The substance in the ivy is called Urushiol Oil.
https://en.wikipedia.org/wiki/Urushiol
https://en.wikipedia.org/wiki/Hapten
https://en.wikipedia.org/wiki/Type_IV_hypersensitivity
”Urushiol is too small a molecule to directly activate an immune response. Instead, it attaches to certain proteins of the skin, where it acts as a hapten, leading to a type IV hypersensitive reaction.”
”Haptens are small molecules that elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one that also does not elicit an immune response by itself (in gen ...
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