Poison ivy leaves are coated with a mixture of chemicals called urushiol.
When people get urushiol on their skin it causes an allergic contact dermatitis.
This is a T cell-mediated immune response, also called delayed hypersensitivity, in which the body's immune system recognizes as foreign and attacks the complex of urushiol derivatives with skin proteins.
The irony is that urushiol in the absence of the immune attack would be harmless.
The most common treatment for severe contact dermatitis is with corticosteroids which diminish the immune attack and resulting inflammation.
A recent recommendation for mild cases is to use manganese sulfate solution to reduce the itching.
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