Question: At home, my backyard leads out to a beautiful copse of trees that provide a very nice place to just relax once in awhile. Unfortunately, I also noticed a plant that looked suspiciously like poison ivy growing nearby. Since I like to be prepared for any situation, I did a Google search on how to recognize poison ivy and treat poison ivy rashes. Common treatments included using calamine lotion and hydrocortisone cream to ease the itch. This led me to wonder how poison ivy worked in the first place – what kind of biochemical reaction is going on in the poison ivy that induces such an inflammatory response in people?


Poison Ivy

Answer: What makes poison ivy such a skin irritant is a chemical that its leaves produce – urushiol. Urushiol is an oil contained in poison ivy’s sap that the plant uses to protect itself from predators. It has a very stable chemical structure and can remain potent for up to 5 years after secretion! (1)


Urushiol Chemical Structure (2)

When skin comes in contact with urushiol, the oil quickly absorbed into the skin, and any attempts to remove it with soap and water (the usual preventative measure against urushiol) are ineffective (1). The oil is then recognized by the dendritic cells of the immune system (Langerhans cells), which travel to the lymph nodes and send a signal to the helper T-cells in the body. If this is the first time the body has encountered urushiol, the helper T-cell will not do anything, but it will remember the oil as a potential antigen. The next time helper T-cells come in contact with urushiol, they will enlist the help of T-lymphocytes. T-lymphocytes then begin to produce cytokines and chemokines in an attempt to rid the body of the oil, at the same time causing a severe inflammatory response in the skin (2).


Poison Ivy Rash (6)

The inflammation is targeted by the hydrocortisone cream, and calamine lotion is used to treat the resulting rash. Hydrocortisone cream is a topical steroid that travels to the nucleus and inhibits or stimulates the mRNA transcription of proteins that induce or prevent inflammation. For example, it stimulates the production of lipocortin, an inhibitor of arachidonic acid, which is a precursor of inflammation inducing proteins like prostaglandins (5).


Corticosteroid inhibition of Arachidonic Acid Production (3)

Calamine lotion, on the other hand, works to soothe the symptoms of the rash by acting as an anti-pruritic (anti-itching) agent, a mild antiseptic, and an anti-astringent to dry weeping or oozing blisters (4).

In some people, the helper T-cells will just ignore the urushiol, and any allergic response within these individuals is averted (2). So, it turns out that the urushiol does not actually harm the body, but it is the body’s own allergic response to the oil that causes the itchy rash.

  • Natalya R.