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  • What’s true in World of Warcraft is also true in your immune system:

  • To defeat your enemy, you have to know your enemy.

  • Uncover its weaknesses. Learn how to see it, before it sees you.

  • Weve already talked about how your innate defense system keeps out, or quietly neutralizes,

  • pathogens without much too much fuss. But sooner or later, a threat’s gonna come along

  • that’s stronger than what the first-responders can handle. That’s when it’s time for

  • the adaptive, or acquired immune system to step in.

  • While your innate system takes its zero-tolerance policy very seriously, and tries to toast

  • any foreign microbe that it encounters, your adaptive system does things differently.

  • It has to be expressly introduced to a specific pathogen, and recognize it as a threat, before it will attack.

  • As its name suggests, youre not born with a working adaptive immune system -- it’s

  • slow to act, in part because it takes time for it to shake hands with so many pathogens

  • and get to know them.

  • These introductions may be organic -- like touching a dirty faucet in the bathroom or

  • walking into a sneeze cloud.

  • Or they may be premeditated, which is why vaccination is pretty much the greatest thing

  • to happen to medicine ever.

  • But once it’s been introduced to a potential threat, your adaptive defenses never forget

  • it. And this ability to remember specific pathogens is one of the key differences between

  • the adaptive and innate defenses.

  • Another main difference is that adaptive immunity is systemic -- rather than being restricted

  • to a particular infection in, say, a sinus or a sliced finger, your adaptive system can

  • fight throughout your whole body at once.

  • And it does this by deploying one or both of its separate, but cooperating, defenses

  • -- your humoral immunity and your cellular defenses.

  • Your humoral immunity -- which you might not have heard of before -- works by dispatching

  • important proteins that I’m sure you have heard of: antibodies.

  • Theyre made by special white blood cells, and they patrol the body’s “humors

  • or fluids like blood and lymph, where they combat viruses and bacteria moving around

  • the interstitial space between your cells.

  • Much of what you know, or have heard about, or think of, when your immune system comes

  • up actually has to do with your humoral immunity.

  • It’s why, if you had mumps as a kid, you probably don’t have to worry about getting

  • it again for the rest of your life.

  • It’s also why doctors and nurses and patients who have been infected with the ebola virus

  • -- a disease once thought to be incurable -- have lived to tell about it.

  • And it’s why vaccinations work.

  • Whether youre protecting yourself from infections or playing an MMO, one of the first

  • steps in any good defensive strategy is to be able to tell your friend from your foe.

  • And in the case of your immune system, that means being able to identify antigens.

  • An antigen could be an invader from the outside world, like a bacterium, virus, or fungus.

  • Or it could be a toxin or a diseased cell within your own body.

  • But in any case, antigens are large signalling molecules not normally found in the body,

  • and they act as flags that get the adaptive immune system riled up.

  • So let’s say a flu virus gets inside of you, and it’s floating around trying to

  • find a good host cell to start multiplying inside of.

  • Before it finds that cell, hopefully it will be paid a visit by one of the stars of your

  • humoral response -- a B lymphocyte.

  • Like all blood cells, these guys originate in your bone marrow. But unlike other white

  • blood cells, they also mature in the bone marrow too.

  • And as a B cell matures, it develops the ability to determine friend from foe, developing both

  • immunocompetence -- or how to recognize and bind to a particular antigen -- as well as

  • self-tolerance, or knowing how to NOT attack your body’s own cells.

  • Once it’s fully mature, a B lymphocyte displays at least 10,000 special protein receptors

  • on its surface -- these are its membrane-bound antibodies.

  • All B lymphocytes have them, but the cool thing is, every individual lymphocyte has

  • its own unique antibodies, each of which is ready to identify and bind to a particular kind of antigen.

  • That means that, with all of your B lymphocytes together, it’s like having 2 billion keys

  • on your immune system’s keychain, each of which can only open one door.

  • So, part of your immune system’s strategy is just to win with overwhelming odds: The

  • more unique antibodies your lymphocytes have, the more likely it is that one will eventually

  • find, bind to, and mark a particular antigen.

  • Once theyve matured, B cells colonize orseedyour secondary lymphoid organs,

  • like your lymph nodes, and start roaming around in your blood and lymph.

  • At this point theyre still naive and untested, and they won’t truly be activated until

  • they meet their perfect enemy match.

  • Which brings us back to the flu virus.

  • When the right B cell finally bumps into an antigen it has antibodies for -- usually in

  • a lymph node or in the spleen -- and recognizes it, it binds to it. This summons the full

  • power of the humoral immune response, and the cell basically goes into berserker mode.

  • Once activated, the B cell starts cloning itself like crazy, quickly producing an army

  • of similar cells, all with the instructions for the exact same antibodies that are designed

  • to fight that one particular antigen.

  • Most of these clones become active fighters, or effector cells. But a few become long-lived

  • memory cells that preserve the genetic code for that specific, successful antibody.

