Jane Brody had a great overview of "poison ivy dermatitis" in the June 17, 2014 NY Times. There are two factual errors, but overall this is an extremely useful overview.
Brody states: "Myths and misconceptions abound about these three plants and the reactions they can cause. Knowing the facts can help to spare you and your family considerable distress." Her fine piece will enable you to avoid some of these. See: "Steering Clear of Poison Ivy"
1) Brody states: "Even people repeatedly exposed who appear to be immune may react to high concentrations of the toxin." Immunity is a state of sensitization or reactivity. The correct term is tolerant, or "exhibit immune tolerance." The immune system has a mechanism of turning off "immune reactivity." Therefore, the sentence should read, "Even people repeatedly exposed who appear to be tolerant may become sensitized."
2. When she states "Nor do you become immune to urushiol through repeated exposures to small amounts. Quite the opposite. There is no way to desensitize a person to urushiol as there is with pollen and peanut allergies. Eating mangoes or cashews will not work." This is not true! In a natural "experiment' with millions of persons, it has been shown that individuals who are reared in areas where mangos grow are most often "tolerant" to the poison ivy resin and related urushiols. It appears that early exposure (especially via the oral route) induces specific immune tolerance. This is difficult, and often impossible, to induce once immunity has been established.
1. The Baleful Weed by Wayne Winterrowd. The author describes how he was desensitized, as a young child, to poison ivy.
2. Exploring the mango-poison ivy connection: the riddle of discriminative plant dermatitis. Hershko K, Weinberg I, Ingber A. Contact Dermatitis. 2005 Jan;52(1):3-5.
Abstract: Our observations suggest that individuals with known history of poison ivy/oak allergy, or those residing in area where these plants are common, may develop allergic contact dermatitis from mango on first exposure. We hypothesize that previous oral exposure to urushiol in the local Israeli population might establish immune tolerance to these plants.