The Fourth of July weekend heralds the season for outdoor fun - idyllic games of golf, mornings of gardening, afternoons of hiking. But beware: This is also prime time for poison ivy. A run-in with this common poisonous plant is likely to force a stint of itching and scratching, not to mention an ugly rash and unsightly blisters, into your summer plans.

According to the American Academy of Family Physicians Foundation, 55 million Americans develop allergic reactions to poison ivy, poison sumac and poison oak each year.

"It's insidious; real nasty stuff," says Susan Pezzolla, a consumer horticulturist with the Cornell Cooperative Extension of Albany, N.Y.

The real culprit is an oil called urushiol (you-ROO-shee-ol), which is found in poison ivy sap, on the leaves, stems and roots of the plant. It flows most actively in spring and summer when the plant is growing. Even tiny amounts of the oil will cause extreme skin irritation for most people.

Contact with the oil can happen when it is brushed onto skin or clothing that touches the plant. Many people have been infected merely by taking off their socks after walking through poison ivy, says Pezzolla.

"People are surprised how easy it is to get the oil on your skin," she says. "It can stick to the fur of your dog or cat, to sports or camping equipment, and garden tools." And, she adds, it can re-infect you if you don't clean everything thoroughly.

The best defense is to learn to identify poison ivy and avoid it. The little rhyme, "Leaves of three, let it be," remains excellent advice, says Pezzolla.

Look for stems with a cluster of three leaves, which can be rounded or elongated, but usually come to a point at the end. The leaves are shiny in spring and early summer, becoming more dull in late summer, and can turn vibrant red or orange in the fall. Poison ivy twines around tree trunks like a vine, straggles over the ground or grows like a small shrub.

"Because it doesn't always look the same, poison ivy can be a little difficult to identify," says Pezzolla. "But any plant with three leaflets should be avoided, always. Don't take any chances."

Pezzolla says it's also important to help children recognize poison ivy.

If you suspect or know you've made contact with poison ivy:

Wash all exposed areas with lots of cold running water as soon as you can reach a stream, lake or garden hose. If possible, use soap, too. The idea is to keep the oily urushiol from penetrating the skin and from spreading to other parts of the body. (Hot water will open your pores and let the oil in.)

Washing your skin with rubbing alcohol may also help remove the oil.

Also wash with detergent all the clothes, tools, equipment, gear, toys or anything the plant or you may have touched. Be careful not to transfer the urushiol to furniture, rugs or car seats.

If infection develops, a reaction will usually appear within 48 hours of exposure - severe itching, redness, and swelling are typical, and blisters may follow.

Treat itching skin with dressings of calamine lotion, Epsom salts, or bicarbonate of soda. Over-the-counter topical corticosteroids (hydrocortisones) are also fairly effective, but only temporary.

Help dry up blisters with baking soda, oatmeal bath powder, aluminum hydroxide gel, calamine and zinc oxide.

If blisters form, call a doctor or a dermatologist. They may prescribe cortisone or other medicines to halt the reaction and help heal the skin.

FACTS ABOUT POISON IVY:

Poison ivy is not contagious. It cannot pass from person to person by touching the rash or blisters. Only urushiol can be spread by contact.

Sensitivity to poison ivy varies from person to person. About 85 percent of all people will develop an allergic reaction. However, a person's sensitivity may change over time.

Dead poison ivy plants are still toxic. Urushiol can remain active for up to several years. Never handle or burn dead plants that look like poison ivy.