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  • The human immune system is the most complex biological system we know, after the human brain,

  • and yet most of us never learn how it works.

  • Or, what it is.

  • Your immune system consists of hundreds of tinyand two largeorgans.

  • It has its own transport network spread throughout your body.

  • Every day, it makes hundreds of billions of fresh cells organized like an army.

  • With soldiers, captains, intelligence officers, heavy weapons, and crazy suicide bombers.

  • It's not some sort of abstract entity.

  • Your immune system is you.

  • Your biology protecting you from the billions of microorganisms that want to consume you,

  • and from your own perverted cells that turn into cancer.

  • It's so manifold that it's impossible to cover in one video,

  • so we'll make a series looking at different aspects of it.

  • Today: What happens when your body is invaded

  • and your first lines of defenses are engaged in a fight for life and death.

  • It's been a normal day,

  • when suddenly the world explodes and an asteroid rips the sky open.

  • Countless alien life forms invade,

  • ready to destroy cities and infrastructure, and eat civilians...

  • Or, this is what your cells experience.

  • You look at your bleeding thumb that you just cut on a dirty twig in the park.

  • How annoying!

  • But inside the wound, a horrible catastrophe has happened.

  • There are dead cells, and blood and dirt everywhere.

  • Even worse, countless bacteria invade the warm caverns between your helpless cells

  • to explore their new home, steal your resources, and poop everywhere.

  • Immediately, the first stage of your defense kicks in.

  • The cells that survive the impact, or are hurt or dying, scream in panic

  • releasing an onslaught of chemical alarm signals that awaken your immune system.

  • The first cells to show up are macrophages.

  • If an average cell were the size of a human, a macrophage would be the size of a black rhino.

  • A stoic cell in principle but you wouldn't want to annoy it.

  • Bacteria -do- annoy them.

  • Within seconds, the large cells attack and begin killing them without mercy.

  • They stretch out parts like the arms of an octopus,

  • and grab the bacteria to swallow them whole and digest them alive.

  • A macrophage can eat 100 bacteria before it's exhausted.

  • But there are too many enemies, so the macrophages call for reinforcements.

  • In your blood, hundreds of thousands of neutrophils pick up their signals and move to the battlefield.

  • Neutrophils are intense suicide warriors that only live to kill.

  • They're so enthusiastic about killing that they kill themselves a few days after birth,

  • so they don't have time to accidentally destroy your body from the inside.

  • As soon as neutrophils arrive, they begin vomiting deadly chemicals at bacteria, or devour them.

  • They are so careless in their attacks that they are causing real damage to your own cells.

  • But collateral damage is not their concern now,

  • or ever.

  • Some neutrophils go so far to push their suicide button and explode,

  • casting a wide and toxic net made from their own DNA filled with deadly chemicals that trap and kill bacteria.

  • Sometimes, they can continue fighting after that, even though they're sort of dead already.

  • This is how much fun they have killing!

  • While the battle rages, your blood vessels let fluid stream into the battlefield like a dam opening up towards a valley.

  • You notice this as inflammation.

  • Your thumb swells up a little and gets red and warm.

  • The fluid brings a silent killer into the battle zone, millions of complement proteins.

  • A sort of automated liquid weapon that stuns and kills bacteria by ripping holes into them.

  • We made a whole video explaining them in detail.

  • We are reaching a crossroad now.

  • If things go well, your first line of defense kills the invaders quickly.

  • But sometimes, the enemies are too strong and would overwhelm your defenses eventually,

  • which means certain death for you, the human.

  • This is the hour of the dendritic cell, your immune system's intelligence officer.

  • While your soldiers were bashing in heads,

  • it was collecting samples by ripping bacteria into tiny parts and covering itself in it.

  • Like a soldier decorating itself in the guts of a dead enemy.

  • The cell leaves the battlefield and enters the superhighway of your immune system

  • that connects all your tissues with your immune headquarters.

  • Your lymph nodes.

  • The dendritic cell coming from the battlefield is looking for a helper T cell,

  • which is a sort of all-purpose commander cell within your immune army.

  • But not any helper T cell.

  • One that happens to have just the right weapon for the bacteria that infected your wound.

  • So it goes around and rubs itself, still covered in bacteria parts, against every helper T cell it meets.

  • Most T cells are a bit disgusted and not interested.

  • But after a few hours, something clicks.

  • A helper T cell recognizes the bacteria parts.

  • This cell is the weapon that's needed right now.

