Immune System, Part 1: Crash Course A&P #45

You may not know it, but your body is engaged
in a never-ending battle. You are literally covered in staph and strep
and e coli, and all sorts of dubious characters that are intent on using you, and your body’s
many resources, to feed themselves, find shelter, and reproduce as much as they want. And, hey, we all gotta make a living. But
it is not your job to give these guys a free lunch. So your body has developed a three-part policy
toward these shady customers, and its enforcement is handled by your immune system. The immune system is different from all the
other systems we’ve talked about this year in that it’s not a specific, tissue-organ-system
kind of system. Instead, it involves a whole bunch of
different tissue groups, organ systems, and specialized-but-widely-distributed defense cells. Together, this league of extraordinary substances
joins forces to perform all of the defense functions your body depends on to keep you
alive in an incredibly germy world. And the first line of defense in this never-ending battle?
That’s your innate, or nonspecific, defense system. Like your average frontline soldier, it’s
prepared to immediately engage with anyone suspicious, and it mostly includes stuff we
were born with, like the external barricades of your skin and mucous membranes, and internal
defenses like phagocytes, antimicrobial proteins, and other attack cells. But some enemies must be fought with special
forces. And here, your body can deploy your adaptive, or specific defense system, which
is more like your Seal Team Six. It takes more time to call in, but it’s
specially designed to go after specific targets. And it keeps files on those bad guys so it
knows how to handle them next time around. But today we’re going to focus on your innate
system, and look at how it uses an arsenal of physical and chemical barriers, killer
cells, and even fever, to keep you healthy. Proving that sometimes, the symptoms we associate
with illness are actually the signs that we’re healing. Just because something is simple doesn’t
mean that it can’t be elegant. I mean, your body is capable of some
incredibly sophisticated things, including defending itself from infection. But occasionally there’s something to be
said for brute force. And a lot of your innate immune system’s
functions aren’t exactly subtle. For example, your body’s very first line of defense is
a simple physical barrier. And it works! Like a wall around a fortress, your skin does
a fantastic job of keeping out all manner of malevolent microorganisms. As long as that tough, keratinized epithelial
membrane doesn’t get torn open or busted up too much, you could probably, like, make
snowballs out of raw sewage and still be alright. Although…no. No. Your many mucous membranes also provide a
handy physical barrier. You’ll remember that they line any cavity that opens up into
the germy outside world, including the respiratory, digestive, urinary, and reproductive tracts. Not only do your skin and mucosa
supply simple physical protection, they also pack some serious chemical weaponry. Eat some questionable leftovers for lunch?
Don’t worry, your stomach is literally filled with acid, so you probably are covered. Walk face-first into your co-worker’s nasty
sneeze cloud? No worries, your nasal passages can whip up a tissue-box worth of sticky mucus
to help trap viruses before they enter your lungs. You’ve also got bacteria-fighting enzymes
in your saliva and lacrimal eye fluid, and peptides called defensins in your skin and
membranes that help keep bacteria and fungi from setting up shop around inflamed or scraped
skin. Which, no matter how careful you are, you’re
gonna get, one way or another. Maybe you shave with a dull blade. Or you
just brush your teeth too hard. And DON’T GET ME STARTED about the dangers of bagel-cutting. So when you’ve breached that first, simple
line of defense, it’s time to call on your second line of internal innate defenses. This is where your body starts pulling strategic
maneuvers like firing up a fever, releasing chemical signals, causing inflammation, or
other defensive tactics that help identify and attack infectious invaders. Some of the first defensive cells on the scene
are your phagocytes. Their name literally means “to eat,” and like Pac-Man, they
indiscriminately chase down intruders and gobble them up. And they come in a few different
varieties: First you’ve got neutrophils, which are
the most abundant type of your white blood cells. They kind of self-destruct after devouring
a pathogen. And, in fact, you’ve actually seen piles of their little dead bodies, because
that’s what pus is made of. But the bigger, tougher phagocytes are the
macrophages. They’re derived from monocyte white-blood cells that have moved out of the
blood stream to occupy tissues. And some are free types that patrol tissues looking for
creepers, while others are fixed — attached to fibers in specific organs, devouring anything
suspicious that passes by. So when a macrophage in, say, the finger I
just cut slicing a bagel, sees a new bacterium coming along, it snares it using cytoplasmic
extensions, reels in it, completely engulfs it, and — essentially — digests it and spits
the rest out. And unlike neutrophils, it can do this over
and over again, like a boss. But not all your defense cells are phagocytic.
