Poison ivy (urushiol) allergy
Blistering rash from urushiol in poison ivy.
About poison ivy (urushiol) allergy
Poison ivy causes an intensely itchy rash through urushiol, an oily resin in its sap. Urushiol is a classic cause of delayed, type IV allergic contact dermatitis, so the rash usually appears 12 to 72 hours after contact, sometimes longer on a first exposure. Poison ivy grows widely in North America, so UK cases are most often seen in travellers or in people handling imported plants; gardeners and hikers are typically affected. The resin is remarkably persistent and can transfer from tools, clothing, pet fur or smoke from burning plants.
The rash is itchy, red and often strikingly blistered, and frequently forms in streaks or lines where the plant brushed against the skin, typically on the arms, legs, hands and face. It can look dramatic and may seem to spread over several days as areas with less exposure react later; the fluid in the blisters itself does not spread the rash. Contaminated clothing or objects can cause fresh outbreaks if not washed. Urushiol also cross-reacts with the closely related poison oak and poison sumac, and with mango skin and cashew shells.
Diagnosis is usually clinical, based on the streaky rash and a history of plant contact; patch testing is available but rarely needed. Management is washing the skin and any contaminated items promptly, avoiding the plant, and easing symptoms with cool compresses, emollients and antihistamines. See your GP if the rash is widespread, involves the face or genitals, or is severe, as stronger topical or oral steroid treatment may be needed.
Common triggers
- Poison ivy (urushiol) and products containing it
Symptoms
- Red, itchy rash (contact dermatitis)
- Dry, cracked or scaly skin
- Blistering or weeping in severe cases
- Burning or stinging where skin met the trigger
Avoidance & management
- Avoid skin contact with the trigger
- Wear gloves or use barrier creams
- Choose products labelled free from the allergen
- Rinse skin promptly after contact
- Use emollients or prescribed steroid creams
Sources & references
Allergy safety: what everyone should know
Mild-to-moderate reaction signs
- Itching, hives or a raised rash
- Swelling of lips, face or eyes
- Tingling mouth, sneezing, runny nose
- Stomach pain, nausea or vomiting
Anaphylaxis — call 999 (UK) or 112 (EU) immediately
- Difficult or noisy breathing, wheeze, persistent cough
- Swelling of the tongue or tightness in the throat
- Dizziness, collapse, pale and floppy (young children)
- Use an adrenaline auto-injector (e.g. EpiPen) at once if prescribed, then call an ambulance even if symptoms improve
Allergy or intolerance? A true allergy involves the immune system and can escalate quickly; an intolerance (e.g. lactose intolerance) is unpleasant but not life-threatening. The symptoms overlap, so self-diagnosis is unreliable.
Getting tested: home-testing kits based on IgG are not diagnostically valid. If you suspect an allergy, ask a GP for referral to an allergy clinic for skin-prick or specific-IgE testing and a supervised food challenge where appropriate.