- Sex and not dying, that's
what biology is all about. 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. I can, however, spend time
wallowing in the filth with my cute bacon-producing friends here and not have to worry about
dying 'cause 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 pigpen than there are in all
of the world's prisons, but I don't have to worry about it because of an elite team
of microscopic assassins that live inside my
body, my immune system. Ah, that was really close to my hand. (upbeat music) 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 that 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. So 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's pretty much 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 as 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 that
you wiggled 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 a bacteria or a fungus,
its job is to just keep the enemy from getting in or once it's in to sneak up behind and
break its neck ninja style. The first line of defense in
keeping sketchy characters out are the skin and the 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 digest tract is also technically the outside of you. Remember how our bodies are basically just 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. Mucous 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 mucous 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
bowel rog on whatever's trying to make its way in. 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 a peanut, and your 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 and the redness and the mucous production and the itching and the
occasionally a little bit of death, so that is why we take
antihistamines to suppress the histamine triggers
so our immune system stops freakin' 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 the 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 its
cells and then it oozes through that gap at the
site of the infection. This is called, get ready for it, diapedesis, from the
Greek for oozing through. There are lots 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 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 body guards 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 grizzly 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. (cheerful music) 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. Look around at today's script with all of its dendritic cells and macrophages and diapedesises, and I think what if we let
1973 name all these things, would we have spikey death
cells and devourerers and oozing action instead? I don't know. Maybe you would have screwed it up, but I don't think we
could have done any worse than all this GD Greek we have
to deal with all the time. Thanks for the Endangered Species Act, Hank. Okay, 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-1, MHC for major histocompatibility 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 its AK-47 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 the
lymph nodes where it passes intelligence about what's
going on in the war front to the acquired immune system. I actually studied dendritic cells in my undergraduate thesis, and I kind of fell in love with them. They're lethal but
they're also intelligent. Great heroes for any Robert Lublin 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
defense 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 telltale 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. 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 sort of 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 dinnertime. You're 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 specific things that they already know about it. There are two major types of lymphocytes, the T cells which form in your bone marrow and then migrate and
mature in the thymus gland right behind your breast bone, and the B cells which originate and mature in the bone marrow. What T and B actually
stand for is a long story, but if it helps you to remember, Ts mature in the thymus,
Bs in the bone marrow. We have two different types of lymphocytes because our bodies have
two different types of acquired immunity, the
cell-mediated response, which is for when the
cells are already infected and the humoral response
for when the infection is just in the humors, the
body's fluid, not in the cells. First let's look at the
cell-mediated response. This process mainly involves
T cells and there are quite a number of different types of them. Helper T cells have a cute sounding name but in a lot of ways, they call the shots for the whole immune system. While they can't kill
pathogens themselves, they activate and direct
the cells that can. If 1973 had named them,
they might have been called admiral T cells or something more awesome. Helper T cells get their
information from other immune cells that are out crackin'
skulls, say for instance, a macrophage finds a
pathogen and destroys it. After the deed has been
done, it has the ability to shred up the proteins from an invader and put a bit of that antigen
on its membrane surface. This is called antigen-presentation because the cell is presenting antigens. A helper T cell can
detect when this happens, and it comes over to attach
itself to the presented antigen. The two cells talk to
each other chemically. The antigen presenting
cell produces a chemical called interleukin 1 which
basically tells the helper T cell boss, I found this guy over
here and then I broke his neck and then he stuck his guts
all over my cell wall. The helper T cell gives it a look and then releases a chemical
called interleukin 2 which is like a bull horn,
an alarm that tells all the lymphocytes in the are
there are problems here, we've got a problem over
here, it's sector 50. This alarm activates a couple
different things all at once. First, the helper T cell
starts making copies, tons of copies of itself. Most of those copies differentiate
into effector T cells which travel around
secreting signaling proteins that stimulate other nearby
lymphocytes to take action. Most of the rest of them
become memory T cells. They're the ones that keep
a record of the intruder and provide us with future
immunity against it. And now for the saddest story of the day. What happens when a cell gets infected, so infected that it
knows that it's a goner, that it in fact is being converted from a healthy useful part of the body to an evil zombie farm pumping out viruses or bacteria suddenly co-opted to help destroy everything it loves? Well, with its last bit of strength, it'll start presenting antigens, not asking to be rescued
but instead asking for a mercy killing. A cytotoxic T cell has the
job of granting that request. Once a cytotoxic T cell gets the message from the helper T cells
that there is an infection to deal with, it starts
patrolling the area for any normal cells presenting antigens. When it finds one, it latches onto it and releases enzymes that create holes in the cell's membrane
and eventually breaks down the whole cell killing the cell and the pathogen in the process. A human cell killing another human cell. And now, for the humoral response. The humoral response is
designed to catch pathogens that are floating around in your body that haven't actually invaded
any of your cells yet. The primary players are B cells which are constantly
patrolling your blood stream like cops walkin' the beat
until they get a signal from a helper T cell
that something's wrong. B cells are covered in
antibodies that can detect and bind to a specific antigen. A single B cell can be covered in a forest of up to 100 thousand antibodies, say for the virus that
causes the common cold, and the B cell next to it will
have just as many receptors for a different antigen, for
chicken pox or something. When a B cell bumps into a
pathogen that it recognizes, it attaches to it and starts
cloning itself like crazy. Suddenly there are tons of that B cell with the same receptor, but
during the cloning process, the clones differentiate into new versions of the original just like the T cells did. Most turn into plasma or effector cells which use the antibody as a blueprint to create a crap ton of antibodies for that specific pathogen,
200 antibodies per second. Once these antibodies are released, they bind to the pathogens like crazy marking them for death until the phagocyte can come along and the dirty work. The rest of the cloned B cells
mostly become memory cells which have the same
receptor and stick around providing future immunity
from this invader. And we are now very out of time
but I really love this stuff so I didn't want to gloss over anything. Mucous, natural killer cells, macrophages, killing things, breaking
them up and sticking them on their cell membranes,
effector cells spewing out antibodies and memory cells making sure that our immune systems hold that grudge all because my absolute
favorite thing to do every single day is not die.