You’ve also got cells with what is by far the awesomest name of any cell in the body:
the natural killer cells. You can call them NK cells if you want to, but like, why would
you do that? Anyway, these tiny assassins patrol your blood
and lymph looking for abnormal cells, and are unique in that they can kill your own
cells if they are infected with viruses or have become cancerous. How can they tell? A normal, healthy cell contains a special
protein on its surface called MHC1, or major Histocompatibility Complex. But if it’s
infected, it stops making that protein. And if an NK cell detects a defective cell,
it doesn’t swallow it whole like a macrophage — it pokes it with an enzyme that triggers
apoptosis, or programmed cell death, which is pretty awesome. So those are some ways your innate immune
cells handle their enemies, but how do they know where to look in the first place? So, let’s talk strategery. So, say you’re in a banana factory and you
slip on a banana peel and scrape your knees. Your outer fortress has been breached, and
the pathogens are just flooding in like orcs through Helm’s Deep. Banana factories are very dirty places. Now your body wants to contain the spread
of pathogens, clean up the mess, and get healing as quickly as possible, so it cues up your
inflammatory response. This is basically an internal fire alarm,
only it uses chemicals instead of sirens to get the message across, and instead of smoke
and fire you sense redness, swelling, heat, and pain. For example, in the event of injury, specialized
mast cells in your connective tissue send out histamine molecules. And histamine is great at calling in the cavalry. For one thing, it causes vasodilation, which
creates redness and heat at the site of the injury. Now, those things might freak you
out a little, but they’re actually signs of healing — the increased temperature, for
example, ratchets up the cells’ metabolic rates so they can repair themselves faster. Meanwhile, histamines and other inflammatory
chemicals also increase the permeability of blood vessels, causing nearby capillaries
to release protein-rich fluids. This causes swelling — which again, is actually
a good thing — because that leaked protein helps clot blood and form scabs, while the
lymphatic system sucks up and filters that extra fluid, cleaning it up before putting
it back into your bloodstream. And of course, like chum to sharks, an inflamed
knee is also going to attract a bunch of local phagocytes — which find it easier to escape
your now-leaky capillaries — and lymphocytes that are also flowing freely, helping to destroy
pathogens while also cleaning up dead-cell wreckage. And don’t forget: During all this, the neutrophils
have been doing their best, but they were the first wave to arrive, so by this time,
they’re starting to die in heaps. They’re triggered when the injured knee-skin
cells release chemicals that begin leukocytosis – the release of neutrophils from the bone
marrow where they’re made into the bloodstream. To attract the neutrophils to the damaged
area, inflamed endothelial cells in the capillaries send out chemicals that act like homing devices–
and when the neutrophils arrive, they cling to the capillary walls near the injury, flatten
themselves out and squeeze through the vessel walls to get to work. Your big monocytes eventually roll up to the
battle, and transform into hungry macrophages, replacing that first line of now-dead neutrophils
and basically just eating up any lingering enemies and then cleaning up the carnage. Now, all this works pretty well in most circumstances.
But you may have noticed if you’ve sustained a more major injury, or are battling an especially
nasty virus or infection, that sometimes your local troops get overrun. When white blood cells and macrophages run
into more foreign invaders than they can handle, they let loose pyrogen chemicals that tap
the hypothalamus and raise your body’s thermostat, calling in a systemic fever to burn everything. The resulting temperature rise increases the
metabolism of your cells so they can heal faster, and it also tells the liver and spleen
to hold onto all of their iron and zinc, so those things can’t contribute to bacterial
growth. But even then, sometimes, well sometimes you
find yourself facing a more formidable foe. That’s when you call in the specialists
— your adaptive immune defenses. And to learn exactly how they save the day,
you have to watch next time. But for now you learned that your immune system’s
responses begin with physical barriers like skin and mucous membranes, and when they’re
not enough, there are your phagocytes — the neutrophils and macrophages. You also learned
about natural killer cells and the inflammatory response, and watched as all of these elements
saved the day when you slipped on a banana peel. Thank you to our Headmaster of Learning, Linnea
Boyev, and thank you to all of our Patreon patrons whose monthly contributions make Crash
Course possible, not only for themselves, but for everybody. If you like Crash Course
and you want to help us keep making videos like this, you can go to This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio, it was written by Kathleen Yale, the script was edited by
Blake de Pastino, and our consultant is Dr. Brandon Jackson. It was directed by Nicholas
Jenkins, edited by Nicole Sweeney, our sound designer is Michael Aranda, and the Graphics
team is Thought Cafe.

