Tuesday, February 3, 2015

Everyone's Favorite Plant to Hate

Written by: Lindsey Whitaker
As a proud outdoor enthusiast, this time of year really makes me love where I live. Rolling GREEN hills (What? Really? I know), scattered old oak trees and the signs of new growth. Like many of us, it makes me want to get out and explore. While exploring the natural wonders of our area might sound delightful, one California native plant might deter you from your adventure… poison oak. 

No other plant presents the same level of fear and caution. Even if you have never experienced the relentless itchy sensation of an encounter with poison oak, you are sure to avoid this pesky plant on your jaunt through the an oak woodland.

Poison oak (Toxicodendron diversilobum), is not alone its quest to irritate you. Plants of the genus Toxicodendron, (of the Anacardiaceae family) are characterized by their ability to produce certain oils that interact with human skin to create contact dermatitis (Corbett, 1975). Contact dermatitis is a fancy doctor term for a red, itchy rash caused by a substance that comes into contact with your skin.  Other members of this unpleasant genus include Poison ivy (Toxicodendron radicans) and Poison Sumac (Toxicodendron vernix) (Corbett, 1975), all equally itchy and fun. 

What is it that makes poison oak so unpleasant, you might ask? Urushiols! Urushiol is allergenic oil that has a strong reaction with your skin. This chemical is found in all parts of the plant including the leaves and stem. It is also found in cashew shells, which go through an intense heating process to make sure they are safe for us to eat. Here is the breakdown of the vicious battle happening on the surface of your skin after you have rubbed up on some poison oak:

Urushiol oils are absorbed easily into your skin just like many oils. The urushiol is recognized by specific urushiol T-cells (CD8+ and CD4+) as a foreign substance and labels it an immunogen (Kelish ,1994). The T-cells then send out signals called cytokines that summon white blood cells called macrophages. Macrophages come rushing to your rescue and destroy the immunogen. After digesting the urushiol, a macrophage will present an antigen (a molecule, most often a protein found on the surface of the immunogen and used by the immune system for identification). That identification helps the immune system make antibodies that mark incoming urushiol so they are easily found and destroyed. Now, your immune system is ready next time you encounter urushiol.

Those hard working macrophages destroyed all of the urushiol and made you very proud. BUT they also have made a giant mess and have damaged surrounding normal tissues.  This results in inflammation of your skin at the site of the now removed urushiol, hence the itchy rash.

I like to think of those hard working macrophages as John Matrix played by Arnold Schwarzenegger in Commando. They are out saving the world but in the process destroy entire cities. 
Scientists have been trying to find a way to make a resistance and lessen the body’s immune response to incoming urushiol. Many approaches have been attempted to make this pesky but extremely useful mechanism chill out. The idea is if you expose your body to urushiol enough, it will not think of it as foreign or a threat. Researches at UC Berkeley found that if you inject modified urushiol molecules into a guinea pig, they develop a specific immune tolerance to urushiol after just two injections (Stampf, 1985). Another study at the University of Mississippi found that oral dose of modified urushiol could also desensitize urushiol-induced rashes (Watson, 1983).

I know what your thinking, why don’t we all have this? There are a few speed pumps in the road to poison oak freedom. First, the guinea pigs where given treatment at least a week before being exposed to the plants…that is a lot of planning. Also, in both scenarios, the treatment does not last very long, 6-8 weeks in both treatments (Watson, 1985 and Stampf, 1985). 

Most importantly, there have been some pretty severe side effects in guinea pigs that took the oral dosage. In order for the oral method to work, it has to be in relatively large doses, which might translate into harmful effects in humans such as gastrointestinal irritation and perianal dermatitis (I will let you look that one up… trust me, you don’t want it)(Watson, 1983). 

On a global scale, there has also been some frightening new research concerning the future of poison oak and it’s family members. A research study from Duke University found that elevated levels of CO2 stimulated growth and increased concentrations of urushiol. This research makes poison oak just a little more alarming in the face of changing climate (Mohan, 2006). 

Increase in biomass in elevated CO2 conditions
More CO2= more urushiol

The take home message of today’s lesson is avoid poison oak at all costs. There is no need to reenact the movie Commando on your skin. If you say, “Oh, I don’t get it,” tell your antibodies that! Those crazy little guys are currently preparing for your next encounter as we speak. Finally, if you wait long enough, scientist might come up with a consumer friendly method to avoid the affects all together. We all will be waiting...impatiently itching.  

Corbett, M.D., and S. Billets. 1975. Characterization of Poison Oak Urushiol. Journal of Pharmaceutical Science 64:1715-1718.

Kalish R.S., J.A. Wood, and A. LaPorte. 1994. Processing of Urushio (Poison Ivy) Hapten by both Endogenous and Exogenous Pathways for Presentation to T Calls in Vitro. The American Society of Clinical Invetigation 93:2039-2047.

Mohan J.E., J.H. Ziska, W.H. Schlesinger, R.B. Thomas, R.C. Sicher, K. George and J.S. Clark. 2006. Biomass and toxicity repsonse of poison ivy (Toxicodendron radicans) to elevated atmoshpeic CO2. Proceedings of the National Academy of Sciences of the United States of America 24:9086-9089.

Stampf J., C. Benezra, V. Byers, and N. Castagnoli Jr.. 1985. Induction of Tolerance to Poison Ivy Urushiol in Guinea Pig by Epicutaneous Application of Structural Analog 5-Methyl-3-n-pentadecylcatechol. The Society for Investigative Dermatology 86:535-538.


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