Why do we itch?

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1:04Hello and welcome to CrowdScience from the BBC World Service, the show that answers your science questions. I'm Anand Jagatia and this week we're devoting an entire episode to a topic that lots of you are curious about. It's a subject that comes up again and again in the inbox. Itchiness. So many of you have got in touch asking about the science behind itching and scratching. What is the sensation of an itch? Where does it come from? Why does scratching feel good? And loads more. So to help answer all of your questions, we've got some experts on hand to discuss them. First, let's meet Sonia Stander, Professor of Dermatology at the University of Münster in Germany.

1:50Hi, Sonia. Hi, hello. Sonja studies the role of our immune systems and the brain in driving severe itchiness. Also joining us is Professor Gil Josipovich. Gil is director of the Itch Centre at the University of Miami and a global leader in decoding the biology of itch. Hello, Gil. Hi, pleasure to be here. And a bit later in the programme, we're going to be doing a little experiment on itchiness that you can take part in wherever you are. But before that, let's get the crowd science itchy and scratchy show going with our first listener question. Hi, I'm Ziqi from China and my question for crowd science is, what exactly is itchiness?

2:32What's happening in our body when we itch? Well, I guess first thing to say is that this is actually a surprisingly big question. There's lots of science to unpack here and we'll be doing that over the next 20 minutes or so. Sonia, how would you answer Ziqi's question? What actually is itchiness? Yeah, so by scratching the skin, we remove parasites or plants or anything that does not belong onto the skin. And so we start to scratch at the very same area where we itch and remove it and it can no longer damage the skin and harm ourselves. In this sense, it belongs to the protection mechanism of the body, like perception of touch and pain and everything.

3:16So without itch, maybe we would not survive. as a human being. Now that's quite a bold claim. So it really makes sense then that we would have evolved this mechanism because obviously you don't want parasites, you don't want things on you that shouldn't be there. But let's move on to you then Gil. I mean there are also other reasons why we itch right? So skin conditions like eczema where people have very dry skin that can become itchy, that's something different. So what's going on there with itch? So in a disease like eczema, There are numerous skin inflammatory diseases that cause itch, and all of them are related to activity of the nerves.

3:56So the protective aspects are extremely important, but sometimes there is what we call overactivity of the nerves and the immune system, because the immune cells could work and cross-talk with the nerves in the skin and induce an overactivity. And it gets sometimes out of control. So that's where it becomes a chronic problem that affects millions of patients. Right. Okay. So you've got itch that, as part of these inflammatory diseases, comes from within the body. And Gil, you mentioned there chronic itch, so itch that lasts a really long time. Sonia, when does an itch become a chronic itch and then a more serious health problem?

4:38So on the one hand, we have a definition what itch is. It's an unpleasant sensation of the skin leading to the urge to scratch. But for the chronic situation, we say, okay, it's a matter of time. We define it as six weeks and more. Chronic itch is very burdensome. Part of this definition was that we added that there are structural and functional changes in the nerve fibers, but also in other cells. and based on this itch can become an own disease and you need own treatments and we hope that in the future we have more and more pharmaceutical developments to help our affected people because at the moment we have only a couple of treatments we can offer.

5:28Okay so I guess the key thing really is that it's a disease in its own right so what is going on to change itch into an actual disease in and of itself? We see that there is a reduction of the nerve fibers and these nerve fibers, they are constantly active and they are firing all the time. You cannot control the ongoing activity in the nerve fibers itself. And this goes up over the spinal cord, up to the brain. We also have hints that brain perception is altered. There are differences in the activity within the brain. So the whole axis from the skin to the brain that transmit the signal itch is disturbed.

6:10Thank you for explaining that Sonia. So I guess now would be a good time then for us to dive into to what an itch actually is as it goes from the skin to the brain. So Gil maybe you could explain the sensation of an itch. So itch has to have skin because it emanates from the skin but the nerve fibers that transmit itch are very similar to the nerve fibers that transmit pain, but there are some unique receptors that are dedicated to itch. So there is an overlap, but there is also unique dedicated nerves that transmit itch. Okay, well that's a bit on the different pathways that the body has that then lead or culminate in an itch and a scratch.

