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 1 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, 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.