Posted on behalf of Mike Scott:
I see some poor sou l has got blisters from Bubon Galbanum, the wild celery bush.
The active poison is Coumarin, used normally as an anti coagulant in Warfarin the Rat poison, but it has a photo-toxic form when it gets onto the skin and is activated by UV. The reaction is a severe response from the immune system hence the huge serious blisters which should not be broken.
Some mild alkali or anti histamine seems to help, but the asker may want to check the Internet to see if better info there to help.
Photo: Mike Scott