  • This ensures that, if and when the antigen returns, there will be a prepared secondary

  • immune response that’s both stronger and faster than the first.

  • This is key to why vaccinations are so brilliant and important, which I’ll come back to in a minute.

  • But while the memory cells are just there to hang back and record things, the effector,

  • or plasma cells, are packed with extra amounts of rough endoplasmic reticulum, which acts

  • as an antibody factory.

  • These cells can mass-produce the same antibodies over and over for that particular invader,

  • spitting them out into the humor at a rate of around 2,000 antibodies per second for

  • four or five days until they die.

  • And the antibodies they make work the same way that the membrane-bound ones do; theyre just free-floating.

  • So they ride the tides of blood and lymph, binding to all the antigens they can find,

  • and marking them for death.

  • Now, antibodies can’t really do the killing themselves, but they do have a few moves that

  • could make it hard for intruders to take hold.

  • One of their most effective and common strategies is neutralization, where antibodies physically

  • block the binding sites on viruses or bacterial toxins, so they can’t hook up to your tissues.

  • And because antibodies have more than one binding site, they can bind to multiple antigens

  • at the same time, in a process called agglutination.

  • The resulting clumps can’t get around easily, which makes it easier for macrophages

  • to come and gobble them up.

  • And not only that, but while all this is going on, antibodies are also ringing a chemical

  • dinner bell, calling in phagocytes from the innate immune system, and special lymphocytes

  • from the adaptive system, to destroy these messy little antigen-antibody clumps.

  • So, the point of all this in the short term is to keep you healthy. But in the long term,

  • this process also adds to your overall immunity.

  • The humoral response allows your body to achieve immunity by encountering pathogens either

  • randomly or on purpose.

  • Active humoral immunity is what we were just talking about -- it’s when B cells bump

  • into antigens and start cranking out antibodies.

  • This can occur naturally, like when you catch the flu or get chickenpox or pick up some

  • nasty bacterial infection, or it can happen artificially -- particularly through vaccination.

  • Most vaccines are made of a dead or extremely weakened pathogen. And they work on the premise

  • that, because a secondary immune response is more intense than a primary response, by

  • introducing a pathogen into your body, youre priming it to fight hard and fast should that

  • antigen show up again.

  • In the case of typically non-fatal infections, like the common flu, this immunity should

  • at least spare you from some of the most severe symptoms.

  • But in the case of more serious diseases, like polio, smallpox, measles, and whooping

  • cough, vaccinations can be truly life-saving.

  • Now, some antigens -- like those for mumps or measles -- don’t really change much over

  • time, so a few immunizations will leave you set for life.

  • But others, like influenza, are constantly evolving and changing their surface antigens.

  • So immunity to last year’s flu probably doesn’t work against this year’s flu.

  • Still, acquired immunity doesn’t have to be active.

  • Babies, for example, naturally obtain passive humoral immunity while still in the womb.

  • They receive readymade antibodies from their mothers through the placenta, and later on

  • through breast milk.

  • And that works pretty well for a few months, but the protection is temporary, because passively

  • obtained antibodies don’t live long in their new body. And they can’t produce effector

  • cells or memory cells, so a baby’s own system won’t remember an antigen if it gets infected again.

  • You can also acquire this kind of temporary passive immunity artificially, by receiving

  • exogenous antibodies from the plasma of an immune donor.

  • This is what recently saved some doctors and nurses who had contracted the ebola virus

  • from infected patients.

  • A serum was made from the blood plasma of other medical workers who had been infected,

  • and survived.

  • The antibodies helped defend the patients from the virus before their own active immunity

  • could identify that particular antigen and start creating their own antibodies.

  • It’s not the same as a vaccine, which immediately engages your B cells, but it can buy a patient

  • some crucial, life-saving time against an infection that would otherwise quickly kill.

  • But B cells and antibodies are only part of the immunity equation. There are plenty of

  • pathogens that quickly worm their way right inside your cells, where theyre safer from

  • the humoral response and free to multiply as much as they’d like.

  • Luckily, your immune system has yet another game plan and new set of players ready to

  • fight that final battle with cell to cell combat.

  • Make sure you catch our final episode next week and learn all about this epic battle royale.

  • But as for today, in our second-to-last episode, you learned how the adaptive immune system’s

  • humoral response guards your extracellular terrain against pathogens. We looked at how

  • B cells mature, identify antigens, and make antibodies, and how antibodies swarm pathogens

  • and mark them for death. We also talked about active and passive humoral immunity, and how

  • vaccines work.

  • Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon

  • patrons. If you are one of those people I just thanked, you make Crash Course possible,

  • for the whole world and also for yourself. If you like Crash Course and you want to help

  • us make videos like this one, you can go to patreon.com/crashcourse.

  • 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. 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.

What’s true in World of Warcraft is also true in your immune system:

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免疫系パート2:クラッシュコースA&P #46 (Immune System, part 2: Crash Course A&P #46)

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    羅紹桀 に公開 2021 年 01 月 14 日
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