That's what biology is all about.

And while the sex part is, I'll grant you,

a little bit sexier,

not dying is also really fantastic...

something that I, personally, like to do every single day.

I, personally, like to not die in all sorts of ways.

Like, I don't jump out of planes, I don't go into active combat zones,

I don't do heroin, but I can, however, spend time wallowing

in filth with my cute bacon-producing friends here

and not have to worry about dying.

Because, somehow, my body can handle a lot

of little devils on my hands, in my air, in my food,

little things that literally want to kill me.

There are more potential human killers in this pig pen

than there are in all of the world's prisons,

but I don't have to worry about it because of the elite team

of microscopic assassins that live inside my body.

My immune system.

Ahh! That was really close to my hand!

You've heard of some of these little ninjas, others maybe not,

but everyone knows the work they do by the trail of dead

that they leave behind.

Pus, being the most disgusting example.

And the work these guys do is pretty hardcore.

They not only identify incoming enemies, they eliminate them,

and then they keep files on them, in case their kind ever comes back.

I don't want to freak you out, but you, and I,

are covered in pathogens right now.

And you really can't blame them for wanting

to get a piece of your action.

Your warm, high-energy, nutrient- rich, salty, watery action.

Your body is a theme park for these guys,

and although the majority of organisms living inside you

actually make your life more comfy,

there are some less-helpful viruses and organisms,

from here on out referred to as pathogens,

that will want to turn your body into a factory for their children.

So let's avoid that!

We have two basic ways of doing it:

innate, or non-specific, immunity that responds to all kinds

of pathogens the same way and very quickly,

whether your body has seen that pathogen before or not.

And your acquired, or adaptive, immunity which develops

more slowly and requires your body to learn the wily ways

of the pathogen before it defeats it.

Every animal has an innate immune system, even sponges!

But only vertebrates have the acquired kind.

You were born with your innate immune system.

And from the second you wriggled your way out

of the sterile environment of your Mom and into this germy,

disgusting world, that system has been protecting you.

The thing about the innate immune system is that

it doesn't care what it's killing.

It doesn't worry about whether it's offing a virus

or bacteria or fungus.

Its job is to just keep the enemy from getting in,

or once it's in, to sneak up behind it

and break its neck, ninja style.

The first line of defense in keeping sketchy characters out

are the skin and mucous membranes.

The skin has so many excellent functions,

like keeping your organs in, that it's easy to forget

that its primary purpose is to keep things out.

It's oily and kind of acidic, and really not easy to penetrate.

And I'm about to rock your world with this,

but your digestive tract is also technically the outside of you.

Remember how our whole bodies are basically

just a built around a tube, right?

Well, the inside of that tube is exposed to as much weird,

grody stuff as the outside of the tube.

So, your body treats the digestive tract like the front lines

of this war, which is one of the reasons why your stomach

takes no prisoners with the whole stomach acid situation.

In addition to things like skin, we've also got mucous membranes

providing another barrier to microbes trying to sneak in.

Mucus membranes line all of your internal surfaces that are

exposed to the outside like your lungs and the inside of your nose,

as well as some other parts of your body like the inside

of your mouth, and your eyelids and your sex organs.

Mucous membranes unsurprisingly produce mucus,

which is a viscous fluid, you've probably heard of it,

and it traps microbes and helps sweep them away.

This is why illness is so often associated with such awe-inspiring

amounts of goop.

Your second line of defense is your inflammatory response.

The honchos here are specialized cells in your connective tissue

called mast cells that constantly search for suspicious objects,

usually unknown proteins, and then release signaling molecules,

like histamine when they find them.

Histamine makes your blood vessels more permeable,

which allows a whole bunch of fluid to flow to the affected area.

And that is what causes inflammation,

but it also brings in a crap-ton of white-blood cells,

infection-fighters, to go all Balrog on whatever's

trying to make its way in.

Now, this is great if you get a splinter in your toe

or a bunch of viruses in your face,

but sometimes something gets into you that's not actually dangerous

like pollen or dust or, like, a peanut

and your innate immune system triggers

an inflammatory response anyway, even though it's not a big deal.

This is what we call an allergic reaction,

and you know what those are like

with the swelling, redness, mucus production, itching,

and occasionally a little bit of death.

So that is why we take antihistamines to suppress

the histamine trigger so our immune systems stop freaking out

about nothing, also, that is why you should always

tell people when there are peanuts in your cookies.

Most of the immune system activity that happens inside

your body's fortress is done by white blood cells, or leukocytes.

Leukocytes are awesome for a lot of reasons,

but one reason is they've got full VIP access to anywhere

in the body that they want to go, with the exception

of the central nervous system: the brain and spinal cord,

which are, for obvious reasons, super high security areas.

