The Science Behind How Sickness Shapes Your Mood | Keely Muscatell | TED

Alright, I'd like you to take a moment.

Think about a time when you were recently sick.

Try specifically to think of a time when,

even though you weren't feeling all that great,

you still felt well enough to get up out of bed,

go about your day.

OK, what was your mood like?

Did you feel a little sad or depressed?

What types of social interactions did you want to have?

Would you have wanted to go to a cocktail party full of strangers

or out on a first date?

I'm a psychology professor

at the University of North Carolina at Chapel Hill,

and for the past 15 years,

I've been studying the interconnections between physiological changes

happening in the body during sickness

and psychological and social well-being.

Specifically my lab studies social psychoneuroimmunology,

which is a mouthful to say,

and is a field of research that's dedicated

to discovering interactions between our social experiences,

psychological processes and the immune system.

Today, I want to tell you about some research showing

that the same physiological changes happening in your body

that cause the physical symptoms you have when you're sick,

are also shaping your mood and your social behavior.

In other words, changes in the immune system

can signal to the brain to cause us to think,

feel and act differently.

And not only that, but also our social experiences can cause changes

in our immune systems.

So purely psychological things happening in our brains

can cause the immune system to ramp up or ramp down.

Because of that, we can get caught in these vicious cycles

where our psychological experiences can cause changes in our immune system,

and those immunological shifts can cause changes

to our psychological experiences.

Now the component of the immune system

that most research in psycho- neuroimmunology focuses on

is the inflammatory response or inflammation.

The inflammatory response is your immune system's

first line of defense against injury or infection,

and it's coordinated by these molecules

that are called pro-inflammatory cytokines.

You can think of cytokines as sort of the chemical messengers

of the immune system.

So they're out there right now, swimming through your bloodstream.

And if an immune cell finds something weird or out of the ordinary,

they'll tell those cytokines to signal to other immune cells

to come and check it out.

So if you think about a time you've had a paper cut,

you may have noticed that the area around the cut swells,

it turns red, it heats up.

That's the inflammatory response in action.

And those symptoms are caused by your cytokines doing their job,

sending out signals to other immune cells to come and heal the cut.

The same thing happens if your immune cells find a virus

or a bacteria in the body.

They send out cytokines to signal to other immune cells

to come and try and eliminate the pathogen.

Now in doing this, cytokines cause

the physical symptoms we commonly have when we're sick.

Things like fever and achiness and fatigue.

So even though we usually think of those symptoms as being caused by a virus

or a bacteria itself,

they're actually caused by our own immune systems

activating to try to eliminate the pathogen.

But in addition to those physical symptoms,

decades of research,

in both animals and humans,

clearly shows that cytokines also cause changes to our mood

and to our social behavior.

So inflammation in the body can signal to the brain

to cause us to feel down,

depressed and even hopeless.

Inflammation can also make us want to socially withdraw from other people

to avoid interacting with individuals in our social networks.

So this research shows the powerful influence that the immune system can have

on our mood and on our social behavior.

Changes in inflammation in the body

can signal to the brain to cause us to feel depressed and even lonely.

OK, so you may be wondering,

why on Earth would your body do this?

Why would you want your immune system to be able to manipulate your brain

and cause you to feel sad and distant from other people in your life?

While we can't know for sure why this happens,

evolutionary theory provides some good food for thought.

The fact is,

revving up and running the immune system takes a lot of energy.

Getting cytokines to swim through the bloodstream

and send signals to immune cells takes calories.

And what else takes calories?

Pretty much everything.

Especially things like going out and seeking pleasurable experiences,

interacting with strangers

and just generally moving about the world.

So the theory is that the immune system is telling the brain to feel depressed

and to withdraw from socializing

because it wants you to stay at home and rest.

And if things that would normally sound fun

just don't seem all that fun,

and if interacting with other people seems exhausting

and maybe even a little threatening,

then we'll be less likely to do those things

and more likely to stay at home

and let our immune systems use our calories.

Now, of course, if this becomes prolonged and inflammation is elevated over weeks

or even years,

that could have really terrible impacts on our well-being.

But in the short term,

we think of this immune-to-brain signaling as adaptive.

Your immune system is basically good at knowing when it would be a good idea

for you to be out interacting with the world and the people in it,

and when it would be better to just stay at home.

But it turns out, the influence of inflammation on our social lives

isn't as simple as always making us feel more disconnected

and socially withdrawn.

