Tetanus and diphtheria vaccines used in the Universal Immunisation Programme are classified as toxoids. What defines a toxoid?
- A An exotoxin treated with formaldehyde so that it loses toxicity but retains antigenicity ✓
- B A live attenuated organism that produces the toxin in vivo at reduced levels
- C The purified receptor-binding fragment of the toxin produced by recombinant DNA technology
- D A heat-killed whole organism bearing the toxin on its surface
Correct answer: A. An exotoxin treated with formaldehyde so that it loses toxicity but retains antigenicity
Explanation
A toxoid is an exotoxin denatured with formaldehyde (formalin) under controlled conditions, destroying its toxic activity while preserving its epitopes so it still induces antitoxin antibodies. Option C describes a true subunit or fragment vaccine such as CRM197, which is a nontoxic diphtheria toxin cross-reactive material used as a carrier protein, not as the diphtheria vaccine itself. Heat killing of whole organisms describes killed vaccines such as whole-cell pertussis.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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