and yet most of us never learn how it works.

Or, what it is.

Your immune system consists of hundreds of tiny—and two large—organs.

It has its own transport network spread throughout your body.

Every day, it makes hundreds of billions of fresh cells organized like an army.

With soldiers, captains, intelligence officers, heavy weapons, and crazy suicide bombers.

It's not some sort of abstract entity.

Your immune system is you.

Your biology protecting you from the billions of microorganisms that want to consume you,

and from your own perverted cells that turn into cancer.

It's so manifold that it's impossible to cover in one video,

so we'll make a series looking at different aspects of it.

Today: What happens when your body is invaded

and your first lines of defenses are engaged in a fight for life and death.

It's been a normal day,

when suddenly the world explodes and an asteroid rips the sky open.

Countless alien life forms invade,

ready to destroy cities and infrastructure, and eat civilians...

Or, this is what your cells experience.

You look at your bleeding thumb that you just cut on a dirty twig in the park.

How annoying!

But inside the wound, a horrible catastrophe has happened.

There are dead cells, and blood and dirt everywhere.

Even worse, countless bacteria invade the warm caverns between your helpless cells

to explore their new home, steal your resources, and poop everywhere.

Immediately, the first stage of your defense kicks in.

The cells that survive the impact, or are hurt or dying, scream in panic

releasing an onslaught of chemical alarm signals that awaken your immune system.

The first cells to show up are macrophages.

If an average cell were the size of a human, a macrophage would be the size of a black rhino.

A stoic cell in principle but you wouldn't want to annoy it.

Bacteria -do- annoy them.

Within seconds, the large cells attack and begin killing them without mercy.

They stretch out parts like the arms of an octopus,

and grab the bacteria to swallow them whole and digest them alive.

A macrophage can eat 100 bacteria before it's exhausted.

But there are too many enemies, so the macrophages call for reinforcements.

In your blood, hundreds of thousands of neutrophils pick up their signals and move to the battlefield.

Neutrophils are intense suicide warriors that only live to kill.

They're so enthusiastic about killing that they kill themselves a few days after birth,

so they don't have time to accidentally destroy your body from the inside.

As soon as neutrophils arrive, they begin vomiting deadly chemicals at bacteria, or devour them.

They are so careless in their attacks that they are causing real damage to your own cells.

But collateral damage is not their concern now,

or ever.

Some neutrophils go so far to push their suicide button and explode,

casting a wide and toxic net made from their own DNA filled with deadly chemicals that trap and kill bacteria.

Sometimes, they can continue fighting after that, even though they're sort of dead already.

This is how much fun they have killing!

While the battle rages, your blood vessels let fluid stream into the battlefield like a dam opening up towards a valley.

You notice this as inflammation.

Your thumb swells up a little and gets red and warm.

The fluid brings a silent killer into the battle zone, millions of complement proteins.

A sort of automated liquid weapon that stuns and kills bacteria by ripping holes into them.

We made a whole video explaining them in detail.

We are reaching a crossroad now.

If things go well, your first line of defense kills the invaders quickly.

But sometimes, the enemies are too strong and would overwhelm your defenses eventually,

which means certain death for you, the human.

This is the hour of the dendritic cell, your immune system's intelligence officer.

While your soldiers were bashing in heads,

it was collecting samples by ripping bacteria into tiny parts and covering itself in it.

Like a soldier decorating itself in the guts of a dead enemy.

The cell leaves the battlefield and enters the superhighway of your immune system

that connects all your tissues with your immune headquarters.

Your lymph nodes.

The dendritic cell coming from the battlefield is looking for a helper T cell,

which is a sort of all-purpose commander cell within your immune army.

But not any helper T cell.

One that happens to have just the right weapon for the bacteria that infected your wound.

So it goes around and rubs itself, still covered in bacteria parts, against every helper T cell it meets.

Most T cells are a bit disgusted and not interested.

But after a few hours, something clicks.

