A child receives the primary series of diphtheria-tetanus-pertussis (DTP) vaccine. The protective antigen in diphtheria component is the toxoid. Which cellular mechanism is MOST responsible for long-term immunological protection after toxoid vaccination?

• A Cytotoxic CD8+ T cells that directly kill Corynebacterium diphtheriae-infected cells
• B Natural killer cells primed by adjuvant to destroy toxin-producing bacteria
• C Mucosal IgA secreted by intestinal plasma cells providing systemic protection
• D Long-lived plasma cells in bone marrow secreting antitoxin IgG continuously, and memory B cells that rapidly expand on re-exposure ✓

Explanation

Toxoid vaccines generate a T-dependent humoral response; the primary immunological correlate of protection is neutralising antibody (antitoxin IgG ≥0.01 IU/mL for diphtheria, ≥0.01 IU/mL for tetanus). Long-lived plasma cells in the bone marrow provide sustained low-level antibody production, and memory B cells ensure rapid, high-titre anamnestic responses on re-exposure or booster doses. Cytotoxic T cells play a minor role in protection against toxin-mediated diseases. Mucosal IgA is the dominant immune response to mucosal vaccines (e.g., OPV), not IM injected toxoids.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.