itchy

J had an allergic reaction this morning right after he climbed a tree in the next door neighbor's yard.
He started scratching and said itchy.

His legs and arm had some hives.

I gave him Benadryl and it dissapiered soon after.
The trees are not new to him because it is almost his daily routine to climb the trees and he never showed any allergic reactions.

Something right there, however, is allergen for him.  I cannot tell what.