A helper T cell recognizes the bacteria parts.

This cell is the weapon that's needed right now.

The dendritic cell is overjoyed and activates the helper T cell.

Okay, wait.

How come your immune system has a cell that has a weapon against the specific bacteria that infected you?

Well, your immune system has a perfect weapon against every possible disease in the universe.

Against the Black Death, the Coronavirus, or an infection that will emerge in 100 years on Mars.

We'll talk about this a bit more in the next video because it's very complex.

So, for now, just know that you have billions of unique helper T cells

that each have weapons against every possible enemy.

After the right T cell is activated, your second line of defense awakes

and rises like a teenager that needs to get up on a school day...

Very slowly.

Your heavy weapons are incredibly effective but they're not fast.

The activated helper T cell begins to clone itself over and over again.

One becomes two, two become four, until there are thousands of them.

Now, they split into two groups.

The first group quickly moves to help out your soldiers.

At the battlefield, things are getting out of hand.

A tired macrophage is ready to give up.

After fighting for days, it just wants to go to sleep like many of its buddies have done already.

But now the helper T cells arrive.

One of them comes to the tired macrophage and whispers something using special chemical signals.

In a heartbeat, the demoralized soldier feels fresh again.

But there's something else.

A hot white anger.

The macrophage knows what it needs to do...

Kill.

Invigorated, it throws itself against the enemies once again.

All over the battlefield, this begins to happen.

Meanwhile, the second group of helper T cells was working on activating another line of defense:

B cells, your antibody factories.

Antibodies are protein super weapons that look like tiny crabs with two pincers to grab enemies.

Just like the helper T cells, there are B cells in your body

that are able to make just the right antibodies for every possible enemy.

And the helper T cell is looking for exactly these B cells.

After a day or two, the right B cell is found, and begins to clone itself.

As soon as enough clones have been made,

each B cell begins pumping out up to 2,000 antibodies per second!

About a week after you injured yourself and bacteria invaded,

your second line of defense finally arrives in full force.

The tiny army begins to saturate the battlefield, pinching and stunning desperate bacteria.

The antibodies clump them together and make them unable to move or fight,

while your soldiers massacre the defenseless victims.

The tide is turning fast.

As the last enemies are cleaned up, your soldiers realize they are no longer needed,

and begin to kill themselves to save resources.

But not all of them.

A few helper T cells remain and turn into memory cells.

They will guard the tissue for years,

making sure the same bacteria will never again gain a foothold here.

Similarly, a few B cells will stay alive

and keep producing a low amount of antibodies,

making you immune against this bacteria,

maybe for the rest of your life.

One day you wake up and notice that the wound has grown over

and left nothing but a faint red mark.

You were completely unaware of the drama your cells had to deal with.

For you, the whole ordeal was a slight annoyance.

While for millions of cells, it was a desperate fight for life and death.

But this is just the beginning of the epic story that unfolds inside you every day,

and is told in full in

"Immune: A Journey into the Mysterious System That Keeps You Alive",

written by Philipp Dettmer, the founder and head writer of Kurzgesagt.

The book is a beautifully illustrated deep dive into the immune system,

the most important thing you don't know enough about,

that will forever change how you think about your body.

Written to be as simple and fun as possible,

it's been an ongoing passion project for almost a decade

because the immune system is just about the most amazing and fascinating topic there is.

So, go on a journey and get to know your own body.

From mystery organs, murder universities, to the largest library in the universe.

Explore how incredibly vast your immune system is and how it actually works.

How it fights enemies from cancer to HIV, or just the flu.

Learn how you can boost it and if that's actually a good idea.

The book will be out in six weeks

and it would mean the world to us and Philipp if you could pre-order it.

Pre-ordering is super important in the world of books,

and in contrast to games, we can guarantee that it's actually finished

and there will be no day one patch.

There's a link in the description.

This is the end of a decade-long personal journey and we'd love to see what you think.

Thank you for watching.