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I've been wondering about this for some time, and I can't come to an answer...

Everybody keeps telling me that scratching one's own skin is bad. It damages the epidermis, creates the hazard of malicious bacteria getting past the skin barrier, and creates the vicious circle of scratching increasing the itching, and itching increasing the scratching (I think I recall reading that this vicious circle also happens even without any special skin diseases, like atopic dermatitis - I might be mistaken here, however)

So, it seems there are hardly any benefits for the body from the scratch reflex. However, if this was true, then "grandma Evolution" wouldn't have given this to us!

What are the benefits of scratching? Are there any benefits of it from the evolutionary point of view? I can come up with just one such benefit, namely fending off insects such as ticks, mosquitoes, fleas or lice. Would this be the evolutionary benefit of scratching?

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    $\begingroup$ Not enough time for a real answer, but I see the scratch reflex as vital. It rids the body of irritants, such as burrs/thorns, insects walking around, poison ivy/oak/sumac/etc. oils, and more. In the cases where the irritant has already affected us (we got bit by the mosquito, a rash from poison has already developed), scratching focuses our attention on the site, and the ensuing irritation works as a strong warning to not keep performing whatever behavior resulted in the irritation in the first place. $\endgroup$
    – MattDMo
    Mar 31, 2015 at 22:42
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    $\begingroup$ 1) Bacteria aren't malicious. 2) Not everything that no longer serves a purpose "devolves". Evolution is not a controlled, intelligent mechanism ("grandma evolution"). 3) Not every itch which is scratched results in an itch-scratch cycle. 4) Scratching an itch is not necessarily bad. There are at least 4 faulty presuppositions in your question. $\endgroup$ Apr 2, 2015 at 22:57
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    $\begingroup$ Humans didn't evolve the scratch reflex. We inherited it from 350 million years of mammal ancestors. $\endgroup$
    – kmm
    Apr 3, 2015 at 3:35

2 Answers 2

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With about 20 square feet of skin constantly exposed to potential irritation, itching must serve an important protective/defensive function.

The "scratch reflex" (I'm not sure I would call it that) is necessary to life and limb. Itch is a major somatic sensation, along with (and different from) pain, temperature, and touch. Itch can be an acute sensation, e.g. a mosquito bite, or a chronic condition (peripheral or internal disease).

Research on itch signaling is still at its infancy, and basic mechanisms by which the itch sensation is transmitted and regulated have yet to be fully understood. It is meant to give us information about how we are interacting with our environment. Loss of such fine sensation, such as occurs with Hansen's Disease - aka leprosy, where patches of skin become painless and do not itch, so are ignored by the patient - leads to festering wounds and resultant problems.

As @MattDMo stated in a comment, sensation allows us to respond appropriately to irritants which, if ignored, might lead to festering wounds: thorns and other foreign bodies, sucking or biting insects (parasites or other, which can act as disease vectors as well), etc. It allows us to learn how to avoid and treat such irritants.

Various types of itch can be classified by the origin of the itch, which could be skin-related, neuropathic, systemic, or psychogenic. The most common cause of itching is dry skin; tiny cracks in the skin cause an inflammatory response, which (within reason) is not a problem in and of itself. The response to the itch, though, can be beneficial or damaging. One can treat dry skin, wash off irritants, treat infections, avoid causes, etc.

Scratching can be benign if done sensibly, or it can injure the skin, allowing infection, etc. However, the benefit of having such sensation under normal circumstances certainly outweighs the inconvenience.

That is not to say that pathological conditions that cause itching are not problematic. They are, and one might reasonably curse the sensation under those circumstances.

Edited to add: Itching, or pruritus, is defined as an unpleasant cutaneous sensation that serves as a physiological self-protective mechanism to prevent the body from being hurt by harmful external agents. (Nature, 2007)

Scratching is simply a reasonable response to itching from any source and is not necessarily harmful. It's not a reflex per se. You can avoid scratching. Some animals must actively seek out a way to scratch. This takes it well outside of the terminology of reflex as neurologically understood.

Itch Signaling in the Nervous System
Pruritus
When you have an itch, what is happening under your skin?