6:54But we had another question from listener Zichi about what itching actually feels like. Does everyone feel itches in the same way? So Sonia, this is, I suppose, more about the experience of itch, what it actually feels like. And people might be aware that obviously we have different sensations of touch, right? We can feel pressure, we can feel kind of tickliness, we can feel heat cold. What about itchiness? Do we all feel itch in the same way? Yeah, that's a question that has so many dimensions. And it starts with the cultural background. We are all the way that every human being has itch. But for assessing itch, also in clinical trials, we always use the itch intensity between zero and 10.

7:40And already there, we experience cultural differences. In some countries, they never would go to a 10. And in other countries, also the lowest level of itchiness is so dramatic for them that they give you a 10. So there are clearly differences in the perception and the perception is in the brain. And so by social and cultural background, this is modified already. I would say also that some unique features in the biology of the skin induces more itch, like in skin of color, We've shown that patients who are African-American or black skin have more severe itch. This most probably is related to some neuroimmune aspects that we're still investigating.

8:29Also, age could be different in terms. And females, the older we get, I have bad news, is that the older we get, we have a higher chance to be more itchy. And also females seem to have more itch than males. Well, that's, I think, a really good overview about what itching actually is. We've talked about acute versus chronic itching and the burden that that can have on people. We've talked about differences around the world and about the different triggers and how they might cause an itch. But we have actually got a question from a listener based in China who's been wondering about itches that seem to come from nowhere.

9:07Hi, I'm Leslie from New York, now living in Shanghai. And I find the sensation of being itchy to be a bit curious. I can somewhat understand when things like clothing or someone else's skin is touching our skin that this could cause an itchy feeling but what is a bigger mystery to me is how an itch can all of a sudden come about when nothing is touching my skin at all. Any insight into this bizarre sensation is much appreciated. So Sonia maybe you could help Leslie solve this mystery about itches that apparently have no cause. What's going on here? Yeah so when you think about your brain you always think that all the signals from all other parts of the body go up to the brain and the brain then perceives all this but sometimes it's too much for the brain and so the brain has also the strategy to inhibit upcoming signals.

10:06We call this descending inhibition. You can imagine that there is a gatekeeper that doesn't let all signals come into the brain. And we found out that the upcoming signal that are usually filtered before entering the brain can all go through and suddenly induce an itching. So why does it happen? This is still a great enigma. But we know that by wearing everyday clothes, this is inducing a lot of stimuli that is trying to go into the brain, but the descending inhibition controls it. And this can be the start that there is a subtle inflammation around the nerve fibers in the spinal cord and disturbing the descending inhibition.

10:54And maybe this can be what Leslie just experienced. So I guess what you were saying there about this inhibition from the brain is that normally our nervous system is suppressing lots of potential signals that just are generated from wearing clothes because they're not important, right? You would go crazy if you were always being disturbed by the clothes on your back. But occasionally some of those actually don't get inhibited and then we get the perception of an itch even though it may not be caused by anything obvious. Right, and under certain circumstances, imagine you are very stressed. then this descending inhibition doesn't work properly.

11:32I definitely know what that feels like. I'm sure lots of listeners do as well. Well, thank you so much, Sonia, for that and to Gil for helping us answer our first set of questions. We've heard that itching isn't just about what happens in the skin, it's also about what's going on in our brains too. And as it turns out, there is a psychological component to itch and that's what we'll be looking at next. This is summer at its peak. Whole Foods Market Summer Fruit Fest is your invitation to eat the season. Fresh, organic, and bursting with flavor. Start your day with peaches and organic blueberries and yogurt.

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12:57You're listening to CrowdScience from the BBC World Service, the show that helps to scratch the itch of your curiosity by finding the answers to your science questions. I'm Anand Jagatia, and this week we're dedicating a whole episode to the many listeners who are clearly itching to know more about the science of itch. So far, we've looked at the different triggers, causes, mechanisms, and types of itch, but we haven't looked at the thing it's all been building towards, scratching. So let's get into that now with another expert guest, Dr Henning Holler, a psychologist at the University of Hull in the UK.

13:32Hi Henning. Hello, nice to be here. Thank you. Henning, you're actually going to help us do a little experiment that listeners can also play along with. What are we actually going to be doing? Yeah, so I thought we could do a little game called Guess the Scratch. So I have a couple of sounds and maybe I could ask the panel and you Anand to just guess what is being scratched here. So you're going to play us some clips and we have to guess which body part is being scratched? Yes, please. Okay, cool. Let's go for it.

