The rash that you get from poison ivy is caused by a chemical in the sap called urushiol. This chemical penetrates the outer layer of skin until it hits the dermis, and in the dermis an allergic reaction to the urushiol occurs. (See How Sun Tans and Sunburns Work for a good description of your skin and how it works.)
From that definition, there are a number of things you can deduce about poison ivy:
- Not all people "get" poison ivy. If your body does not mount an allergic reaction, then you can swim in urushiol and it will have no effect. It turns out, however, that the majority of people's immune systems react to urushiol after several exposures.
- You cannot get poison ivy unless you come in contact with the sap that contains urushiol. However, it is incredibly easy to come in contact with it. You can get it from the plants itself. You can get it from touching your shoes or pants if they have rubbed against poison ivy plants. You can get it from your dog's or cat's fur if they walk through poison ivy. One of the worst cases I ever got came from my dog, who apparently walked through some at a rest stop during a trip and then fell asleep in my lap for the rest of the ride...
- The urushiol has to penetrate the skin to get to the dermis, so thin skin will show symptoms before thick skin will.
- Urushiol does not spread through the body (although it may appear to because of the delay in symptoms caused by differing skin thicknesses). The blisters that form are also not "contagious." They do not contain urushiol.
- If you come in contact with poison ivy, washing off the sap will limit your reaction to it. You have to wash it off before any significant penetration occurs.
According to this article, the mechanism that causes the itching and swelling is a complex immune response to the urushiol:
Delayed hypersensitivity does not start to be noticeable until several hours to a full day after exposure to the antigen. It may last for over a week. T lymphocytes recognize the foreign substances, usually after the antigen is eaten, degraded, and presented (in pieces) by so-called antigen-presenting cells such as Langerhans cells in the skin, or macrophages. Urushiol metabolites are presented by this and other mechanisms. The T lymphocytes pour out inflammatory signal substances called cytokines. These call in armies of white blood cells called monocytes, which become macrophages. The macrophages become activated by the cytokines and attack everything in the vicinity, and can cause severe tissue damage.
To learn more about your immune system, see How Your Immune System Works.
Here are some interesting links: