Why we itch
Why do we itch? Itching seems at first blush a counterproductive phenomenon: In the United States, 6.4 million people with dermatitis scratch their way to the dermatologist's office each year, according to a National Ambulatory Medical Care Survey. Yet itching is universal enough that chances are it has been selected for. So what is its selective advantage?
In our original habitat, itching most commonly followed a bite. Bites can be poisonous. Thus, the urge to itch, if carried to the point of bleeding, causes us to bleed the toxins from the bite off. This is consistent with another biological response to an itching stimulus, namely inflammation: The overall function of inflammation is to neutralise and destroy any toxic agents at the site of an injury and to restore tissue homeostasis.
You have to wonder, though, how bad the toxins that make us itch must have been to outweigh the risk of infection by scratching oneself to the point of bleeding. Perhaps the purpose of scratching is to stimulate inflammation?
So next time you wonder why you are scratching to the point of bleeding, stop pitying yourself and realize that in an evolutionary point of view, getting rid of toxins may well be worth sacrificing a little blood.
Alex Backer, Ph.D.
P.S. This explanation was inspired by a discussion with Adam Sadowsky where he did most of the talking.