I’m not asking for the technical physiological reason. What I’m wondering about is why people would have evolved (or been created) with this urge, since it doesn’t seem to serve any purpose, and is apparently damaging.
Unless it’s just a byproduct of some other aspect of human physiology.
Why do you say it’s damaging? In some cases, yes, particularly with allergic reactions or certain disorders in which scratching doesn’t stop the itch. In such a case, I guess sometimes people can scratch enough to injure themselves, or introduce infection in the skin.
But it seems to me that scratching at a bug might kill it or get it off, right? Also, wouldn’t scratching the skin increase blood flow to the area, mebbe helping it heal a little faster?
Plus, don’t discount the benefit of an action that feels so darned good. Prolly releases all kinda nifty endorphin-type stuff, and it’s a bonding experience between humans or between human and pets, for example. I imagine it’s physically beneficial to do something that feels so satisfying.
Honey, did your mama tell you not to scratch ever? 'Cause I think it’s mostly ok.
Please tell me it’s mostly ok…
Is it “damaging” to the point of preventing people from reproducing? Nothing else is relevant.
Of course it serves a purpose. Tickling sensation = insect crawling on your skin that could potentially bite and cause disease. Therefore scratch it, dislodging or squishing said insect.
Itching is also caused by irritants such as grass spicules. Once again, scratching it and dislodging the material is a Good Thing.
Also caused by infection and minor injury. I’ve lost count of how many splinters, ingrown hairs and so forth I’ve located when I scratched them. Once again, locating and dislodging the problem is a Good Thing.
I kind of don’t understand it from this aspect: Histamines. Especially in the case of poison ivy which seems to exist solely to exploit this glitch in the human system (since it seems most other animals have no reaction and in fact eat the leaves and berries of the plant without ill effect).
Also, I know histamines are really important from the perspective that they’re neurotransmitters, but why would we want to be incapacitated with allergies during mating season and harvest season?
I haven’t read it in a while but this article, which is kind of gross, is an exploration of the itch mechanism by an MD. It also touches on phantom limbs.
Also, the compulsion to pick at scabs and zits.
Sigh.
We don’t.
Histamines serve as chemical signals that the body has been invaded by a foreign organism. That’s worth knowing.
Histamines are part of the immune system, and they help our body to fight disease in numerous ways.
Allergies are NOT beneficial reactions, by definition. They are false-positives of the immune system, things that our bodies ‘fight’ even though they won’t really hurt us.
I don’t know enough to get into the case of poison ivy in detail, but I doubt it exists solely to stimulate an allergic reaction in us. 
If it’s not a defense mechanism that it uses against many predators, then maybe it’s a purely coincidental effect.
Histamines are more than just a part of your immune system
itching can has causes like chemicals released with cell death. it is a useful indication of a local problem that is less than pain.
Oh my, you’re not kidding. I still can’t figure out how she…
…scratched all the way through her skull and into her BRAIN!!!
Humans have a mating season?!
Allergies are a modern reaction for the rich/industrialized of the world. I don’t know what the mechanism is, but you can see all kinds of research on it. People who live where they might actually harvest something are far, far less likely to have allergies. In fact, there wasn’t even a word for “allergy” until 1905. That’s how recent a phenomenon it is. Here’s just one study that talks about this.
More succinct, but less scientific:
My guess? Constant irritation at the surface had resulted in weakened bone. I know that infection and inflammation can do that, over time, to bone.
I’m just amazed that no one did anything more drastic sooner. I mean, forget wearing bandages or caps to bed… I’d be wearing boxing gloves and a motorcycle helmet if that’s what it took.
sorry, I made a wrong assumption about how humans used to live.