What causes poison ivy?

The rash from poison ivy is caused by a chemical in the ivy's sap called urushiol. This chemical can penetrate the outer layer of skin and can hit the dermis. In the dermis an allergic reaction to the urushiol can occur. A number of things can be deduced about poison ivy:

• Not all people are susceptible to poison ivy's ill effects. If someone's body does not mount an allergic reaction, he or she can swim in urushiol and will see no effects. Unfortunately, the majority of people's immune systems will react to urushiol after several exposures.
• No one can get poison ivy unless they come in contact with the sap that contains urushiol. Unfortunately, it is incredibly easy to come in contact with poison ivy. You can get it from the plants itself; get it from touching shoes or pants if they have rubbed against some poison ivy; or get it from a dog or cat's fur if the pet walked through poison ivy.
• The urushiol must penetrate the skin to get to the dermis. Thin skin, therefore, will show symptoms before thick skin.
• Urushiol does not spread through the body. Sometimes, however, it may appear as if it has spread through the body because of the delay in symptoms caused by different skin thicknesses. The blistering that forms is also not "contagious." Those blisters do not contain urushiol.
• If someone comes in contact with poison ivy, washing off the sap can limit the reaction to it. The sap must be washed off before any significant penetration occurs.