00:00Every second of your life you are under attack.
00:03Billions of bacteria, viruses and fungi are trying to make you their home.
00:08Still, our bodies have developed a super complex little army with guards, soldiers, intelligence, weapons factories and communicators
00:14to protect you from, well, dying.
00:17For this video, let's assume the immune system has 12 different jobs.
00:22For example, kill enemies, communicate, etc.
00:25And it has 21 different cells and 2 protein forces.
00:29These cells have up to 4 different jobs.
00:32Let's assign them.
00:34Here are the interactions.
00:36Now, let's make this understandable.
00:38First of all, let's add colors to the jobs.
00:41Now let's illustrate the cells.
00:43The central color represents the main job of the cell, while the surrounding ones represent secondary duties.
00:50Now the immune system looks like this.
00:53Now the interactions.
00:55Isn't this complexity just awesome?
00:59For this video, we will only talk about these cells and ignore the rest.
01:03So, what happens in the case of an infection?
01:06It's a beautiful day when suddenly a wild rusty nail appears and you cut yourself.
01:12The first barrier of the immune system is breached, your skin.
01:16Nearby bacteria seize on the opportunity and enter your wound.
01:20They start using up the body's resources and double their numbers about every 20 minutes.
01:25At first, they fly under the radar, but when a certain bacteria population is reached,
01:30they change their behavior and start to damage the body by changing the environment around them.
01:34The immune system has to stop them as fast as possible.
01:38First of all, your guard cells, known as macrophages, intervene.
01:42They are huge cells that guard every border region of the body.
01:46Most of the time, they alone can suffocate an attack, because they can devour up to 100 intruders each.
01:52They swallow the intruder whole and trap it inside a membrane.
01:56Then the enemy gets broken down by enzymes and is killed.
01:59On top of that, they cause inflammation by ordering the blood vessels to release water into the battlefield,
02:04so fighting becomes easier.
02:06You notice this as a very mild swelling.
02:09When the macrophages fight for too long,
02:11they call in heavy backup by releasing messenger proteins that communicate location and urgency.
02:16Neutrophiles leave their patrol routes in the blood and move to the battlefield.
02:21The neutrophiles fight so furiously that they kill healthy cells in the process.
02:25On top of that, they generate barriers that trap and kill the bacteria.
02:29They are indeed so deadly that they evolved to commit suicide after five days
02:34to prevent them from causing too much damage.
02:36If this is not enough to stop the invasion, the brain of the immune system kicks in.
02:41The dendritic cell gets active.
02:43It reacts to the signals of the soldiers and starts collecting samples from the enemies.
02:47They rip them into pieces and present the parts on their outer layer.
02:51Now, the dendritic cell makes a crucial decision.
02:54Should they call for antivirus forces that eradicate infected body cells or an army of bacteria killers?
03:00In this case, antibacterial forces are necessary.
03:03It then travels to the closest lymph node in about a day.
03:06Here, billions of helper and killer T-cells are waiting to be activated.
03:11When T-cells are born, they go through a difficult and complicated training process
03:15and only a quarter survives.
03:17The surviving cells are equipped with a specific setup.
03:20And the dendritic cell is on its way looking for a helper T-cell with a setup that's just right.
03:26It's looking for a helper T-cell that can bind to the parts of the intruders
03:29which the dendritic cell has presented on its membrane.
03:32When it finally finds one, the chain reaction takes place.
03:36The helper T-cell is activated.
03:38It quickly duplicates thousands of times.
03:41Some become memory T-cells that stay in the lymph node
03:44and will make you practically immune against this enemy.
03:46Some travel to the field of battle to help out.
03:49And the third group goes on to travel to the center of the lymph node
03:52to activate a very powerful weapons factory.
03:55Like the T-cells, they're born with a specific setup.
03:58And when a B-cell and a T-cell with the same setup meet, hell breaks loose.
04:02The B-cell duplicates rapidly and starts producing millions of little weapons.
04:07They work so hard that they would literally die from exhaustion very fast.
04:11Here, helper T-cells play another important role.
04:14They stimulate the hard-working factories and tell them,
04:17don't die yet. We still need you. Keep going.
04:20This also ensures that the factories die if the infection is over
04:23so the body doesn't waste energy or hurt itself.
04:26But what is produced by the B-cells?
04:29You've heard of them, of course. Antibodies.
04:32Little proteins that are engineered to bind to the surface of the specific intruder.
04:36There are even different kinds of antibodies that have slightly different jobs.
04:40The helper T-cells tell the plasma cells which type is needed the most in this particular invasion.
04:45Millions of them flood the blood and saturate the body.
04:49Meanwhile, at the site of infection, the situation is getting dire.
04:53The intruders have multiplied in number and start hurting the body.
04:57Guard and attack cells fight hard, but also die in the process.
05:02Helper T-cells support them by ordering them to be more aggressive and to stay alive longer.
05:06But without help, they can't overwhelm the bacteria.
05:09But now the second line of defense arrives.
05:12Billions of antibodies flood the battlefield and disable lots of the intruders,
05:16rendering them helpless or killing them in the process.
05:19They also stun the bacteria and make them an easy target.
05:22Their back is built to connect to killer cells,
05:25so they can connect and kill the enemy more easily.
05:28Macrophages are especially good at gnoming up the bacteria which antibodies have attached to.
05:33Now the balance shifts.
05:35In a team effort, the infection is wiped out.
05:38At this point, millions of body cells have already died.
05:42No big deal, the losses are quickly replenished.
05:45Most immune cells are now useless, and without the constant signals they commit suicide,
05:50so as not to waste any resources.
05:53But some stay behind.
05:55The memory cells.
05:56If this enemy is encountered ever again in the future,
05:59they will be ready for it and probably kill it before you even notice.
06:03This was a very, very simplified explanation of parts of the immune system at work.
06:08Can you imagine how complex this system is?
06:10Even at this level, when we ignore so many players and all the chemistry?
06:14Life is awfully complicated, but if we take the time to understand it,
06:18we always encounter endless wonders and great beauty.
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