House Call Doctor explains the plant behind the rash.
Poison ivy (and its partners in crime, poison oak and poison sumac) is a plant that has oil on its leaves. Some people develop an allergy to this oil, which results in the typical poison ivy rash. There are a number of other less common substances that can also cause a rash, which doctors call contact dermatitis, including Neomycin (a common skin antibiotic), nickel (which is present in many belt-buckles) and something called Balsam of Peru.
There is no difference between the rash caused by poison ivy and those of poison oak and sumac. The typical rash from contact dermatitis is red, raised, sometimes blistering, and very itchy. It also occurs in crops or patches on the skin rather than in a generalized rash. That is because the rash only happens where the skin has contacted the offending oil. Note: the rash will only spread is if the oil from the plant spreads to new parts of the skin.
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