed until 9-12 months of age. - Represents the **gold standard** of effective maternal antibody transfer. *Mumps* - Maternal antibodies provide **good passive immunity** in early infancy. - Mumps in young infants is rare due to this maternal protection. *RSV (Respiratory Syncytial Virus)* - Maternal antibodies provide **limited but present** passive immunity. - Unlike pertussis where protection is nearly absent, RSV maternal antibodies can **reduce severity** of illness. - However, RSV remains a major cause of bronchiolitis in infants despite this partial protection. - The key difference: RSV has **some** maternal protection (inadequate but present), whereas pertussis has **almost none** (most inadequate).
Question 1064: Which type of antibody is primarily involved in the secondary immune response?
- A. IgA
- B. IgG (Correct Answer)
- C. IgM
- D. IgE
Explanation: ***IgG*** - **Immunoglobulin G (IgG)** is the predominant antibody in the **secondary immune response**, offering long-term immunity and protection. - It is produced in large quantities and has a longer half-life compared to other immunoglobulins, allowing for sustained defense against pathogens. *IgA* - **Immunoglobulin A (IgA)** is primarily found in **mucosal secretions** like tears, saliva, and breast milk, providing local immunity. - While important for first-line defense, it is not the main antibody elevated during a secondary systemic immune response. *IgM* - **Immunoglobulin M (IgM)** is the first antibody produced during the **primary immune response** to a new pathogen. - Although it indicates an immediate immune reaction, it is gradually replaced by IgG in the later stages and during secondary responses. *IgE* - **Immunoglobulin E (IgE)** is primarily involved in **allergic reactions** and defense against **parasites**. - Its levels significantly increase during these specific conditions, but it does not play a major role in a typical secondary immune response to common pathogens.
Question 1065: Which interleukin is specifically secreted by Th17 cells?
- A. IFN Gamma
- B. IL6
- C. IL-17 (Correct Answer)
- D. IL-22
Explanation: ***IL22*** - Th17 cells predominantly secrete **IL-17** and also produce **IL-22**, which is significant in mucosal immunity and inflammation [1]. - **IL-22** plays a crucial role in the response to infections and in the pathogenesis of inflammatory diseases. *IL16* - IL-16 is primarily associated with **chemoattractant and regulatory functions** for lymphocytes and not directly secreted by Th17 cells. - It is involved in **eosinophil and T cell activation**, which is not characteristic of the Th17 response. *IFN Gamma* - IFN-gamma is mainly produced by **Th1 cells** and is critical for **cell-mediated immunity**, which is distinct from the function of Th17 cells. - It plays a role in activating **macrophages**, unlike Th17 cells which focus on **neutrophil recruitment** and inflammation. *IL6* - While IL-6 is a pro-inflammatory cytokine that can be involved in various immune responses, it is not primarily secreted by Th17 cells. - It is produced by a variety of cell types including fibroblasts and macrophages, acting as a mediator in the **acute phase response**. **References:** [1] 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. 158-160.
Practice by Chapter
Cells and Organs of Immune System
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Innate Immunity
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Complement System
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