Wikipedia tells me that the stem of the Urtica dioica bears short and long hairs. The long hairs are little needles that inject acetylcholine, histamine, 5-HT (serotonin), moroidin, leukotrienes, and possibly formic acid. (My nettle sting did feel a bit like an ant-bite at first.)
"Urtication" is the process of flogging with nettles. It's not done only to torment; it is considered a folk remedy for rheumatism. Similarly, a beekeeper of my acquaintance reports that it is common knowledge among his colleagues that beekeepers have no autoimmune diseases, a fact they attribute to being routinely stung in the course of their duties. I have heard reports of people cured of crippling arthritis after a dangerous bout with multiple bee stings. So maybe I should be out there grasping nettles, or annoying bees, but I think I'll pass.
Nettles lose their sting upon being cooked, and are said to taste like spinach. Late in their season, however, they produce a gritty molecule that can irritate the urinary tract, so harvest them young. They are used to flavor some cheeses, such as Yarg and Gouda. Their stems produce a fiber somewhat like linen, but coarser. Their roots produce a useful yellow dye. Their presence indicates highly fertile soil, and they are excellent sources of nitrogen for compost. They are one of the few plants that can tolerate and even flourish in soils rich in poultry droppings. We certainly have those. Major chicken-poop operation going on here, though I must say that the nettles were growing quite a distance from the chicken coops.
It's been about 27 hours, and now the effect is finally wearing off.