The leaves of young box elder trees look similar, as does Virginia Creeper. The oily resin in poison ivy that produces the rash is incredibly long-lived. Plants stored at room temperature for five years are almost as potent as fresh specimens, and the toxic ingredient called urushiol is so potent that a drop the size of a pinhead can set off a rash in 50 people. Like most plant oils, the chemicals in poison ivy readily adhere to human skin. After a maximum of 48 hours, the rash breaks out.
First episodes are periods of sensitization in which the body prepares itself for future attacks.
The poison ivy chemical has to bind with proteins in the skin to make a strong enough chemical to cause the reaction. The oozing you see is the reaction of the white blood cells trying to get rid of the poison ivy. Julius says that a hot, soapy shower immediately after poison ivy exposure might minimize, but won't prevent, the rash, which itches for about two weeks before turning back into healthy skin. And as for home remedies like sticking the rash under hot running water or bathing it in bleach, these only offer temporary relief from the itching. According to Julius, even antihistamines only treat the symptoms of poison ivy, not the root cause.