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77 Responses

  1. Kris says:

    I have a really strong immune system, but believe it or not I really think it's annoying.. when I get a cold like literally any normal person, it would just go away in a few hours, so I never gets days off of school because it goes away so fast.

    Yea.. I may have an immune system that instantly removes diseases, but still.. no days off really sucks.

  2. onni hh says:

    Oi im gonna make u OPS dash faster than flash or imma ching you up.
    Dont dirsespect Nottingham Hill

  3. Gamerlibs says:

    I was watching this in class and screamed when I saw the Master Sword 0:22

  4. LaShanna Braswell says:

    So entertaining

  5. Brandon Blaze says:

    7:35 is that what I think it is?

  6. music eralicia says:


  7. Viv a.k.a. Raymond says:

    This was so cool. Haha.
    You guys should put these up on Netflix.

  8. maria joana baptista barbosa says:

    good for study education

  9. Preston Choong says:

    Who came here from cells at work ?

  10. KT LoveBTS says:

    7:20 for a sec I thought I was watching at a higher speed

  11. Azariah SHYIRAMBERE says:

    Is an MK cell a ctoxic T cell?

  12. theMata94 says:

    I love the animations. Pac man, how funny!

  13. Paloni Koticha says:

    Just loved your animations sir.. truely helpful

  14. Meriem Ouchene says:

    good job ! but , vitesse de lecteure 0.75 don't speak fast please so i can understand you

  15. THE Vicman says:


  16. jean-paul fornes says:

    At minute 4:58 its not exactly true, MHC1 is presented on the cell that are infected by a virus, cancel cells etc, its not true that all the cell have MHC1, but whatever, i love your videos

  17. Coltafanan Studios says:

    how come when the central nervous system (such as the brain) when it gets inflamed, it's deadly? If the brain swells up such as meningitis, the histamine reaction can kill you which is the opposite of what your body should be doing

  18. Saumya gg says:

    You are awesome!

  19. Alexandra Eukers says:

    U talk way too fast.

  20. Anna Booher says:

    “The pathogens are flooding in like orcs from helms deep” that had me dying ?

  21. MyChilepepper says:

    The immune system is hijacked if you have bullous pemphigoid.

  22. The Daily Digestion says:

    Great Video!!!!

  23. Namrata Jha says:

    Nick system : i am creating a group
    Tony bone marrow : what group
    Nick system: * avengers theme in background* immune system
    avengers theme

  24. Alice V. says:

    those white cells are SO CUTE

  25. curly_joni says:

    To anyone watching these… try watching the anime Cells at Work (it also have a manga), that helps too and it's another entertaining way to learn.

  26. Orbital Potato says:

    5:05 One Punch Cell

  27. Andrew P says:

    Dude… Yur awesome, adding humor as well to help emotional retention. Great stuff.

  28. Tiki MVP Health and Beauty says:

    I luv how he says, “proving that sometimes the symptoms we sometimes associate with illness is actually a sign of healing” ?? ?? simple and elegant ?

  29. Ibn Alam Porosh says:

    best animation award

  30. sol10 says:

    Goddamn we're all amazing!

  31. Baj Sisters says:


  32. metushelach8 says:

    The Neutrophils have various ways to kill bacteria, not just phagocytosis. They can "throw" Neutrophil extracellular traps or NETS (which is kinda the spiderman version of catching bacteria) or through the process of Degranulation (Basically "shooting" chemicals at the bacteria). They are also very important in performing Chemotaxis and have a vital role in activating and "recruiting" other neutrphils and white blood cells to the area of infection. In addition, the argument that once they performed phagocytosis they die is simply misleading. The facts of the matter is The ability of microbes to alter the fate of neutrophils is highly varied, can be microbe-specific, and ranges from prolonging the neutrophil lifespan to causing rapid neutrophil lysis after phagocytosis. In Wikipedia there is a video depicting a Neutrophil perform phagocytosis on several Condida bacteria (I counted around 10) over a period of 2 hours. They usually live 1 – 2 days in the tissues. Imagine how much they can do. You totally glossed over one of most the important white blood cell in the body.

