A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 65-year-old male with a history of COPD presents to the emergency department with dyspnea, productive cough, and a fever of 40.0°C (104.0°F) for the past 2 days. His respiratory rate is 20/min, blood pressure is 125/85 mm Hg, and heart rate is 95/min. A chest X-ray is obtained and shows a right lower lobe infiltrate. Sputum cultures are pending and he is started on antibiotics. The patient has not received any vaccinations in the last 20 years. The physician discusses with him the importance of getting a vaccine that can produce immunity via which of the following mechanisms?
-
A
T cell-dependent B cell responseCorrect. Pneumococcal conjugate vaccine (PCV13/15/20) couples capsular polysaccharide to a protein carrier, eliciting a T-cell-dependent B-cell response with class switching to IgG and durable memory — superior to the T-cell-independent response of plain polysaccharide vaccines.
-
B
Natural killer cell responseIncorrect. Natural killer cells are part of innate immunity and target virally infected or stressed cells via NKG2D and missing-self mechanisms; they are not the mechanism of vaccine-induced adaptive immunity.
-
C
Mast cell degranulation responseIncorrect. Mast cell degranulation is the basis of immediate (type I) hypersensitivity reactions and IgE-mediated allergy, not protective vaccine immunity.
-
D
No need to vaccinate, as the patient has already had a pneumonia vaccineIncorrect. The patient has not received any vaccines in 20 years and almost certainly has not had PCV13/15/20 (newer products); vaccination is needed.
-
E
Type 1 hypersensitivity response with IgE productionIncorrect. Type 1 hypersensitivity with IgE is the mechanism of allergic reactions to vaccines (e.g., egg-protein anaphylaxis), not the protective immunity goal of vaccination — protective pneumococcal vaccine immunity is mediated by T-cell-dependent IgG class switching.
↑ Tap an answer to reveal the reasoning
Answer: A. An older patient with COPD and bacterial pneumonia who has not been vaccinated in 20 years urgently needs both the pneumococcal vaccine and the annual influenza vaccine. Two pneumococcal vaccine formulations exist with different immune mechanisms:
1) Pneumococcal POLYSACCHARIDE vaccine (PPSV23): contains 23 capsular polysaccharide antigens. As pure polysaccharides, they elicit a T-cell-INDEPENDENT B-cell response, producing IgM with poor immunologic memory and limited efficacy in infants and immunocompromised hosts.
2) Pneumococcal CONJUGATE vaccine (PCV13 or PCV15/PCV20): conjugates the polysaccharide to a protein carrier (CRM197, a diphtheria toxin variant). The protein carrier engages helper T cells, eliciting a T-cell-DEPENDENT B-cell response with isotype switching to IgG, affinity maturation, and durable memory B cells.
For adults ≥65 or younger adults with chronic lung disease (this patient has COPD), the ACIP recommends PCV20 alone or PCV15 followed by PPSV23. The benefit of the conjugate vaccine — which is the immunologic improvement being emphasized — is precisely its T-cell-dependent mechanism, producing more robust and durable protective immunity.
**Why each option:**
**A.** Correct. Pneumococcal conjugate vaccine (PCV13/15/20) couples capsular polysaccharide to a protein carrier, eliciting a T-cell-dependent B-cell response with class switching to IgG and durable memory — superior to the T-cell-independent response of plain polysaccharide vaccines.
**B.** Natural killer cells are part of innate immunity and target virally infected or stressed cells via NKG2D and missing-self mechanisms; they are not the mechanism of vaccine-induced adaptive immunity.
**C.** Mast cell degranulation is the basis of immediate (type I) hypersensitivity reactions and IgE-mediated allergy, not protective vaccine immunity.
**D.** The patient has not received any vaccines in 20 years and almost certainly has not had PCV13/15/20 (newer products); vaccination is needed.