Spring is here, and that means one thing: poison ivy. Okay, that’s a bit melodramatic, as spring also kills off winter, brings my garden alive, and reminds us that summer is around the corner. But I happen to have horrid reactions to poison ivy – as well as insects. And besides passing on my genes of stunning good looks, superior intellect, and sarcastic wit to my girls, I also passed along my allergic reactions. So we stay away from poison ivy.
Poison ivy, Toxicodendron radicans, belongs to the Anacardiaceae family of plants. Notable members of this family are the cashew, pistaschio, and mango trees. Yes, mango. Poison ivy’s genus, Toxicodendron, gives a little information about it right off the bat. Toxicodendron is Latin for poisonous (toxico) tree (dendron), and every species in it wants to hurt me. These include poison oak, poison sumac, and various Asian lacquer trees.
I use two sayings to identify poison ivy – there are more, but these are what I preach to the kids:
“Leaves of three, let it be.” (Which also works to identify poison oak)
“Hairy vine, no friend of mine.”
Look at the photo, and you’ll understand. There is a cluster of three leaves, all coming from a main stem, and hairy aerial roots that help it adhere to trees, buildings, sheep, etc. In case you haven’t figures it out yet, poison ivy grows as a vine, and can be quite invasive. The trees in my backyard and in the forest behind my house are full of poison ivy. Another identifying characteristic are its gray-white berries. These berries may be safe for birds and other wildlife, but I learned long ago that white berries are never safe to eat. There are exceptions to this, but I’m still not chancing it.
Poison ivy is native to North America, and its first description is from John Smith. THE John Smith of American colonization (in)fame(y) and Pocahontas
legend myth. While in what is now Virginia, he wrote :
“The poisoned weed is much in shape like our English ivy, but being but touched, causeth redness, itching, and lastly, blisters, and which, howsoever, after a while pass away of themselves.” John Smith downplayed the noxiousness of poison ivy by writing “it hath got itself an ill name, although questionless of no ill nature.” (1)
There are many that would disagree, as the symptoms of itchy rashes that morph into weeping open sores affects 10-50 million Americans each year (2). Any part of poison ivy or oak can cause allergic contact dermatitis (ACD), from the leaves and stems, right down to it’s roots. When cut open, the sap oxidizes to a black lacquer which is particularly painful. But just staying physically away from poison ivy and oak isn’t enough. The toxic chemical can be volatilized from burning material, and thus become airborne. Such poisoning affects a large number of wilderness firefighters each year, irritating lungs, and accounts for 1% of the State of California’s workman compensation claims (2).
The chemical responsible for this large scale mess? Urushiol, an oily fatty-acid like compound. From looking at the structure, it greatly resembles soap, with a long alkyl chain for a tail and a polar, acidic catechol head.
There are several variants of urushiol, all involving that long side-chain, but the one shown above, pentadecylcatechol, is the one most prevalent in poison ivy and oak. The mechanism of urushiol induced ACD involves the oxidation of urushiol to a quinone, which then acts as a hapten and binds to antigen presenting (Langerhans) cells in the skin. [A hapten is an immunology term that describes an allergen that elicits an immune response only when bound to a larger biomolecule, such as a protein.] The cells then migrate to lymph nodes and present antigens to T-lymphocytes, starting the allergic reaction cascade (3). In short, this is like most allergic reactions. The body senses something foreign in the body and mobilizes its troops to expel the invader at whatever costs necessary.
The whatever costs necessary just happens to look like this:
That is exactly how I react. Gross, huh? The good news is that all of the weepy, pus-filled, goodness is not contagious, so it does not spread. The bad news is that a reaction may not occur for up to several days, so one affected area may not yet have generated an immune response. This can lead to round two, on another part of the body.
As with many allergens, some are not affected by urushiol at all. It is estimated that up to 30% of people are not sensitive to urushiol, and in fact “outgrow it” (4). Unfortunately urushiol sensitivity, like most allergens, is strongly predicted by parental sensitization. The same sensitization has been found in identical twins (5). I have twins, non-identical, but identically beautiful, that behave very different when exposed to allergens such as insect bites and stings.
The cruelest of tricks however, is that there is what is called a “sensitization phase,” which is just your body recognizing that there is an invader, and then formulating a plan for what to do next. So at the first exposure, you may be fine, but the second one is going to get you. This is what happened to a companion of Peter Kalm, a student of Carl Linnaeus, the Godfather of Taxonomy. Kalm’s friend Yungstroem, when they traveled in America from 1748-1751, touched the leaves and wood of poison sumac in the summer of 1749. The following winter he laughed at the “fable” of the toxic nature of sumac. But the following summer he became a believer, as he experienced “a violent pain and itching in his eyes, as soon as he touched the tree” (1). The lesson? “Great, kid. Don’t get cocky.” – Han Solo.
I suppose I should offer some hope for people that have been exposed, especially if they’ve made it this far. Wash the affected area with a large amount of hand or dish soap, wash the clothing you were wearing, and try not to scratch too much. It will go away. But the best bet it to not become exposed at all, so remember “Leaves of three, let it be.”
[Homepage featured image of poison ivy by Stilfehler (CC BY-SA 2.0)]
1. Rostenberg, A. “An Anecdotal Biographical History of Poison Ivy.” Archives of Dermatology 72.5 (1955): 438-45.
2. Gladman, Aaron C. “Toxicodendron Dermatitis: Poison Ivy, Oak, and Sumac.” Wilderness & Environmental Medicine 17.2 (2006): 120-28.
3. Kalish, R. S. “Recent Developments in the Pathogenesis of Allergic Contact Dermatitis.”Archives of Dermatology 127.10 (1991): 1558-563.
4. Allen, P. “Leaves of Three, Let Them Be; If It Were Only That Easy.” Pediatric Nursing 30:2 (2004): 129-135.
5. Thestrup-Oederseb, Kristian. “Contact Allergy in Monozygous Twins.” Contact Dermatitis 36.1 (1997): 52-53.