Poison ivy experiment

Dr Thales Martens came to Yale on sabbatical from the U. of Rio De Genaro where he was a professor to study the migratory habits of eels. Strange in that they do not have eels in Brazil. Thais has nothing to do with that research other than while we were gathering elvers (baby eels) he asked me about the plant called poison ivy. Nearby was a fence with a luxurious growth of that vine and I pointed it out. "Ah, let us try an experiment." he said and picked a leaf of ivy and tore it in half and handed it to me. He rubbed his half on the back of his left hand and suggested I do the same. I did as he did. A week later we had our scheduled quest and I asked him about the experiment. My hand had reacted as I thought it would with a swollen hand that was almost recovered in the 7 days after the experiment. His hand was normal and mine was still swollen slightly by then. He was so upset by the reaction he all but insisted we go to the emergency room of our hospital. I knew what the results would most likely be as it is well established that a person has to be sensitized to the toxic material before it can cause a problem. Years went by- perhaps 20 when one bright spring day when I noticed new sprouts of poison ivy sprouting and wondered if the new leaves were toxic and rubbed a leaf on the back of my left hand. No reaction. Two weeks later when the plant was in handsome leaf I repeated the experiment and still not a bit of reaction. Again two weeks later I repeated the test with still no indication of a reaction. Wondering if the plants from which I had taken for the study were somehow non poisonous I selected a leaf from a vine a distance away growing up a tree and there was absolutely no ill effects. I wonder if after becoming sensitive to the toxic substance with age or by some other mechanism others loose sensitivity as I apparently have done. I am past my 90th birthday.