  33. Sara McGaha says:

    My good lord you are awesome!!!! Thanks so much for your vids!!!!!

  34. Mahum Habeeb says:

    Please do microbiology!!!!!!

  35. Nicole Lynn says:

    "Banana factories are very dirty places" LMAO

  36. MY HELPER says:

    Anyone else rewinds the video for listening to his pronounciation of certain words. They sound like music. Anyways these lectures can probably help me pass my exam. Thanks?

  37. Goobistash tfm says:

    what about the complement system? (great vid)

  38. yutbacount says:

    liked the LOTR reference!

  39. Jaime Kelleher says:

    Why have these adorable little cells not been made in to stuffed animals?! I'd buy that CrashCourse merch! <3 SOOO cute!

  40. Dorothy Isidro says:

    They protecc
    They atacc
    But most importantly
    We want our body bacc

  41. Mutinta Kanyimba says:

    crash course makes everything sooo cute to studyy

  42. Mitch DeLaRosa says:

    i like the immune system 😀

  43. frosty wall says:


  44. saskia kristofedes says:

    After microbial killing who cleans the residual body ??? Ty

  45. Anna Pitkevich says:

    I like all your videos, but will you be kind to talk slower please? I don't mind to watch your videos even if they'll be 5 minutes longer

  46. Kim Wallis says:

    I can't explain just how much your videos are helping me! I'm also 62 but just getting into this education lark (I hated school when I was a kid, now I'm a big kid I don't think I'll ever really grow up) If only this sort of thing was available then! I am training yes at a late stage to be a reflexologist and need to understand anatomy and physiology you have made this all possible as I was struggling just reading books. Thank you, thank you, thank you.

  47. Dhruv Veda says:

    I love crashcourse

  48. Guy Unger says:

    You guys are awesome!!! You truly make learning fun!! And Hank you present info “Like a boss”!!!

  49. John Heath says:

    What’s with the random Erlenmeyer flask with the pampers nappy juice in it on the bench?

  50. Minati Lodh says:

    I understood that

  51. Divya Mahesh says:

    Hank explains things so well.

  52. publick says:

    I completely lost it at pokemon hahahahaha

  53. Pam says:

    They are so cute ❤lol

  54. EvilBees says:

    The animator/s really outdid themselves on this episode, I love it to bits! (No pun intended)

  55. Ahamed Ahamed says:

    U guys makes best ever vedios

  56. E Dab says:

    What i imagined: EL HERMANO VS GOHAN BLANCO
    What actually is: LUNCH

    What i thought about the immune system

  57. E Dab says:

    BTW after i got a wound i put saliva in it

  58. E Dab says:


  59. Dan Etienne says:

    Im sick ?

  60. Christyann Riggs says:

    they need to make t-shirts or plushies with these immune cells on them! so cute!

  61. watchgoose says:

    my own immune system was sent into overdrive when brown recluse venom entered an artery. I FELT it moving from my foot back to my heart. since then, I have become allergic to many things I wasn't before.

  62. Dorothy Isidro says:

    WW 8,000,000,000,000
    Is happening right now

  63. kabir jalota says:

    See, I do own Captain America's shield.

  64. Nicole Malmberg says:

    This was awesome and so entertaining compared to my boring pp't notes. I also think NKC is an awesome name.

  65. Alexis Veal says:

    Exam is tomorrow, like this for good luck ?

  66. Jeremy Cornwell says:

    When over clocked because of disease, you feel good at first.

  67. llifeform says:

    Haha.. this was fun 🙂

  68. Jordan Laliotis says:

    the moving molecule on the left of the screen is very distracting

  69. Hannah C says:

    Is anyone else here because they're sick?

  70. Ramy Al Farhat says:

    Watching this 1 day before the test

  71. a ahir says:

    so should stop taking paracetamol for fever it's killing virus

  72. Theresa Geiger says:

    Lier 80 percent is in your gut

  73. Shanna B says:

    The animation in this video is GOLD!!!

  74. B J says:

    Too much info at once. Wow.

  75. Greta Garbo says:

    crash course won't be needed now theres an anime with big tiddie macrophages and hot neutrophils

  76. Ellen Cain says:

    thank you for doing such quick and effective videos! love watching you guys before starting a chapter just to get a quick overview before digging deeper!!! 20/10 recommend!

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