14:13Okay. So you are playing a human skin or is it also on animals? Yes, no, strictly human. Strictly human. It sounds like hairy skin, so I would go for the scalp. Yeah, I think it's beard or something, because it does sound like there's hair there. Gil, what do you think? I actually thought there was some clothing that he was scratching on some kind of plastic or something. I don't know. It didn't seem so hairy. It is definitely hairy skin, and Anand was spot on, so it is indeed a beard scratch that we were listening to. Wow. Well, I'm an expert. I'm always scratching my beard. You are the expert here.

14:49Even though I don't have any actual credentials. Okay, have you got another clip for us then, Henning? Yes. So let's listen to the next one. What is being scratched here?

15:03Any suggestions? Yes. It sounds like a little bit thicker skin, so I thought about palm soles or the lower arms or the back. The back. Interesting. Yeah, that didn't really sound very natural to me. It sounded more like somebody was scratching plastic or something, or I don't know, like clothing on the outside. What's the actual answer, Henning? Yeah, so Sonja was really spot on there. So this is indeed a rubbing of palms. Okay. I've never really thought much about the sounds of different scratching, but they are very distinctive. Okay, and should we play the last one? Yes, so here's actually a bit of fabric that is being scratched.

15:43Maybe you can try to guess which fabric is being scratched.

15:51that's jeans

15:55jeans and sweater i'm a bit of a loss here i'm afraid yeah i think it's a trowel jeans yes it was indeed a jeans scratch you got it all yes yes so after 20 years of listening Henning, I passed the test. Wow. Henning, we did surprisingly well actually on that, but there's a twist here, isn't there? Yes. So actually the background to this is whether listening to these sounds makes you feel itchy. Did anyone notice any feelings of itchiness or maybe anyone started to scratch spontaneously while you were listening to them? I definitely did. I 100 % did. Did either of the panel, did either of you notice that?

16:42We are desensitised most probably. We are desensitised. You're numb to that sound. Okay, so Henning, it wasn't really a game to see if we could get it right. This is something that you use in your research when you want to see if you can induce itchiness just via sound, right? Yes, that's right. So maybe if we go back a step. So we started out with the question, can we catch a scratch? So similar to how you can catch a yawn. we wanted to know whether it basically is socially contagious. So we did a study where participants watched videos. So the videos showed people either scratching themselves or just tapping their arm.

17:24And we asked people to rate after each video how itchy does watching this make you feel. Additionally, we videotaped them while they were doing the task so that we could score how often they started to scratch themselves spontaneously. and we also scanned their brain activity while they were doing the task. And what we found in this study was, yes, these scratching videos indeed made people feel itchier. They also started to scratch themselves more spontaneously. So the answer is basically, yes, you can catch a scratch. And when we looked at the brain activity, we basically found that the same brain network that is active when people experience the itch that starts on the skin.

18:10We see the same brain area being active during this contagious itch. So the brain constructs the itch experience. So the itch that people perceive, it's not just a passive readout of the signal that arrives at the brain, but a signal is interpreted, expectations factors enter it, attention factors into it. And under certain situations, we can experience an itch without any skin stimulation. Okay. Do you have any theories for maybe why this plays out? Is it that if somebody else near you is scratching, that maybe they have a parasite, maybe you have a parasite, so it's good if you scratch too? Yes.

18:52Maybe contagious itch serves like a social warning call there. I see someone else who has an itch, I might be in the same situation with them. I might also be exposed to whatever threatens their skin. So it's like a social safety mechanism, possibly. Well, thanks, Henning, for taking us through all that and for doing that experiment with us. Now it's time for our next set of listener questions. Hi, I'm Roby from Chapel Hill in the United States. I've always wondered why we get these little itches on our skin, but then with just a bit of a scratch, they go away. So why do itches go away when we scratch them?

19:30That's a good question, Roby. And we had a similar question from Luke in the UK. Why does scratching help relieve an itch? There's nothing worse than having an itchy foot when you got your shoes on. This question was inspired by a very itchy foot while I was on my daily commute. And another from Morgan in Norway. What actually happens when you scratch an itch? It itches so you scratch it, which makes it better. But what happens in the body? so Gil what actually happens when you scratch an itch why does it seem to make the itch go away so scratching induces inhibition of the itch and also there's a significant component of some kind of pleasurability and we've shown in the brain an area of anterior cingulate cortex which is involved also in the unpleasant sensation of itch, was inhibited by the act of scratching.

