How rare is it to be resistant to poison ivy, or become resistant. I ask only because when younger I was very allergic, but now I can pull it out with bare hands or roll around in it without even an itch.
The poison ivy plant itself is not what makes people itch. It's an oil called urushiol, which is inside the leaves, stems and roots of the plants. That's why some of the sources I quote below use the word urushiol, as that's the technical term for what causes the reaction.
Most of the research I found indicates that between 15% and 25% of people are resistant to the effects of poison ivy.
This source puts the number at 15%:
Have you ever wondered: Can I get poison ivy? What you're really asking is: Am I allergic to the plant?
Not everyone is. Up to 85% of Americans are allergic to poison ivy, leaving at least 15% resistant to any reaction.
Dr. David Adams, a dermatologist at Penn State in Hershey, Pennsylvania, puts the resistant figure at around 25%. He's quoted in an article published in Science Daily in July, 2015:
Three-quarters of the population will get an itchy red rash if exposed to the urushiol oil inside the plant's leaves, stem and roots. One-quarter of people will not have any reaction to exposure.
Even if you were in the majority group who were allergic as a child, there is research to back up your experience of being less allergic, or even not allergic at all, now that you're older.
Sensitivity to poison ivy, poison oak, and poison sumac varies from a mild to severe reaction, and may not cause any reaction at all the first time you're exposed. Some adults who reacted to poison ivy as children may find that they are now less sensitive. Some may even lose their sensitivity altogether.
From Live Science:
Question: I used to get poison ivy a lot when I was a kid. As an adult, I can't remember getting it. Do you develop an immunity to poison ivy?
Answer: Most people have some level of sensitivity to toxic plants. It's the world's most common allergy.
Sensitivity to poison ivy, oak and sumac tends to decline with age. People who got rashes as children usually see their sensitivity decrease by early adulthood. People who were once allergic to poison plants may even lose their sensitivity entirely later in life.
It can go the other way too.
From Myths About Poison Ivy:
Myth: Once allergic, always allergic to poison ivy.
Fact: A person's sensitivity to urushiol reaction changes over time, and even from season to season. People who were sensitive to poison ivy as children may not be allergic as adults and vice versa.
Most of us are born into this world immune to poison ivy. But that can change at any time, even well into adult hood. When that happens, it almost always means that the allergy lasts for the rest of a lifetime. However, there are contrary claims: Some people who as a child suffered a severe rash allegedly develop immunity as adults.
I don't know how rare resistance is, but it seems the real question is about resistance coming and going.
Yes, resistance can come and go for individuals over their lifetime. Exposure, or long periods of non-exposure, can make a difference, but the result seems to not be predictable.
Most anecdotes are from people that were, or thought they were, resistant at some point, then have a sudden and unpleasant reaction one day. Of course those are the anecdotes we are more likely to tell and pass along than the other way around, so meaningful data is hard to tell.
However, people being resistant, then one day not any more, has happened to three people I've personally talked to. It can definitely happen.
The moral of the story is to always treat poison ivy (or any of the urushiol-bearing plants) as if you are allergic, because you can't ever rely on not being allergic in any one case.