One of the most important discoveries

that we've made in this area of research recently

is that inflammation might actually make us more motivated

to seek some social interactions,

specifically those with the people who we're closest to.

So it’s not that inflammation makes us less social across the board,

it may just make us more selective about who we want to socialize with.

To discover this, my colleagues and I ran an experiment.

We asked participants in our study

to send us digital photographs of someone who they could go to

if they needed help or support.

As you would expect, people sent us photos of their romantic partners,

their closest friends and their family.

Then we had our participants view those images of their support figure

while we tracked their brain activity using MRI scanning.

Now, critically, prior to the MRI scans,

we exposed a random half of the participants in our study

to a bacteria which cause levels of inflammation in the body to increase.

The other half just had normal, low levels of inflammation.

And what we found is that for the people who we exposed to that bacteria,

who had lots of cytokines in their body,

they actually showed more activity in a reward-related region of the brain,

called the ventral striatum,

when they viewed images of their support figures

relative to the people who had low levels of inflammation.

So what this suggests

is that while inflammation might make social interactions with strangers

seem less rewarding,

it doesn't necessarily make all social interactions seem less rewarding.

In fact, inflammation may make us more motivated to seek interactions

with people who could provide us with comfort or care.

Those who could be a shortcut to chicken soup.

But of course, it's not every day

that we come into contact with a virus or a bacteria,

or that we have a wound

that causes our levels of inflammation to increase.

So how much of a day-to-day influence on our social lives

does the immune system actually have?

Research shows that the answer is a lot more than you might think.

That's because one of the most robust findings

in all of psychoneuroimmunology research

is that stress causes inflammation.

So purely psychological things happening in the brain

can cause increases in cytokines in the body.

This is true across species,

from rodents to primates to humans.

In my lab and in others across the globe,

we study the effects of stress on inflammation

by asking people to come into the lab and give a ten-minute,

off-the-cuff speech

about why they'd be a good candidate for their dream job.

And then they have to do challenging mental math out loud.

And they do this all in front of a panel of experts who's judging them.

Yeah, people report that this is stressful.

And critically,

in response to this psychologically stressful experience,

we see that levels of cytokines in the body have increased.

Let's take a second and really appreciate that remarkable finding.

In the absence of anything physically wrong with the body --

no viruses, no bacteria, no wounds --

a purely psychological stressor

is causing the inflammatory response to activate.

So it's not just that our immune systems are sending signals to our brain,

but also our brains are sending signals to our immune systems

telling them to activate in response to stress that we encounter.

OK, so now that you know

that your immune system has this very active social life

and that your brain and your immune system are constantly communicating

in ways that could make you feel depressed or lonely,

what should you do?

For me, knowing about the social life of the immune system

has changed my life,

in part because it gives me an explanation

for why I might feel physically crummy at the end of a stressful day,

or why I might feel a little down

and disconnected from other people when I'm feeling physically unwell.

So, for example,

the experience of giving this talk today has been really rewarding

and also quite challenging.

You might even say it was, at times, stressful.

And I'm sure you've had similar experiences in your lives.

Maybe it was a difficult task you had to complete at work,

a confrontation with a spouse or a family member,

or just some problem that cropped up in your life

that you had to quickly figure out how to solve.

Based on what I've told you here today,

I hope you're predicting that in response to these stressful experiences,

we're likely to have high levels of inflammation in our bodies.

Our cytokines will have come out in full force

in response to the stress we've experienced.

And because of that,

if we then feel exhausted

and a bit like we've failed

and we don't want to go anywhere or see anyone or do anything,

and we just want to stay at home and relax on the couch

with the people who we're closest to,

we'll know why.

And because we know that those symptoms are likely just our immune systems

telling our brains to feel depressed

and to withdraw from social interactions

so that we stay at home to rest, relax and recuperate,

we don't have to agonize

over why we're maybe feeling a little down on ourselves,

or a little distant from other people.

We can rest assured that

in the coming days,

we'll be feeling much more like ourselves again

as our cytokine levels go back down.

So in the future,

when you encounter stress in your life

and you feel those reins of inflammation start to take hold,

I hope you remember what I've told you here today.

And you remind yourself that those symptoms you're experiencing,

that's likely just your immune system

doing its job,

telling your brain to feel depressed and withdrawn

as evolution intended,

to keep you healthy in the long term.

Thank you very much.

(Applause)