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Both poison ivy and poison oak can cause a red rash on the skin due to an allergic reaction to the chemical urushiol in their oily sap. I've heard people say that they are allergic to either one or the other before but not both. It seems to me that since the chemical responsible for the reaction is the same in both plants one should be susceptible to both or neither at all.

Is it possible to only be allergic to one, and if so, how is this possible?

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According to the American Academy of Dermatology its the same oil in both poison ivy and poison oak along with sumac,

What you see and feel on your skin is caused by urushiol (you-ROO-shee-all). Urushiol is the oil in poison ivy, oak, and sumac. You find this oil in all parts of the plants — the leaves, stems, and even the roots.

Source

So there are a couple of possibilities,

  • They incorrectly identified a harmless plant as one of the harmful ones and didn't get the reaction they were expecting.

  • One gave a much smaller amount of oil and they didn't react to it.

  • Reactions get worse the more you are exposed, the first plant wasn't nearly as bad as when they were exposed the second time to the other plant.

In any case, I really don't think this is an experiment that's worth testing, especially as subsequent exposures lead to worse reactions.

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