The Facts
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 first episode of poison ivy rash usually occurs 14 to 21 days after contact
with the plant. Subsequent episodes usually appear within two days after exposure. The
rash peaks after five days, and begins to fade after a week or ten 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 two similarly nasty relatives: poison oak and poison sumac.
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 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 ten 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 can remain on clothing, shoes, and tools for months.
Thankfully, scratching the rash won't usually spread the urushiol poison to
other parts of the body, although it can spread the rash. 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.