About Poison Ivy: Causes
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 two ways - irritation (irritant contact dermatitis) or allergic reaction (allergic contact dermatitis).
With an allergic reaction like 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 ten days. But once someone is sensitized and fully allergic, their next contact with poison ivy could cause itching and a bad rash within four to twenty-four hours. Urushiol resin can be transferred by fingers or animal fur, and can remain on clothing, shoes, and tools for months. Thankfully, scratching the rash won't 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.