Histamine
The "stings" on the nettle inject chemicals into the skin; one of these, histamine, is thought to be behind the initial sting. Others, serotonin and choline, are implicated in the longer term pain. The chemicals also include formic acid but in low concentration, thought to be too low to cause the initial "sting". Phospholipases are found in snake venoms. Melittin is a pain-causing chemical in bee stings
