Dr Michelle Dickinson: nanotechnologist reveals why it feels good to scratch

We've all felt the instant relief of scratching an itch, but what if that fleeting moment of bliss was actually doing more than just satisfying an itch?  

Scientists have just published new research in the journal Science which shows how scratching activates an immune response that can help protect the skin against harmful infections. 

Scratching has long been thought to serve a practical function: removing irritants like insects, dust, or allergens. But some itches, such as those caused by mosquito bites, persist long after the culprit is gone. This suggests that scratching might serve another purpose beyond simply removing irritants. 

Researchers induced an itchy allergic contact dermatitis on the ears of mice have of which were allowed to scratch the itch and half that couldn't due to wearing a tiny headcollar. The mice that were allowed to scratch developed increased swelling and a surge of immune cells called neutrophils at the site. However, the mice that were prevented from scratching, had significantly lower inflammation and fewer immune cells in the affected area. 

The researchers found that scratching activates pain-sensing neurons, which then release a powerful neurochemical called substance P. This messenger molecule wakes up mast cells which are immune cells that play a key role in allergic reactions. Once activated, mast cells recruit neutrophils, amplifying inflammation at the scratched site. 

Previously, scientists believed that mast cells were only triggered by allergens, however this study revealed an entirely new pathway: scratching itself can activate these immune responses. While this might sound like a bad thing, it turns out that it has a hidden benefit. 

The team discovered that scratching isn’t just about relief, it also helps to keep dangerous bacteria at bay. In their experiment, they found that mice who were allowed to scratch had lower levels of the potentially harmful bacteria Staphylococcus aureus on their skin compared to those prevented from scratching. This suggests that scratching might have antibacterial effects, helping to remove harmful microbes before they cause infection. 

Of course, not all scratching is beneficial. Chronic scratching, as seen in conditions like eczema or diabetes, can lead to skin damage and increase the risk of infection. In these cases, the same immune response that helps in the short term can become a problem if it’s constantly activated. 

Interestingly, researchers identified two separate nerve pathways, one that signals an itch and another that controls the immune response from scratching. This discovery opens the door to potential treatments that could block the itch-scratch cycle while preserving the immune benefits of scratching. 

Scientists hope that by targeting specific nerve pathways, they can develop new therapies that stop the discomfort without shutting down the body’s natural immune defences. 

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Episode Transcript

Speaker 1 (00:06):
You're listening to the Sunday Session podcast with Francesca Rudgin
from News Talks.

Speaker 2 (00:11):
It'd be and do you know what I mean now
is doctor Michelle to concern with our science study of
the week of Good morning Michelle, good morning. So this
is really interesting what you've discovered about scratching, or what
some scientists have discovered about scratching.

Speaker 3 (00:26):
I'm always amazed at what we don't know and then
it's so obvious. So for those who want to read
this at home, this is published in the journal Science.
Its open source, you can read it. It's a lovely study.
But previously, up until this study, we always thought that
we scratch to remove irritantce So maybe an insect has
landed on your arm, this and dust, maybe you've touched

(00:47):
a leaf that was you were allergic to until you
try and scratch the allergen out. But these scientists said, well,
if that was true, then you would stop scratching once
that irritant had gone. Okay, so you've got the bug off,
then you stop scratching. But we don't because anybody who's
had a mosquito bite knows that even if the mosquito
has gone, you are still scratched, which made them think,

(01:08):
maybe we scratched for another reason. That's what this study
is all about. And to do this study. Sadly, it
was a mice study, and some of these mice did
not have a good day. So what they did is
they applied a substance to the ears of mice that
induced an itchy contact dermatitis to their ears. And half
of those mice were fine, they could scratch freely, and
they were studied. Half of the mice suddenly had to

(01:30):
wear a tiny little head collar. You know when you
take your pet to the vet and they've got those
little Elizabethan collars, one of those, but a tiny one
made for mice, which meant that they couldn't scratch their ears.
And so even though they had itchy ears, there was
nothing they could do. And they studied these mice, and
the mice that could scratch freely developed swelling in their ears.

(01:51):
Their ears swelled up, and that was because their immune
response basically kicked in, so there's a rush of immune
cells to their ears they called neutrophils at the site
they were scratching at their ears. Interestingly, the little mice
with the colors on that couldn't scratch there is didn't
swell up, and there was very little inflammation. And that's
sort of what's important about this study because what they

(02:14):
found is if you scratch your skin, what you do
is you activate a neuron and this neuron releases a
powerful neurochemical they call it substance P in this paper,
and this wakes up massed cells. And these are immune
cells that basically play a key role in allergic reactions.
And so what's happening is that you're scratching, you're actually

(02:36):
kicking off an immune response in your body to help
try and get rid and attack anything that might be
at that site, which is why their ear swelled up.
That's why when you scratch your mossy bites, your sort
of arm swells up a little bit if it's on
your arm. Now why is that important, Well, what they
found is that by kicking in your immune response, and
your immune response, which is part of your white blood cells,

(02:58):
is what kills anything that's nasty around you, they're able
to actually measure that they were getting dangerous bacteria at
the set in killing it and keeping it at bay.
So not only were the scratching mice allowed to scratch
and sort of get that itch feeling better. They actually
measured they had lower levels of harmful bacteria at the

(03:18):
scratching site, so staphalocarcosorus was one of the ones that
they measured. So what they found which they didn't know before,
is the act of scratching itself can actually activate your
immune response to help your body to fight whatever it
is that might be attacking you. And the reason why
this is different is previously they thought the immune response

(03:39):
was only kicked off by the allergen itself, so the
thing that was touching your skin. And the reason why
that's important is because actually scratching is designed to protect
you and help you tell your body you need to
get here now to get rid of whatever this is
and bring some white blood cells here to fight against that.
And that is the first time they've ever found that.
They thought that all you scratch for was to get

(04:01):
rid of the physical object. And the reason why that's
interesting is because in this study they were able to
find two different nerve pathways when it comes to scratching.
So one of these pathways signals the itch, so you
scratch it, but one of them controls the immune response
and the reason why that's interesting is now they know
what these pathways are. If you have something like exma,

(04:22):
where you have a chronic scratching problem, they might be
able to turn off one of these nerve pathways to
help stop the itch. Because if you scratch too much,
you can break your skin and you can just cause
some infections and other problems like that. So these poor
mice very itchy. Is I actually have discovered something really important?

Speaker 2 (04:40):
Yeah, a bit of a bad day in the office.
Thank you so much, Michelle. Appreciate that I've just spent
the whole time sitting here sort of mentioning itching, just
scratching myself, But thank you so much for it.

Speaker 1 (04:50):
For more from the Sunday session with Francesca Rudken, listen
live to News Talks it'd be from nine am Sunday,
or follow the podcast on iHeartRadio.