Tech & Science

Why Poison Ivy Causes a Rash—And What It Has To Do With Psoriasis

Poison ivy (Toxicodendron radicans) causes blisters and rashes by interacting with a specific immune cell found throughout the skin, but new research suggests it may be blockable. D. Gordon E. Robertson / WIKIMEDIA COMMONS

Poison ivy causes nasty rashes thanks to a chemical component called urushiol. This oily allergen binds to the skin, causing an allergic action. That’s why a lucky few are not susceptible to these rashes—they aren’t allergic.

But most of us, 85 percent in fact, are susceptible. Until now, however, exactly how this chemical caused an allergic reaction was unknown.

In a study published in the journal Nature Immunology, researchers from Australia’s Monash University and Harvard Medical School have pinpointed the exact immune cell that urushiol binds to. It’s called the CD1a protein, and it’s found widely throughout the skin and mucous membranes in so-called Langerhans cells. After the allergen binds, it sets off a chain reaction of events that attracts helper T cells to the area, which then secrete proteins that cause inflammation.

In the paper, the scientists show that mice without these CD1a proteins don’t show a strong reaction to urushiol, while those with the protein do react, with a rash and blisters similarly to those in humans. However, when antibodies were given to the mice that block the reaction between urushiol and CD1a, they didn’t break out in a rash.

Moreover, this protein also seems to be the culprit in the autoimmune skin disease psoriasis, says study co-lead author and Harvard Medical School researcher Florian Winau. Investigation of blood cells of humans with the condition showed an elevated level of T cells sensitized to this protein. Antibodies against this protein also prevented mice from getting psoriasis in a mouse-model of the condition. Winau thinks what causes this is that lipids in the skin interact with CD1a in a way that causes a “false alarm” and triggers T cells to attack.

He and his colleagues are working with industry to develop medication that act directly on the CD1a protein, and he hopes it will be able to treat both psoriasis and poison ivy.

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