Narrator: Every second of your life youare under attack. Billions of bacteria, viruses, and fungi are trying to make youtheir home. So our bodies have developed a super complex little army with guards,soldiers, intelligence, weapons factories and communicators to protect you from uh,well, dying. For this tutorial, let's assume the immune system has twelve different jobs. For example, kill enemies, communicate etc. And it has 21 different cells and twoprotein forces. These cells have up to four different jobs. Let's assign them. Here are the interactions. Now let's make this understandable. First of all, let's addcolors to the jobs. Now let's illustrate
the cells. The central color represents the main job of the cell, while the surrounding ones represent secondary duties. Now the immune system looks like this. Now the interactions. Isn't this complexity just awesomeé For this tutorial we will only talk about these cells and ignore the rest. So what happens in the case of an infectioné Music It's a beautiful day when suddenly a wild rusty nail appears and you cut yourself. The first barrier of the immune system isbreached: your skin. Nearby bacteria sieze
on the opportunity and enter your wound.They start using up the body's resources and double their numbers about every 20 minutes. At first they fly under the radar but when a certain bacteria population isreached, they change their behavior and start to damage the body by changing the environment around them. The immune system has to stop them as fast as possible. First of all your guard cells, known as macrophages, intervene. They are huge cells that guard every border region of the body. Most of the time they alone cansuffocate an attack because they can devour up to 100 intruders each. They swallow the intruder whole and trap it inside a membrane.
Then the enemy gets broken down by enzymesand is killed. On top of that, they cause inflammation by ordering the blood vesselsto release water into the battlefield so fighting becomes easier. You notice this as a very mild swelling. When the macrophages fight for too long, they call in heavy backup by releasing messenger proteins that communicate location and urgency. Neutrophiles leave their patrol routes in the blood and move to the battlefield. TheNeutrophiles fight so furiously that they kill healthy cells in the process. On top of that, they generate barriers that trap and kill the bacteria. They are indeed so deadly that they evolved to commit suicide
after five days to prevent them from causing too much damage. If this is not enough to stop the invasion, the brain of the immune system kicks in. The dendritic cell gets active. It reacts to the signals of the soldiers and starts collecting samples from the enemies. They rip them into piecesand present the parts on their outer layer. Now, the dendritic cell makes a crucial decision. Should they call for antivirus forces that eradicate infected body cells,or an army of bacteria killersé In this case, antibacteria forces are necessary.It then travels to the closest lymph node in about a day. Here billions of helper andKillerT cells are waiting to be activated.
When TCells are born they go through adifficult and complicated training process and only a quarter survives. The survivingcells are equipped with a specific setup. And the dendritic cell is on its way lookingfor a helper Tcell with a set up that's just right. It's looking for a helper Tcellthat combines the parts of the intruders which the dendritic cell has presented on its membrane. When it finally finds one, a chain reaction takes place. The helper Tcell is activated. It quickly duplicates thousands of times. Some become memory Tcells that stay in the lymph node and will make you practically immune to this enemy.Some travel to the field of battle to help
How parasites change their hosts behavior Jaap de Roode
Which of these entities has evolved the ability to manipulate an animalmany times its sizeé The answer is all of them. These are all parasites, organisms that live on or inside another host organism, which they harm and sometimes even kill. Parasite survival depends on transmittingfrom one host to the next, sometimes through an intermediate species.
Our parasites elegantly achieve thisby manipulating their host's behavior, sometimes through direct brain hijacking. For example, this is the Gordian worm. One of its hosts, this cricket. The Gordian worm needs water to mate,but the cricket prefers dry land. So once it's big enough to reproduce, the worm produces proteins that garblethe cricket's navigational system. The confused cricket jumps around erratically,
moves closer to water, and eventually leaps in,often drowning in the process. The worm then wriggles out to mate and its eggs get eaten by little water insects that mature, colonize land, and are, in turn, eaten by new crickets. And thus, the Gordian worm lives on.
And here's the rabies virus,another mindaltering parasite. This virus infects mammals, often dogs, and travels up the animal's nerves to its brain where it causes inflammation that eventually kills the host. But before it does, it often increasesits host's aggressiveness and ramps up the productionof rabiestransmitting saliva, while making it hard to swallow. These factors make the host more likely to bite another animal
and more likely to pass the virus on when it does. And now, meet Ophiocordyceps,also known as the zombie fungus. Its host of choice is tropical antsthat normally live in treetops. After Ophiocordyceps spores pierce the ant's exoskeleton, they set off convulsions that make the ant fall from the tree. The fungus changes the ant's behavior,compelling it to wander mindlessly until it stumbles onto a plant leaf withthe perfect fungal breeding conditions, which it latches onto.
The ant then dies, and the fungus parasitizes its bodyto build a tall, thin stalk from its neck. Within several weeks, the stalk shoots off spores, which turn more ants into sixlegged leafseeking zombies. One of humanity's most deadly assailantsis a behavioraltering parasite, though if it's any consolation, it's not our brains that are being hijacked. I'm talking about Plasmodium,which causes malaria.