Leukocytes can move through the circulatory system

and when they get to a place where they're needed,

they can basically send a signal to ask the capillary

to open a gap between it's cells and then it oozes through that gap

to the site of the infection, this is called,

get ready for it, diapedesis.

From the greek for "oozing through."

There are lot of different kinds of leukocytes,

like different branches of your own personal microscopic army.

The kind specific to the innate immune system are phagocytes,

more greek, this time Phago, meaning eating.

And they're just any cells that ingest microorganisms

through the process of phagocytosis.

Phagocytes are pretty cool.

They can literally chase down the invading cells,

grab them and then completely engulf them.

And some, like the super-abundant neutrophils,

move around the bloodstream, and can quickly get

to where the action is.

Once a neutrophil kills an invading microbe,

they basically just roll over and die.

Dead neutrophils collect together into what we lovingly call pus.

The biggest and baddest of the phagocytes are the macrophages,

the "big eaters," which don't generally travel a lot,

but instead hang out like bodyguards in your various organs.

Not only do they kill outside invaders,

they can also detect when one of your cells has gone rogue,

like a cancer cell, and kill those, too.

And they, unlike the neutrophils, don't die once they've

killed a bacterium.

They can eat up to 100 before they die.

BIG EATER!

Of all the grisly stuff that goes on in the never-ending

street war that is your immune system, some of the most

gruesome stuff is done by a kind of cell called

"Natural Killer Cells" which reminds me,

I think it's time for our very first Open Letter.

An open letter, to 1973.

Dear 1973,

You had a lot going on, the Vietnam war ending, Roe v Wade,

Watergate...it was a tumultuous time.

But part of me wishes that you, 1973, had an opportunity

to name everything in biology because you got one chance

to name a new type of immune cell, and you named it

"The Natural Killer Cell" and I freaking love that.

I look around at today's script with all of it's dendritic cells

and macrophages and dieapudeesises and I think,

what if we let 1973 name all these things,

would we have Spikey Death Cells, Devourerers,

and Oozing Action instead?

I don't know...maybe you would have screwed it up,

but I don't think you could have done any worse

than all of this GD greek we have to deal with all the time.

Thanks for the Endangered Species Act! - Hank

Ok! Natural Killer Cells, more than just a great name,

also the only phagocyte in the innate immune system

that destroys other human cells.

When your cells are healthy, they have a special protein

on their surface called MHC I

MHC for major histocompatability complex.

But when your cells are infected, say with a virus,

or when they're cancerous, they stop producing that protein.

So the Natural Killers are always going around checking up

on each of your cells, and when it finds one that's not normal,

it pulls out it's AK47 and unloads.

Actually, it just binds with it and then secretes

an enzyme that dissolves its membrane, but still. Killing.

Finally, dendritic cells are a type of phagocyte

that hangs out on the surface of much of your body

that comes in contact with the environment

in your nose, on your skin, in your stomach and intestines.

They eat up pathogens and then carry information about them

back to the spleen or lymph nodes, where it passes intelligence

about what's going on on the war front to the acquired immune system.

I actually studied dendritic cells in my undergraduate thesis

and I kinda fell in love with them.

They're lethal... but they're also intelligent,

great heroes for any Robert Ludlum novel.

To be fair though, macrophages can do this too.

The activity of these cells give us a chance to transfer

from the innate immune system to the acquired immune system,

which is going to make things a little more complicated.

The acquired system has to learn as much as it can about every

pathogen it interacts with, store that information,

and then use it to invent defenses against them.

It's your super-elite, double-secret strike force delta.

You get to work building your acquired immune system

immediately after you're born, harvesting bacteria and other stuff

not just good bacteria that can help your guts out,

but also harmful ones that your body learns from

and stores information about.

That system keeps an eye out for any foreign substance:

a toxin, a virus, a bacteria, even parts of those things

that could be tell-tale signs of a bad guy.

We call those signs antigens,

a word that comes from antibody generator.

An antigen is anything that causes your immune system

to ID a pathogen and then create an antibody against it.

Now antibodies aren't cells, they're highly specialized

proteins produced by B cells to recognize and help lay

the smack-down on intruders.

But antibodies can't kill invaders themselves.

They're just little proteins after all.

The best that they can do by themselves is just swarm all over

the invader, making it harder for it to move,

and to excrete toxins, or otherwise infiltrate healthy cells.

But more often, antibodies serve as "tags,"

attaching themselves to the scumbags and then releasing

chemical signals to nearby phagocytes,

alerting them that it's dinner time.

Your acquired immune system also has its own type

of white blood cells.

Not phagocytes, which go after everything that looks

a little bit sketchy, but lymphocytes, which go after