Poison Ivy, Poison Oak and Poison Sumac contain an oily substance, urushiol, which causes irritation and blistering of the skin. Human reactions vary from extreme susceptibility to near immunity. Just becoming older can have a tempering effect on the severity of the rash and the condition is most serious on children and young adults.
After identifying the plant you should not touch any portion, including the roots and berries. However, in many cases contact is unavoidable and you have to assume that some exposure has occurred.
When working around poison oak or poison ivy, wash exposed skin as soon as possible with soap and cold water, or a special soap such as "TechNu," specifically designed to remove urushiol. Follow washing by liberally applying rubbing alcohol or a solution of alcohol and water in equal proportions to dissolve all the unabsorbed poison. There is also a "blocking agent" called Ivy Block that is applied before coming in contact with the plant.
Contaminated clothing can carry the poisonous oil for years. Poisoning may occur even after laundering. If you are extremely sensitive, dry cleaning of clothing is recommended. Do not wash contaminated clothing with other clothing. Take care to rinse out the washing machine thoroughly after laundering contaminated clothing.
The ultra-violet part of sunlight is thought to help break down urushiol. You actually may want to dry or expose your clothes to the sun.