Just as insect bites can cause itching, skin contact with plant toxins can also cause itching  Studies on itching of oak leaves have found that when they come into contact with plant toxins, skin cells release histamine that causes redness, swelling and itching  Antihistamine drugs such as diphenhydramine can treat this kind of itching, but there is no suitable drug to treat itching caused by allergic eczema
Reducing skin friction helps to inhibit histamine activity
 For the latter, the best advice is wear loose clothes and don't scratch with your hands under any circumstances