The Poison Ivy Allergic Reaction: Allergic Contact Dermatitis

Caution: Poison IvySummer is upon us it is time to be outside enjoying the great outdoors. Amidst the seasonal allergies and sunburns, another risk lurks at the edges of roads, fields, parking lots, and forests: poison ivy. Toxicodendron radicans, a main species of poison ivy, is a woody vine or sub-shrub that can be found in just about every region of the United States except the Southwest, Alaska, and Hawaii. Avoiding this plant can be difficult as it grows in different forms, lacks a consistent leaf shape, and grows in the same habitat as other similar looking plants.

What to look for? Poison ivy may have a darker in appearance vine accompanied by the famous three shiny leaves. However, the presence or lack of three leaves is not a perfect indication of poison ivy; but if in doubt, avoid the plants.

Binomial Name Common Name Habitat Identification Aids
Toxicodendron radicans Common or Eastern Poison Ivy Eastern half of the United States and Southern Canada Groups of three ovate (widest below center) leaflets, each 5- to 15-cm long. Central leaflet usually bigger, with longer stem. Leaflet margins can be toothed, lobed, or flat. Apex pointed. Stems usually with aerial rootlets. Grows along roads, trails, and streams. Usually climbs as a woody vine, but can be shrublike.
Toxicodendron rydbergii Northern or Western Poison Ivy Western half of the United States (sans CA) and Northern Border States Similar to T radicans. Thicker stems, less commonly with aerial rootlets. Grows along roads, trails, and streams. Small shrub ~ 1m high
Toxicodendron diversilobum Western Poison Oak Pacific Coast Groups of 3-5 oval leaflets, each 3-7cm long. Leaflet margins wavy to deeply lobed. Apex rounded. Stems smooth, stiff, shrublike with aerial roots. In canyons and riparian habitats, commonly grows as a climbing vine with aerial rootlets that adhere to the trunks of oaks and sycamores. Also forms dense thickets in chaparral and coastal sage scrub. Regenerates quickly after disturbances, such as fire or clearing of land.
Toxicodendron pubescens Eastern Poison Oak Southeastern United States Similar to T diversilobum. Variable leaf appearances that can mimic white oak leaves. No aerial roots. Grows in disturbed areas, especially common in sandy soils. Usually shrublike but can climb.
Toxicodendron vernix Poison Sumac or Dogwood Eastern third of the United States Groups of 7-13 leaflets, arranged as pairs along central rib, with single leaflet at the end. Damp, swampy areas. Medium-sized bush, grows 2-6m high.
(Table from Gladman 2006)

The culprit of this allergic reaction is the catechol-oleoresin, urushiol. Urushiol is a oil that is created by some members of the plant family Anacardiaceae - the Sumac family. *On a side note, cashews and mangoes are of the same family, Anacardiaceae, and there may be some indications of cross sensitivity to these. Urushiol is absorbed by the skin in as few as 3 minutes, and once absorbed it is very difficult to rinse off. The oil itself is not the complete culprit. The oil must bind with the proteins of the skin, acting as a hapten. A hapten is a small molecule that will usually induce an allergic response when bound to a larger molecule such as a protein. This combination of urushiol and protein initiates an immunologic response resulting in:

  • Redness and extreme itching;
  • Rash erupts on areas exposed to the resin, and it is often in the pattern of streaks or patches resembling the pattern of where the plant touched the skin; and,
  • Rash is in the form of red bumps (papules) and may also form large weeping blisters.

These symptoms will occur after the body has been sensitized to urushiol. This means that the individual has already had previous exposure to urushiol (and in some cases mild symptoms occur on first exposure).

Poison Ivy allergic contact dermatitis is NOT contagious, and the fluid from the papules and/or blisters will NOT spread the dermatitis (except in rare cases that they contain more urushiol oil).

Be cautious of broken skin as these areas will be more susceptible to infection, which will implicate further complications.

The principle active ingredients in various poison ivy products are:

Antimicrobial: Benzyl Alchohol
Antiseptic: Phenol, Isopropyl Alchohol, Benzalkonium Chloride, Camphor, Menthol, Zinc Oxide
Protectant/Astringent: Calamine, Zinc Oxide, Coconut Oil, Petrolatum
Local Anesthetics/Analgesics: Benzocaine, Camphor, Pramoxine, Menthol, Phenylcarbinol (Benzyl Alchohol), Methyl Salicylate
Antipruritics: Benzyl Alchohol, Camphor
Antihistamines: Diphenhydramine
Topical Corticosteroids: Hydrocortisone

As with any rash, if it appears near the eyes or anywhere on the face seek medical attention as this can lead to serious medical complications. Also, if it persists or worsens contact medical attention.

SO, what to do this Summer? Avoid the aforementioned areas where poison ivy and other various anacardiaceae (poison oak and poison sumac) thrive. Be careful when burning brush, as burning these plants is extremely dangerous. Clean clothing, bags, and equipment, as the resin will linger for extremely long periods of time. Should you become exposed, wash with soap and water as soon as possible to avoid spreading the oil, and minimize absorption. And should you have a reaction resist the temptation to scratch as this will increase risk of infection. Over-the-counter remedies can help to alleviate symptoms, but for more severe cases, medical attention is required.


Jeremy KW Spiewak, Pharmacy Intern, CPhT, MA RPhT, BA Chemistry, Doctor of Pharmacy Candidate at Massachusetts College of Pharmacy and Health Sciences


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Further Reading:

CDC Workplace Safety & Health Topics: Poisonous Plants


Gladman, Aaron C. (2006) Toxicodendron Dermatitis: Poison Ivy, Oak, and Sumac. Wilderness & Environmental Medicine, 17:2, pp120-128.

Goodman, R., and Hollimon, D. (2010) Allergic Contact Dermatitis: Poison Ivy. Dermatology Nursing, July-Aug, pp 26+.

Kalish, Richard M. (1995) Antigen Processing: The Gateway to the Immune Response. Journal of the American Academy of Dermatology, 32:4, pp640-652.

Kalish, Richard M., Wood, Jonathan A., LaPorte, Axel. (1994) Processing of Urushiol (Poison Ivy) Hapten By Both Endogenous and Exogenous Pathways for Presentation to T Cells In Vitro. Journal of Clinical Investigations, 93:5, pp2039-2047

Penn Medicine. Poison Ivy - Poison Oak -Poison Sumac. (2007) [online] Available through Accessed May 2012.

Poison Ivy Treatment Products. Drug Facts and Comparisons® eAnswers [online]. 2012. Available through Wolters Kluwer Health, Inc. Accessed May 2012.

W. Steven Pray (2007) Poison Ivy: The Classic Contact Dermatitis. US Pharmacist, 32:4, pp 11-15.

Original Published:May 2012

Revised:May 2012