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    $\begingroup$ Great. But, itching causes scratching. Scratching is not a good way to deal with dry skin (it only worsens the problem), and not a good way to deal with thorns or insects (thorns should be pulled off, insects smashed). You say: "Scratching can be benign if done sensibly". But I can't see how? $\endgroup$
    – gaazkam
    Apr 1, 2015 at 22:59
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    $\begingroup$ OK, so scratching doesn't need to be malicious. But what are the benefits? $\endgroup$
    – gaazkam
    Apr 2, 2015 at 11:46
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    $\begingroup$ So, to sum up, scratching is good just because it relieves itching. Great. But why did such mechanism evolve, since scratching can do nothing about the cause of the itch? ... all right. One possibility is this mechanism is a leftover from earlier mammals... perhaps scratching is the best way for a cat or a dog to fend of insects or remove thorns from their fur, but definitely not for humans! Yet this now useless mechanism (of scratching, not itching) has not yet devolved. $\endgroup$
    – gaazkam
    Apr 2, 2015 at 18:12
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    $\begingroup$ I'm not arguing with your answers. At least, that's not what I'm trying to do. I see nothing wrong with what you claim here, and I find all of this very interesting. I'm sorry if you find my behavior inappropriate; it's just that if I'm curious about something, I tend to belabour just until I can finally fully understand it. I know, however, that my interpersonal skills leave very much to be desired, and I should definitely work on this. $\endgroup$
    – gaazkam
    Apr 4, 2015 at 18:04
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    $\begingroup$ @endolith - what a silly comment! How do you explain racing, skydiving, shooting heroin, and the other hundreds of potentially lethal activities humans engage in because they feel good? Write a better answer with sources. $\endgroup$ Aug 3, 2017 at 19:55
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Short answer
People scratch to alleviate the itchiness.

Background
Itch (pruritis) is caused by a moderate stimulation of pain receptors. There is not such a thing as an itch receptor. Due to the neural circuitry involved, rubbing a painful bruise actually dulls the pain due to stimulation of the tactile sense (Kakigi & Shibasaki, 1992). This may explain the familiar first action one does when injured - grabbing the injured spot and rub it (cats lick their wounds, likely dulling the pain and cleaning it up at the same time). Given the similarity with the neural processing of pain, it is not surprising that a very similar mechanism applies to scratching - scratching relieves the itchy sensation (Nillson et al., 2003).

So while evolutionary explanations may be valid, they can never be proven. And in fact, too much scratching can be harmful, and I quote from Dermnet:

Itch [...] provokes the desire to rub or scratch the area to obtain relief. [...] Constant scratching to obtain relief can damage the skin (excoriation, lichenification) and reduce its effectiveness as a major protective barrier.

Hence, from a behavioral and neurophysiological point of view, we simply want to get rid of itchiness and we scratch. Whether this is evolutionary advantageous other than to provide a means to temporarily reduce debilitating itchiness, is questionable.

References
- Kakigi & Shibasaki J Neurol Neurosurg Psychiatry 1992;55:282-6
- Nillson et al. J Eur Acad Dermatol Venereol 2004;18:37–43

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    $\begingroup$ OK, so now I know how scratching relieves itching. But what are the benefits of scratching to the whole system? Or, more precisely - what are the benefits of the mechanism of scratching relieving itching? $\endgroup$
    – gaazkam
    Apr 2, 2015 at 11:52
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    $\begingroup$ @gaazkam - my point is, as I conclude in the last sentence, we cannot tell what then benefits are of scratching. My guess is that it feels good, as it relieves the itch. The evolutionary advantage regarding ticks and flees etc. is the fact that we feel itching. That warns us and lets us undertake action. If this action is scratching, it is important to realize that settled ticks don't care about scratching, flees are notoriously hardened critters and mosquito bites only start to itch when the deed is done; scratching in itself has no obvious advantage, other than to relieve itching. $\endgroup$
    – AliceD
    Apr 2, 2015 at 12:38
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    $\begingroup$ So, to sum up, scratching is good just because it relieves itching. Great. But why did such mechanism evolve, since scratching can do nothing about the cause of the itch? ... all right. One possibility is this mechanism is a leftover from earlier mammals... perhaps scratching is the best way for a cat or a dog to fend of insects or remove thorns from their fur, but definitely not for humans! Yet this now useless mechanism (of scratching, not itching) has not yet devolved. $\endgroup$
    – gaazkam
    Apr 2, 2015 at 18:12
  • $\begingroup$ Are pain and itch the same mechanism? I have read otherwise. $\endgroup$ Apr 2, 2015 at 22:45
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    $\begingroup$ @anongoodnurse: you may both be right to some extent: "In general, pruritus involves activation of the pruriceptors of specialized nerve cells called C-fibers. These C-fibers are identical to those associated with the sensation of pain, but they are functionally distinct and only convey the itch sensation—they comprise about 5 percent of the total C-fibers in human skin. " $\endgroup$ Mar 16, 2019 at 1:00

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