20:26In reality, the short-term activity of scratching is very helpful. Unfortunately, if you scratch too much, you damage the skin and it causes a vicious circle of itch scratch. Right, okay. So it might be partly because the scratching is sort of activating other neurons that then inhibit the itch, but also maybe that feels good, but long term it can create this cycle that perpetuates. Correct. And we also had another question about itches that seem to come back again. Hello, I'm Nathan from England, and my question for CrowdScience is, why doesn't scratching always make an itch go away? It seems strange that something meant to relieve an itch can sometimes make it come back again.

21:14Thank you, Nathan. And we also had a question from Jonathan in the UK who sent us a similar one. Why is it that when you start scratching your back, more areas itch? So Sonia, what's going on there? Because it's like your itches are playing a game of cat and mouse. Like you scratch one, it disappears, it turns up somewhere else. Do you want to have a go explaining what might be happening there? Yeah, so the nerve fibers, they are organized in different hierarchies and they are all connected to each other. And when you start to scratch the skin, you activate nerve fibers by this mechanical stimulus that are in the hierarchy above the nerve fibers that transmit the signal of itch.

21:58So in the spinal cord, the scratching and activation of these nerve fibers leads to an inhibition of these upcoming signals from the slower conducting itch fibers. And so the signal of itch doesn't run to the brain and is perceived in the brain. But on the other hand, when you are activating your nerve fibre network in the skin by this mechanical stimulus of scratching, you might also activate other nerve fibres that transmit itching. And so you have the perception that you scratch the itch, it helps on the one hand, but the itching can occur just around the scratching area. Well, maybe that explains why sometimes you just can't seem to scratch or get rid of an itch.

22:49We've also had another question about scratching that I personally really want to know the answer to, and it's from a listener called Colin in Navano, Japan. Recently, I was bitten by a buyo, which is the Japanese word for blackfly. Their bites aren't usually serious, but do cause a small red spot to appear on the skin. It gradually swells and gets unbelievably itchy. The urge to scratch is very strong, but if you give in, although the immediate feeling is one of relief, inevitably the skin becomes more irritated and takes longer to heal. As far as I can remember, I've always been told not to scratch these kinds of itches, for example mosquito bites.

23:32Is there ever a case when scratching is actually beneficial? And if not, then why does our body play such a cruel trick on us? so gil why do we feel such a strong desire to scratch these insect bites could there be some benefit in the long run or is it just a cruel trick as colin says well i think that evolutionary we mentioned that scratching of the insect was to remove it out and what happens here is that we can't control that urge. It's like telling someone to stop yawning or it's very similar in terms of that behaviour. So there's some kind of also reward of chemicals that are involved and inducing that kind of feeling of pleasurability and repetitive scratching.

24:28So it's sort of a mismatch between what we've evolved to do and what's actually useful. Like it might have been useful in the past, but it's not useful anymore. Yeah, the system got out of control. That's what happens with Insect Bite. And I'm quite sure that all of us have experienced that pleasurability of scratching this Insect Bite. So we continue doing it. It's very rewarding. We've talked about how itchiness is this sort of compulsion, right? It's this urge to scratch that you really can't stop yourself from doing. And I just wondered if we know anything about how that's generated and whether it has anything in common with other kinds of compulsive behaviors that we can't control.

25:11Sonia, have you got any information on that? Yes, that's a very good question. And with the scratching, there is an amplification in the spinal cord and it's transmitted to the brain. and in the brain, you have this feeling that it's a very pleasant sensation. It's very rewarding when you scratch. So I always do with my students a test. I tell them, you do not have an itch. Scratch your skin and sense how it feels. It feels unpleasant. And now, memorize the last time you had a mosquito bite and then you scratch. This really feels good. And how long do you scratch until it's bleeding, until you destroyed the nerve fibers of the skin.

25:57And imagine you scratch and scratch and scratch and these signals are amplified and it feels even more rewarding. So the sensation of rewarding is sometimes very important for the patients and they really look forward to the next scratching. That's really interesting. Thank you for that. Now, we're getting towards the end of the show, but we have one more question about why itchiness seems to get worse towards the end of the day from Vary in Berlin in Germany. Hi, I've noticed that when I get bitten by mosquitoes, when the bites are a few days old, they are much itchier at night. Why would this be?