The molecules on their "nose" that helps puncture the skin have toxins on them, and leave behind some of the molecules when they're drinking. Because of the loss of blood in the punctured area, it becomes irritated, causing you to scratch. And when you scratch, it spreads the molecules around the area, irritating the spot even more; making you itch even more. It's really just a viscious cycle that only stops if you stop scratching and put an anti-bacterial medicine on it. Hope this helps! :)
the poisoned area gets stimulated
Scratching spreads the toxins transferred during the bite to a bigger area.