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Edward Jenner's smallpox vaccine worked on which principle?
A{'text': 'Inactivated bacterial toxin'}
B{'text': 'Killed pathogen suspended in oil'}
C{'text': 'Genetic engineering of the pathogen'}
D{'text': 'Cross-protection by a related virus'}
Answer & Solution
Correct answer: D. {'text': 'Cross-protection by a related virus'}
1. Jenner observed that milkmaids who caught **cowpox** rarely got smallpox.
2. He deliberately exposed people to cowpox material, conferring immunity to smallpox.
3. This is **cross-protection**: a related but milder virus stimulates antibodies that also recognise the deadly virus.
4. The toxoid principle (formaldehyde) wasn't known yet; killed-pathogen and recombinant vaccines came much later.
5. So the principle is **cross-protection by a related virus**.
_Source: Selina Concise ICSE Class 8 Biology Ch 8 "Diseases and First Aid", Vaccine preparation types_
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