Atul Gawande in The New Yorker:
Scientists believe that itch, and the accompanying scratch reflex, evolved in order to protect us from insects and clinging plant toxins—from such dangers as malaria, yellow fever, and dengue, transmitted by mosquitoes; from tularemia, river blindness, and sleeping sickness, transmitted by flies; from typhus-bearing lice, plague-bearing fleas, and poisonous spiders. The theory goes a long way toward explaining why itch is so exquisitely tuned. You can spend all day without noticing the feel of your shirt collar on your neck, and yet a single stray thread poking out, or a louse’s fine legs brushing by, can set you scratching furiously.
But how, exactly, itch works has been a puzzle. For most of medical history, scientists thought that itching was merely a weak form of pain. Then, in 1987, the German researcher H. O. Handwerker and his colleagues used mild electric pulses to drive histamine, an itch-producing substance that the body releases during allergic reactions, into the skin of volunteers. As the researchers increased the dose of histamine, they found that they were able to increase the intensity of itch the volunteers reported, from the barely appreciable to the “maximum imaginable.” Yet the volunteers never felt an increase in pain. The scientists concluded that itch and pain are entirely separate sensations, transmitted along different pathways.