  • The dendritic cell is overjoyed and activates the helper T cell.

  • Okay, wait.

  • How come your immune system has a cell that has a weapon against the specific bacteria that infected you?

  • Well, your immune system has a perfect weapon against every possible disease in the universe.

  • Against the Black Death, the Coronavirus, or an infection that will emerge in 100 years on Mars.

  • We'll talk about this a bit more in the next video because it's very complex.

  • So, for now, just know that you have billions of unique helper T cells

  • that each have weapons against every possible enemy.

  • After the right T cell is activated, your second line of defense awakes

  • and rises like a teenager that needs to get up on a school day...

  • Very slowly.

  • Your heavy weapons are incredibly effective but they're not fast.

  • The activated helper T cell begins to clone itself over and over again.

  • One becomes two, two become four, until there are thousands of them.

  • Now, they split into two groups.

  • The first group quickly moves to help out your soldiers.

  • At the battlefield, things are getting out of hand.

  • A tired macrophage is ready to give up.

  • After fighting for days, it just wants to go to sleep like many of its buddies have done already.

  • But now the helper T cells arrive.

  • One of them comes to the tired macrophage and whispers something using special chemical signals.

  • In a heartbeat, the demoralized soldier feels fresh again.

  • But there's something else.

  • A hot white anger.

  • The macrophage knows what it needs to do...

  • Kill.

  • Invigorated, it throws itself against the enemies once again.

  • All over the battlefield, this begins to happen.

  • Meanwhile, the second group of helper T cells was working on activating another line of defense:

  • B cells, your antibody factories.

  • Antibodies are protein super weapons that look like tiny crabs with two pincers to grab enemies.

  • Just like the helper T cells, there are B cells in your body

  • that are able to make just the right antibodies for every possible enemy.

  • And the helper T cell is looking for exactly these B cells.

  • After a day or two, the right B cell is found, and begins to clone itself.

  • As soon as enough clones have been made,

  • each B cell begins pumping out up to 2,000 antibodies per second!

  • About a week after you injured yourself and bacteria invaded,

  • your second line of defense finally arrives in full force.

  • The tiny army begins to saturate the battlefield, pinching and stunning desperate bacteria.

  • The antibodies clump them together and make them unable to move or fight,

  • while your soldiers massacre the defenseless victims.

  • The tide is turning fast.

  • As the last enemies are cleaned up, your soldiers realize they are no longer needed,

  • and begin to kill themselves to save resources.

  • But not all of them.

  • A few helper T cells remain and turn into memory cells.

  • They will guard the tissue for years,

  • making sure the same bacteria will never again gain a foothold here.

  • Similarly, a few B cells will stay alive

  • and keep producing a low amount of antibodies,

  • making you immune against this bacteria,

  • maybe for the rest of your life.

  • One day you wake up and notice that the wound has grown over

  • and left nothing but a faint red mark.

  • You were completely unaware of the drama your cells had to deal with.

  • For you, the whole ordeal was a slight annoyance.

  • While for millions of cells, it was a desperate fight for life and death.

  • But this is just the beginning of the epic story that unfolds inside you every day,

  • and is told in full in

  • "Immune: A Journey into the Mysterious System That Keeps You Alive",

  • written by Philipp Dettmer, the founder and head writer of Kurzgesagt.

  • The book is a beautifully illustrated deep dive into the immune system,

  • the most important thing you don't know enough about,

  • that will forever change how you think about your body.

  • Written to be as simple and fun as possible,

  • it's been an ongoing passion project for almost a decade

  • because the immune system is just about the most amazing and fascinating topic there is.

  • So, go on a journey and get to know your own body.

  • From mystery organs, murder universities, to the largest library in the universe.

  • Explore how incredibly vast your immune system is and how it actually works.

  • How it fights enemies from cancer to HIV, or just the flu.

  • Learn how you can boost it and if that's actually a good idea.

  • The book will be out in six weeks

  • and it would mean the world to us and Philipp if you could pre-order it.

  • Pre-ordering is super important in the world of books,

  • and in contrast to games, we can guarantee that it's actually finished

  • and there will be no day one patch.

  • There's a link in the description.

  • This is the end of a decade-long personal journey and we'd love to see what you think.

  • Thank you for watching.

The human immune system is the most complex biological system we know, after the human brain,

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免疫系統如何確實運作(How The Immune System ACTUALLY Works – IMMUNE)

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    ivycandy 發佈於 2021 年 09 月 06 日
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