LITTLE is known about itching. Editors know that "itching" is a much safer noun to use than "pruritus," a word misspelled by about half the medical profession and their amanuenses in the conviction that the ultimate syllable contains an inflammatory "i." People in general know that mosquito bites cause itching. So do, say the experimenters, histamine, kinins, certain proteolytic enzymes and cowage ("A tropical woody vine `mucuna pruriens' having crooked pods covered with barbed brittle hairs which cause intolerable itching. Also, these hairs, mixed with honey or other vehicle, used as vermifuge").
Although substances have been identified that do cause itching, the neurochemistry of the common spontaneous itch remains obscure. Similarly uncertain is the modality of the sensation. The conventional view, summarized as long ago as 1941, is that an itch is a nonpainful pain. In other words, itching is the result of weak stimulation of pain receptors. According to
Scratching the Surface of the Itch. Arch Dermatol. 1969;99(2):133-134. doi:10.1001/archderm.1969.01610200005001