The reaction to the allergen in the skin is of a type IV hypersensitivity nature. The immune system reacts to the allergen with lymphocytic T cells, causing a cell-mediated response. After contact with the skin, urushiol (from poison ivy, oak, or sumac) causes a delayed hypersensitivity reaction that humans do not have at birth. Persons exposed to urushiol before the age of five are not as likely to develop a sensitivity to it as is someone first exposed between five years of age and adulthood. Even if exposed during this time period, roughly 10% of the population cannot develop sensitivity to urushiol, and others will not if the level of exposure is not great enough (36,41,42).

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