Plants that roar: poison ivy causes lots of childhood pain.

Say "poison ivy," and everyone grins-everyone, that is, except mothers whose children have endured the ravages of the annoying plant with the familiar-leaf clusters. That's because the name recalls a special brand of suffering-a grown-up, X-rated version of the measles, in which two to three weeks may pass while large patches of the child's skin ooze, blister, and itch with the intensity of a fresh mosquito bite.

Poison ivy, poison oak, poison sumac, and poison wood are closely related members of the same family of plants, so what's said here about poison ivy applies to the others as well.

The extremely potent oil that causes the characteristic weeping sores is called urushiol. A blend of several powerful skin irritants, its composition varies only slightly from one branch of the family to another. Poison sumac wins first prize for potency.

Urushiol is carried to all parts of the plant, the roots being most abundantly supplied, making them more dangerous than the leaves. But it doesn't get on the surface of a plant unless the plant is damaged. That's highly likely, however, since bug chewings and bad weather open holes in nearly all the plants. THAT AWFUL ITCH. When urushiol touches the skin, the body's immune system goes into action to repel the foreign invader, causing the red, raw, itching sores. That's why cortisone, which suppresses the body's immune defenses, remains the most effective treatment-though cortisone isn't a true cure, since it merely suppresses symptoms.

Dr. Ilona Frieden, assistant clinical professor of pediatrics and dermatology at the University of California Medical Center in San Francisco, says, "I think the most important aspect of treatment is to make children aware of what the plants look like. That's not as easy as it sounds, since poison ivy looks very different at various times of the year. In the spring the leaves may be dark or light green, and in the fall they turn red, orange, yellow, and brown. In winter the bare branches, which can also cause a severe rash, are brown with a slight tinge of orange or red."

For very mild cases Dr. Frieden recommends prompt treatment with over-the-counter cortisone creams. "Benadryl, which is available over the counter, can also be taken to decrease itching," she says. "We don't recommend the use of Caladryl (a combination of calamine and benadryl) or Solarcaine, since there's a small possibility that they may cause further irritation.

"If the rash becomes extensive or starts to blister, then the child should be taken to a physician for evaluation. I think it's safe to use systemic steroids (cortisone) to treat young children for severe poison ivy, as long as it's done under proper supervision, and if there are no other illnesses that place the child at risk from the treatment."

Dr. Alexander Fisher, professor of dermatology at New York University's graduate school of medicine, warns that preparations containing benzocaine, zirconium, or antihistamine are "notorious sensitizers" and may cause rashes of their own. Fisher says that over-the-counter desensitizers that contain urushiol (homeopathic pills, etc.) should never be used during a case of the rash, as they only increase the suffering. PREVENTION. Researchers have discovered that the quicker urushiol is removed from the skin, the less the child will suffer. Ordinary hand soap and water will do the job-cold water because it closes the pores, preventing further penetration. Water also reacts chemically with urushiol, changing it into a relatively harmless substance. So if you're out in the woods and a youngster brushes against poison ivy, have him soak the affected area in a stream for a few minutes.

It usually takes the rash about 24 hours to appear, though the interval may varY from six hours to several days. Meanwhile, the original contact area has been touched and the urushiol has been spread to far-flung regions of the body. "You'd think I'd rolled in it!" many an indignant sufferer cries. However, once you've washed off the irritant oil, the affliction will spread no farther.

Urushiol is incredibly stable. Botanists have contracted the rash after handling plant specimens hundreds of years old. So after your child has played in known habitats, throw clothes in the washer and wipe shoes with a soapy cloth. Also, remind children to wash their hands after holding pets that may have romped in the bushes (animals are immune). Scrub toys and hand tools thoroughly with soap and water after contact with the plants.

Not only is urushiol potent and stable; it can pass through just about any barrier of clothing you can devise, including leather. And because it's oil soluble, it can even penetrate rubber gloves. NATURAL REMEDIES. Herb gatherers have a saying: "If you stand next to a poisonous plant and stretch out your arm, you'll find the antidote within reach." And in fact, jewelweed, which grows more or less in the same range as poison ivy, is indeed the most potent natural remedy. (Jewelweed doesn't grow within the range of poison oak, unfortunately.) The late Euell Gibbons, author of books on wild edible plants, claimed that after adding handfuls of jewelweed to his bathwater, he could walk through poison ivy with impunity. Fresh-cut jewelweed is also brewed as a tea, which is applied to the sores and taken internally.

Aloe vera juice, available in health food stores, soothes the itch and may accelerate healing. But the most effective itch stopper is also the simplest: plain water.

If contact with poison ivy is unavoidable, ask your druggist about products such as Kerodex 51 (Ayerst Laboratories), Toxic-Guard (Reynes Products, Inc.), or Ivy Shield, (Interpro, Inc.).

Dr. Williwn Epstein, considered the world's foremost authority on poison ivy, says that once a person has the rash, the quickest relief comes from a family of new drugs called fluorinated steroids. Used primarily in treating skin diseases such as psoriasis and available only by prescription, these steroids are suspended in a chemical gel that delivers the drug to the immune system with optimal efficiency. But, warns Epstein, you must be terribly careful with these powerful new steroids, and they're particularly dangerous if misused in treating small children."

Dr. Ilona Frieden concurs. Fluorinated steroids are safe only if used for brief periods under a doctor's supervision. They're much more potent, and they involve potential risks such as thinning of the skin, so they shouldn't be used on the face. If they're applied to large areas of the body, there can be systemic absorption, which is the equivalent of taking steroids internally. And that can be particularly dangerous in very young children."

Epstein believes he could develop a cure for poison ivy in five years, if a pharmaceutical company would fund the necessary research. "Our experiments with animals have shown that we can produce an urushiol-like substance that doesn't have the sensitizing, rash causing potential, but might immunize," he says. If the proposed tests were successful, the result would be a safe, effective poison ivy vaccine.

Meanwhile, that surest of remedies avoidance-is still the best. To help your children identify these pretty but dangerous plants, you might like to teach them this folk rhyme, which was recited by frontier mothers a hundred years ago:

"Berries white, poisonous sight ! Leaves of three, let me be!"