What makes nettles hurt is the histamine in the nettle “hair” which is tiny and penetrates your skin, and an acid which burns (acetylcholine), along with seratonin. Histamine is just an irritant, which is why you want to scratch. Applying an anti-hystamine will counter this effect – the dock leaf is a natural antihistamine but If you rub too hard you will disturb the nettle sting and make it worse. The best way to deal with the problem (apparently) is to break off some dock leaf and chew it for a few seconds, then gently rub the saliva/dock sap mixture around the sting. The antihistamine from the dock leaf together with the natural healing properties of saliva will ease the stinging sensation.
There are two things about this – #1 is that the dock leaf may already have been urinated on by a passing dog, so this strikes me as dangerous and highly unpleasant. #2 is better I think – by the time you have hunted around looking for the age-old dock leaf you have probably forgotten all about the nettle sting (this is probably why it’s recommended – by your Granny!!)
Strangely enough, docks tend to grow near nettles and so people just learnt that rubbing a dock leaf onto a nettle sting works. They just went by a process of trial and error and happened to find that docks work. Dock leaves are relatively alkaline and one of the things in a nettle sting is formic acid (the same as in a red ant bite – in fact “formic acid” gets its name because it was first isolated by distilling red ants “formica rufa” – ugh!!!!) so the alkaline dock juice neutralises the irritating formic acid. If not, teh best thing is to go home and give the sting a thorough scrubbing with soap – which is also alkaline. It will also clean the area at the same time.