Leaves of three, let it be, Berries white, poison in sight.

Although Scouts recite this verse like a hiker's catechism, millions of people each year fall prey to the pretty leaves that cause an awful itch.

Poison ivy grows "just about everywhere in the Washington area, says poison ivy expert Dayton L. Klingman, chief of the Weed Science Lab at the Beltsville Agriculture Research Center.

"It leaves branch off in threes, and they have a smooth usually shiny upper surface," says Klingman, who notes that poisoning incidents are highest in June July and August, "It is more apt to grow in moist, shady spots, and gets white berries in the fall and red leaves in October."

Poison oak is generally found in the South, up through the middle of Virginia, Klingman says. A low-growing shrub with leaflets that occur in threes, it's named for its oak-like, lobed leaves.

Poison sumac tends to grow in swampy, wet areas, It is a coarse, woody shrub or small tree with tapered leaves that pair out in series of seven to 13 leaflets, with the odd, single leaflet on the branch's end.

The itchy red rash we call poison ivy is actually a type of allergic reaction, says Dr. Andrew Margileth, outpatient department director at Children's Hospital. Roughly 70 percent of the population is susceptible to irritation from a substance called urushiol, found in the plant's sticky sap.

You don't have to touch the plant to get a reaction. The urushiol can affect your skin if you touch some clothing, a garden implement or a pet that has come in contact with the plant, or if you are exposed to the smoke of burning plants.

"The first time you touch it you may react mildly or not at all," says Margileth. "But once you're sensitized, the next time you touch it you might break out badly.

"If you touch it, immediately wash the area with soap or wipe it with rubbing alcohol," advises Margileth, who says washing the skin within two hours of contact may help you escape with only mild symptoms.

The red, itchy rash and blisters can appear anywhere from six to 36 hours after contact. To combat itching, take aspirin and ask a pharmacist to recommend on over-the-counter oral antihisiamine. "Plain old calamine lotion" is still the best topical lotion, marigleth says.

Cool, soothing baths, two to three times a day may help. Use mild soap or add a teaspoon of corn starch or bicarbonate of soda per quart of water. Cut your fingernails short to guard against infection from scratching.

Symptoms should be gone in seven to 14 days. If the rash gets worse, not better, or if there is yellowish drainage or extreme pain, see a doctor.

Prevention is best, says Margileth, who says that "children as young as 2 1/2 can be taught to recognise the plants and stay away." CAPTION: Illustration 1, Poison ivy; Illustration 2, Poison sumac; Illustration 3, Poison oak