Your Immune System: Natural Born Killer – Crash Course Biology #32

By Adem Lewis / in , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , /

Sex and not dying. 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 specific things that
they already know about. There are two major
types of lymphocytes: the T cells, which form
in your bone marrow and migrate and mature
in the thymus gland, behind your breastbone,
and 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: T’s mature in the thymus,
B’s 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 cracking 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, “Uh, boss, I found this guy over
here and then I broke his neck and then I stuck his guts
all over my cell wall.” And the Helper T cell gives it a
look and then releases a chemical called Interleukin 2,
which is like a bullhorn, an alarm that tells all the
HERE IN SECTOR 15!” This alarm activates a couple of
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. The 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’s an
infection to deal with, it starts patrolling the area for
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. 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 bloodstream like cops walking 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 100,000 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 creating a crap-ton of antibodies for
that specific pathogen like 200 antibodies per second! Once these antibodies are released,
they bind to the pathogens like crazy, marking them
for death until a phagocyte can come along and
do 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. Mucus, 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. If you want to review anything
we discussed in this episode there’s a table of
contents over there. If you have any
questions for us, we’ll be down in the comments
or on Facebook or Twitter. And we’ll see you next time.

100 thoughts on “Your Immune System: Natural Born Killer – Crash Course Biology #32

  1. I just wanted to say without these videos (I feel like I have almost watched them all) my A & P class would be impossible! You do such a great job explaining the human body in a way everyone can understand and they are always a great kickstarter for new material and clarify anything I don’t understand. Thank you!!

  2. Thank you very much the Crash Course team for making such wonderful videos which helps us in understanding our topics so well.

  3. My body hosts an autonomous microscopic defensive swarm that will do anything to protect me. I have no ability to restrain it and I don't know my own power. My swarm of autonomous killer cells literally can't be reasoned with

  4. This video was SO helpful. Everything seemed theoretical until I saw this, took notes, and really understood the animations…thank you soo so much

  5. AHH Hank made a mistake oh no
    He said "stuck its guts all over my cell WALL"
    watch the video for context otherwise I realize that sounds a little weird…

  6. Best Crash Course video I've seen. Wasn't gone through so fast. I enjoy all of the CC videos, but most of them are too fast for me to absorb.

  7. This was explained so well! The analogies really helped me remember the material I'm learning. These videos are a blessing. Thank you!!

  8. "Uhm boss, I found this guy over here and then I broke his neck and then I stuck his guts all over my cell wall" Still laughing, gets me every time! Absolutely love it

  9. way to go Hank, you stay talking about viruses and you made my anxiety over stomach viruses go up.

    okay i’m not trying to be mean but my anxiety really did go nuts owo

  10. To let you there is a mistake in the dutch subtitles, the subtitles calls antigens and antibodies by the same name which is not the case, antibodies should be translated with "antistoffen"

  11. This guy literally is reading his book, he is so fast that I cant process any of the information he is giving us .. too fast .. he is not explaining nothing either he doesn't make it easier for us

  12. I just finished GCSEs (physics paper 2) and I realised I failed science, so I’m here to get a head start on a levels

  13. Thank you for this video! I just read a chapter on the immune system and the book was rather confusing explaining all the different types of immune cells, but the way you phrase it helped me understand immensely! You make a good teacher!

  14. Not a big deal but there was a typo (only 1 that I could see) – major histocompatibility (not ability – 'i' instead of 'a'). Otherwise this video is amazingly perfect! Love the entire series!

  15. Also, petition to replace greek names? Anglish for science? And a video-game developer gets to name them. Or name them after roles in a police precinct.

  16. ahem, for an asexual i find that first comment unnecessary and very atypical for human behavior that i don't wanna hear again

  17. I started this video with full volume thinking it would make my mom proud instead she burst in asked what i was watching

  18. End of Season 1: The terrorist group of the phagocytes were successfully eliminated, the only thing remaining the memory cells. It is the funeral of the lymphocytes that died honorably in battle.
    "Sir, you have to take a look at this"
    "Can't you see I am busy-" "Sir, we have a more dangerous terrorist group that is infiltrating our headquarters"
    "The headquarters? Don't tell me- The DNA of the special ops; the lymphocytes?"
    "Yes, the figured out our messaging system, the RNA, they are using it to stop the production of Antibodies"
    "This lowers the amount of lymphocytes we have-", "Yes"
    "This could lead to the destruction of our whole organisation, the IS- Immune System, this means we could face our ghost protocol, What is the name of the terrorist group?"
    Continued in Season 2

  19. I suggest watching the anime Cells at Work after this. It's really helpful and a fun way of learning biology btw!

  20. On behalf of the British biology cohort of 2019 I’d like to Thankyou for saving our ALevels ✊🏻

  21. u have guided me through high school and idk where i would have been without those videos so thanks to you, hank 💙

  22. The open letter and tHE NATURAL KILLER CELL SHSHSHSH that was so funny!

    Anyway, I love this channel so much I'm able to learn new things while having fun! 💕

  23. My education and passion is finance and this was my first introduction to my immune system.

    Damn that's cool.

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