Poison ivy is a common wild plant that causes an extremely irritating allergic reaction when you touch it or brush against it. Exposure to it may result in what doctors call contact dermatitis - a blistering rash that spreads over the skin in the area touched by the plant. In severe cases, this rash can develop into extremely painful, swollen areas of skin filled with fluid.
The rash usually appears within 2 days after exposure, but may take longer to appear with the first exposure. The rash peaks after 5 days, and begins to fade after a week or 10 days. While some people can become exposed and suffer little or no effect, being totally immune to poison ivy is unlikely. People who seem immune at one time and place may have an intense reaction the next time they encounter the plant. Poison ivy has 2 similarly nasty relatives: poison oak and poison sumac.
The leaves, stems, and roots of poison ivy contain a resin called urushiol. It's so toxic that tiny amounts on exposed skin can trigger an inflammatory allergic reaction. Doctors call this reaction contact dermatitis, which simply means an inflammation caused by contact with a foreign substance. Foreign substances can cause inflammation in 2 ways: irritation (irritant contact dermatitis) or allergic reaction (allergic contact dermatitis).
With an allergic reaction such as poison ivy causes, even repeated exposure to the plant may not cause a rash at first. This is because the body is registering its new sensitivity, a process that can take up to 10 days. But once someone is sensitized and fully allergic, their next contact with poison ivy could cause itching and a bad rash within 4 to 24 hours.
Urushiol resin can be transferred by fingers or animal fur, and it can remain for months on clothing, shoes, and tools. Thankfully, scratching the rash won't usually spread the urushiol poison to other parts of the body. Allergic contact dermatitis is most often confined to a specific area and usually has clearly defined boundaries. Scratching can prolong the discomfort and cause an infection.
The effects of the contact dermatitis caused by poison ivy range from a mild, short-lived redness to severe swelling and blisters.
Often, the rash contains linear streaks of tiny, itching blisters (vesicles). The rash area may be very small or may cover a large area of the body. It rarely appears on the soles of the feet or palms of the hand. Usually, the rash first becomes noticeable as patches of red, itchy skin. Small blisters then form, filled with a clear fluid, and eventually break open. In severe reactions, the rash will develop into swollen, extremely painful areas of skin filled with fluid. Itching and a temporary thickening or scaliness of the affected skin may last for days or weeks.
Exposure to poison ivy can cause severe allergic complications, such as a more general swelling, headache, fever, or infection. A doctor should be consulted if the rash stays red and itchy for more than 2 weeks, if the rash is over most of the body or near the eyes, or if fever is present. Also, the urushiol toxin in poison ivy is not killed by fire. As a result, being exposed to or inhaling the smoke from burning poison ivy can cause a severe allergic reaction, inside the body as well as on the skin.