Why Are Some People More Susceptible To Poison Ivy Than Others?

Why Are Some People More Susceptible To Poison Ivy Than Others?

- in Health
Poison Ivy Allergic

Why is it that some people can lay in poison ivy for hours and nothing happens, yet others have severe reactions with the slightest exposure?

To find out, we’ll need to take a closer look at what causes the reaction.

The Cause

Poison ivy (as well as poison sumac and poison oak) contain an oil called Urushiol. This oil is what causes the allergic reaction associated with poison ivy. Urushiol has a dermatitis-producing principle, pentadecylacatechol. This chemical does not evaporate and will dry quickly on objects it comes into contact with.

This chemical can remain potent for a year or more. It is important to wash any objects, especially clothing after contact with poison ivy. The resin will remain active on these articles and can cause a rash months, or even years, later.

The word ‘urushiol’ is derived from ‘urushi’, The Japanese name for lacquer. The Japanese lacquer tree contains small amounts of urushiol in its sap, and those who are sensitive to the oil can even react when they come into contact with lacquered furniture from Japan.

So why are some people more sensitive than others?

Roughly 85% of all humans will acquire or have an allergy to poison ivy. The remaining 15% are completely immune, able to roll around in it without having a reaction.Poison Ivy There is no guarantee however, that a person who is resistant won’t become sensitive later on down the road. This is because people aren’t born with allergies; they develop them after exposure. Repeated exposures can make one even more sensitive to the irritant or allergen.

It is very uncommon for those under the age of five to get the itchy rash and blisters associated with a poison ivy reaction. Sensitivity for many can also fade with age, and quite a few adults who suffered severe reactions as children will find that they are no longer allergic or have a very mild reaction to poison ivy later in their life.

This means that the people who don’t seem allergic to poison ivy might lie in the 15 percent who are resistant, or they may just no longer be as sensitive as they were as a child.

Is there any way to prevent minimize exposure?

Aside from avoiding the plants entirely and using common sense, many forest service employees spray antiperspirant deodorant on thier exposed skin before they go out if they they think they might come in contact with poison ivy. The aluminum chlorohydrate may help prevent penetration of the oil through the skin.

If you do come into contact with poison ivy, the best preventive method to minimize exposure is to wash with lots of soap and water within 15 minutes of contact. After 15 minutes, the chemical that causes the reaction has already bonded to the proteins in your skin.

If a severe reaction develops, contact a dermatologist immediately, or go to an emergency room. Prescription medication may be needed to reduce itching and the swelling.

Hawley’s Condensed Chemical Dictionary, 14th Edition, 2002, John Wiley & Sons, Inc.
Donald G. Barceloux (2008). Medical Toxicology of Natural Substances: Foods, Fungi, Medicinal Herbs, Plants, and Venomous Animals. John Wiley and Sons. pp. 681
Robert Alan Lewis (1998). Lewis’ dictionary of toxicology. CRC Press. pp. 901


  1. Be aware that poison ivy will attack you no matter what age. I am 82, and just recovering from one of the worst attacks of my long life. Note that the P. I. leaves are red, yellow, and orange during the fall, which is what attracted me because I was collecting fall leaves for an art project. Remember, to ID the poison ivy VINE, look for hairs on the stem.

  2. This article actually doesn’t at all explain why the 15% aren’t affected. There’s some level of false advertising happening here.

    1. I have yet to find anyone who discusses the ‘why’. Only the ‘how’ and ‘how many are effected’. Funny, ‘why’ is in the title, but isn’t really answered, unless you want to attribute it to increased exposure, but that doesn’t really explain it, IMO. I used to never be sensitive to poison ivy, then became so, then wasn’t again later. I attribute it to my immune system’s handling of stress, and this may be a key. Generally, as people age, their immune system takes more and more hits. Older people may lose sensitivity, as often the stress of performance decreases.

    2. I have always been one that couldn’t get it… It’s true that I haven’t “tested” my immunity lately but I lived near a deep wooded area which was laden w/ the stuff I’d play Army for hours in those woods and upon noticing that all of my friends would get it. I did seek these areas as hiding spots and never again never developed Poison ivy.

      I believe I have at least part of the answer; I have always had very, as in extremely, oily skin as a youngster and well into my 30s and it is my belief that the oil somehow “protected” me from the effects.

  3. My father used to brag about his immunity to poison ivy. He said he he would play in it barefoot as a child. He would pick leaves and rub them on his arm, and nothing. One day, when he was in his late 50’s, he was picking blackberries that were covered with poison ivy(bad idea because unless he washed them well, the toxin could gotten into the preserves my mother was making). THIS time, he got a reaction. I suppose that this one last time is what sent him over. I’ve 44 and have never had Poison Ivy but my ex-wife could get it by just being near the stuff. I’ve weed whacked poison ivy in shorts(by accident) and no response, but as rule, I avoid contact. The stuff is nasty, the oil can stay around for years. Say you use a hoe to hack a plant, then place in in your garage without washing, you don’t use the hoe for a year…. then pick it up, if you’re sensitive, you can still get a reaction. Also interesting is that it’s only Humans that appear affected. Your pets won’t get poison ivy, but they will get the oil on their fur, and you can get a reaction by petting them.

  4. 15% don’t have a response just like 15% HAVE a response to latex. To 20% of the population Cilantro tastes like soap. It’s genetic. The numbers aren’t exact of course, it’s a rough estimate. What’s unique is that reactions can work backwards with this stuff. You’re ‘immunity’ in weakened with each exposure. See the story about about my dad. You can actually build up a sensitivity over time. I won’t say I’m immune but I’m not as sensitive as some.

  5. I have always been allergic to poison ivy. I get it from handling firewood and from my dogs.They dig in the woods which is full of poison ivy and the roots I know several times I got it from their paws. I’ve never had a mild case of it until the last few years. I looked and looked for something better than Calamine lotion. I thought something had to be better than that. I read that vinegar was good to wash the oil off. So far that has worked great. I’ve experienced it enough to know what that first little itch feels like before anything is visible. It has stopped it every time and I was surprised to learn it will even stop it after a rash begins. Before I started using vinegar it would get worse and worse and easily last a for weeks or a month. I have no doubt it has worked for me.

  6. Charles_Miller

    Starting at the age of 15, I worked in a tree-cutting crew, and I quickly became “the golden child” when the rest of the crew discovered that I was immune to poison ivy, poison oak, and poison sumac. It thus became one of my tasks (a very important task) to go in and rip out all the toxic plants barehanded and dispose of them, clearing the way for the other guys to do their jobs. All of the other guys were allergic, but I never had so much as an itch from contact with any of these plants. Today I’m 55 years old, and I’m still ripping out poison ivy, poison oak and poison sumac around my property and for my neighbors, doing so barehanded with no reaction whatsoever. My old doctor explained to me once that about two-thirds of the human population of this planet are allergic to poison ivy and similar toxic plant oils, while my newest doctor claims that about 85% of all humans are allergic. Yes, there are some people who have less severe allergic reactions than others, but true “immunity” is NOT a myth. There are, indeed, a small number of people in the world who are completely and permanently immune to poison ivy, poison oak, and poison sumac.

    While natural immunity may sound desirable, it actually has some huge drawbacks; mainly, the fact that I can go hiking or rolling around in poison ivy with no reaction means that I’m not always AWARE that I’m carrying the toxins on my skin and clothing. Thereafter, everything I touch and everyone with whom I come in contact can become contaminated with the toxic oils. This generally means that (when I’m returning from the wilderness) I have to strip down and rinse myself with a concoction of water, liquid dish detergent and rubbing alcohol and throw my clothing in the back-porch washer BEFORE I ever enter the house or even touch the doorknob. Sometimes being immune isn’t as advantageous as it sounds at first.

    1. Anyone else in your family resistant? People often say it’s genetic, but if that were true you’d expect it to run in families.

  7. I am one of those people who are completely immune to poison oak, ivy, and sumac. The reason? I have O-negative blood. This blood type is very rare; it was said that 15% of all people in the world have Rh negative blood. However, only 6.6% have O-negative blood.
    I donate Anti-D plasma to make the Rhogam shot for pregnant women. There are also some of us that have two markers that make us immune to HIV/AIDS.

    Everyone that I have met that has O-negative blood is immune to poison ivy, oak and sumac. I have not met one that wasn’t, yet. Only 6.6% of the population has O-negative blood.
    I have never had an infection of any kind, never had stomach problems. Not even a tummy ache or heartburn. My eye site is 20-10 now; it used to be 20-5 and that always baffled the doctors, especially when doing my eye test at the DMV, and I would read all the lines including the company name that created the chart and ‘Made in Korea” at the bottom! I only recently learned that it is my blood type that is the cause. It is crazy. I just thought I was strange, but the more people I meet when donating Anti-D plasma, the more I know I am not alone.

    If you have this blood type, please donate blood and plasma. I make quite a bit of money donating Anti-D plasma. There are many clinical trials that are ongoing. They are testing the use of O negative blood for an HIV vaccine and other diseases/conditions.

    My daughter is also immune she has O negative blood.

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