Why do we itch? - Emma Bryce

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The average person experiences dozens of individual itches each day. We’ve all experienced the annoyance of an inconvenient itch — but have you ever pondered why we itch in the first place? Is there actually an evolutionary purpose to the itch, or is it simply there to annoy us? Emma Bryce digs deep into the skin to find out.

Lesson by Emma Bryce, animation by Sashko Danylenko.

You're standing at
the ready inside the goal

when suddenly, you feel an intense itch
on the back of your head.

We've all experienced the annoyance
of an inconvenient itch,

but have you ever pondered why
we itch in the first place?

The average person experiences
dozens of individual itches each day.

They can be triggered by all sorts
of things,

including allergic reactions,

dryness,

and even some diseases.

And then there are the mysterious ones
that pop up for no reason at all,

or just from talking about itching.

You're scratching your head right now,
aren't you?

Anyhow, let's take one of the most
common sources: bug bites.

When a mosquito bites you,

it releases a compound into your body
called an anticoagulant

that prevents your blood from clotting.

That compound,
which we're mildly allergic to,

triggers the release of histamine,

a chemical that makes
our capillaries swell.

This enables increased blood flow,

which helpfully accelerates
the body's immune response

to this perceived threat.

That explains the swelling,

and it's the same reason pollen
can make your eyes puff up.

Histamine also activates the nerves
involved in itching,

which is why bug bites make you scratch.

But the itchy sensation itself
isn't yet fully understood.

In fact, much of what we do know

comes from studying
the mechanics of itching in mice.

Researchers have discovered that
itch signals in their skin

are transmitted via a subclass of
the nerves that are associated with pain.

These dedicated nerves produce a molecule
called natriuretic polypetide B,

which triggers a signal that's carried
up the spinal cord to the brain,

where it creates the feeling of an itch.

When we scratch, the action of our
fingernails on the skin

causes a low level pain signal
that overrides the itching sensation.

It's almost like a distraction,
which creates the sensation of relief.

But is there actually an evolutionary
purpose to the itch,

or is it simply there to annoy us?

The leading theory is that our skin
has evolved to be acutely aware of touch

so that we're equipped to deal with risks
from the outside world.

Think about it.

Our automatic scratching response
would dislodge anything harmful

that's potentially lurking on our skin,

like a harmful sting,

a biting insect,

or the tendrils of a poisonous plant.

This might explain why we don't
feel itching inside our bodies,

like in our intestines,

which is safe from these external threats,

though imagine how
maddening that would be.

In some people, glitches in the pathways
responsible for all of this

can cause excessive itching
that can actually harm their health.

One extreme example is a psychological
condition called delusory parasitosis

where people believe their bodies
are infested with mites or fleas

scurrying over and under their skin,

making them itch incessantly.

Another phenomenon
called phantom itching

can occur in patients
who've had amputations.

Because this injury has
so severely damaged the nervous system,

it confuses the body's normal
nerve signaling

and creates sensations in limbs
that are no longer there.

Doctors are now finding ways
to treat these itching anomalies.

In amputees, mirrors are used to reflect
the remaining limb,

which the patient scratches.

That creates an illusion
that tricks the brain

into thinking the imaginary
itch has been satisfied.

Oddly enough, that actually works.

Researchers are also searching
for the genes involved in itching

and developing treatments to try
and block the pathway of an itch

in extreme cases.

If having an unscratchable itch
feels like your own personal hell,

Dante agreed.

The Italian poet wrote about
a section of hell

where people were punished by being
left in pits to itch for all eternity.