Plants of the genus Toxicodendron are found throughout East Asia, North America, and South America. Five species common to North America cause more cases of allergic contact dermatitis than all other contact antigens combined. These species are known best by their common designations:
1. Poison ivy (two varieties).
2. Poison oak (two varieties).
When these plants are bruised or injured, they emit a sap called urushiol, which contains a mixture of highly allergenic, cross-reacting catechols. Contact with this sap in sufficient quantity can induce immune recognition (sensitizing dose), and with subsequent exposure (eliciting dose), a delayed hypersensitivity reaction will occur. Once allergic, an immunologically competent person will continue to react with any threshold reexposure to the offending antigen.
Some portion of the plant must be damaged for exposure to occur. Smoke from wood that contains part of a toxicodendron plant can contain enough antigen to cause severe exposure. The plant resin dries rapidly under fingernails and on skin, clothing, tools, and sporting equipment. It will retain antigenicity indefinitely unless removed. Toxicodendron dermatitis is important not only because of its frequency, but also because it serves as a model for understanding other types of allergic contact dermatitis.
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