I am going to tell you a story.
It was a hot, humid day during a summer in the mid 1970’s and after a morning of caving we found ourselves in the Green River cooling off and washing the mud out of our clothes; twelve packs of beer and watermelons tied together in the cold water and tethered to the bank. Despite the chill of the water, each of us peeled the layers of mud and clothing off until, much to our surprise, we found ourselves skinny dipping and worrying about what a mixed group of long haired hippie looking twenty year old college students would have to endure should the local law show up.
We were having a high old time and presently found ourselves sitting on the river bank dripping wet, drinking beer, eating huge slices of watermelon and spitting seeds at each other. We had worked our way through several melons when one of the crew noticed that the section of bank we had chosen for our repast was covered of poison ivy and here we all sat naked to the world, the vegetation our only picnic blanket.
Fear entered our hearts as our genitals contracted into our bodies seeking the safety of concealment. Each and every one of us launched ourselves head first back into the river, watermelon husks hanging like walrus tusks from our mouths. Fearing the worst, we splashed, scrubbed, chafed, wished for soap, used our semi-mudded clothes for washcloths bemoaned our cursed luck and eventually made our way back to our trucks; dreading the coming night an subsequent weeks of torture we knew were to follow.
To our surprise, despite having sat in, among and on the poison ivy, not a single one of our party came down with a rash.
I swear that all that I have related is truth but I cannot tell you how or why we were so fortunate.
Perhaps it was the watermelon; perhaps the cave mud or the cold water; it could have been the Lone Star or maybe the cheap Mexican pot. I cannot say which or what combinations protected us and not a single member of the group ever volunteered to test out many the possibilities, so you are on your own here, glean whatever useful information you can.