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Every living being needs to fight off other living beings that want to feast on them.​
So as multicellular life evolved over billions of years, it came up with ways to defend itself.​
Today humans have a sophisticated defense network,​
like physical barriers, defense cells, and weapons factories.​
But one of the most important defenses of our body is largely unknown:​
The Complement System​
It evolved over 700 million years ago and is an army of over 30 different proteins​
that work together in a complex and elegant dance to stop intruders.​
All in all, about 15 quintillion of them are saturating every fluid in your body right now.​
Guided by nothing but chemistry,​
these proteins are one of the most effective weapons we have against invaders.​
Many other parts of the immune system are just tools to activate the Complement System.​
But it's also really dangerous.​
Imagine having trillions of little bombs inside your blood that could go off at any moment.​
So our cells use numerous mechanisms to prevent​ ​complement from accidentally attacking ​​them​.​
Okay, what exactly does it do and what makes it so dangerous?​
In a nutshell, The Complement System does three things:​
it cripples enemies, it activates the immune system, and it rips holes in things until they die.​
But, how?​
After all, these are mindless proteins randomly drifting around without will or direction.​
Well, this is actually part of the strategy.​
Complement proteins float around in a sort-of passive mode.​
They do nothing, until they get activated and change their shape.​
In the world of proteins, your shape determines what you can and cannot do.​
Because shape determines what you can interact with and in what way.​
For example, in your passive shape you might do nothing.​
In your active shape, however, you might, for example, change the shape of other proteins​
activating them so they can activate others.​
Mechanisms like this one can start cascades that spread very quickly.​
Imagine the complement proteins as being like millions of matches very close together.​
Once one catches fire, it ignites the ones around it.​
They ignite more and suddenly you have a big fire.​
To show the actual mechanisms of The Complement System is a tad complicated and overwhelming.​
So, we'll simplify here.​
Now, let's imagine you cut yourself and a bunch of bacteria enter the wound​
and make it into the surrounding tissue.​
Our complement attack begins with C3.​
C3 is the first match, the initial spark that will start our fire.​
And to do that, C3 needs to switch from passive to active.​
How this happens is complex, but let's just say it can happen randomly​
through other complement proteins that bind to enemies or through antibodies.​
All you really need to know is that C3 breaks into two smaller proteins,​
C3a and C3b, that are now activated.​
One of these parts, the C3b protein, is like a seeker missile specialized in bacteria, fungi, and viruses.​
It has a fraction of a second to find a victim or it will be neutralized by water molecules.​
If C3b does find a target, it anchors itself very tightly to its surface and doesn't let go.​
By doing so, the protein changes its shape again.​
In its new shape, it's now able to grab other proteins and start a small cascade,​
changing its shape multiple times, adding other complement proteins to itself.​
Finally, it transforms itself into a recruiting platform known as C3 Convertase.​
This platform is an expert at activating more C3 proteins that start the whole cycle anew.​
An amplification loop begins.​
Soon, thousands of proteins cover the bacteria.​
For the bacteria, this is very bad.​
It can cripple the bacteria and make them helpless, or slow them down.​
Imagine being covered by thousands of flies.​
But, there's more.​
Do you remember the other part of C3—the C3a protein?​
C3a is like a distress beacon.​
Thousands of them flood away from the site of battle screaming for attention.​
Passive immune cells notice the C3a proteins,​
and awaken from their slumber to follow the protein tracks to the site of infection.​
The more alarm proteins they encounter, the more aggressive they get.​
This way, complement guides reinforcements exactly to the place where they're needed the most.​
So far, the complement has slowed down the invaders and called for help.​
Now, it's beginning to actively help to kill the enemy.​
The first immune cells to arrive at the battlefield are ​​phagocytes​.​
Which means, cells that swallow you whole, trap you in a tiny prison, and then kill you with acid.​
But, to swallow an enemy, they need to grab it first.​
Which is not easy because bacteria prefer not to be grabbed and are sort-of slippery.​
But now, the complement that has anchored itself to the bacteria​
acts as a sort-of glue that makes it easy for the immune cells to catch their victims.​
But it gets even better.​
Imagine being covered in flies again.​
Now, imagine them turning into wasps.​
Another cascade is about to begin.​
On the surface of a bacteria, the C3 recruitment platform changes its shape again​
and begins to recruit new proteins.​
Together, they begin the construction of a bigger structure: a Membrane Attack Complex.​
Piece-by-piece, new proteins shaped like long spears anchor themselves deep into the bacteria's membranes,​
until they rip a hole into them that can't be closed again.​
Fluids rush into the bacteria and their insides spill out.​
They bleed to death.​
The remaining bacteria are maimed and distracted by the complement,​
and quickly taken care of by the arriving immune cells.​
The invasion has been nipped in the bud before it had the opportunity to become dangerous.​
You probably didn't even notice it.​
But while bacteria are not happy about complement,​
the enemies it might be the most useful against are actually viruses.​
Viruses have one problem: they need to travel from cell to cell.​
Outside of cells, they're basically hoping to randomly bump against a cell to infect by pure chance.​
Here, they're completely defenseless.​
And here, complement is able to intercept and cripple them,​
so they become harmless and guide the immune system to devour them.​
Without complement, virus infections would be a lot more deadly.​
But wait, if we have such an effective weapon, why do we ever get sick?​
The problem is that in a war, both sides adapt.​
For example, when the ​​vaccinia virus​ infects a cell,​
it forces it to produce a protein that shuts complement activation down.​
This way, the virus creates safe zones around the cells it infects.​
When it kills them, and tries to infect more, it has a higher chance of being successful.​
Or some bacteria, for example, can grab certain molecules from the blood that keep the complement system calm​
and make themselves invisible.​
So the complement system, while being extremely important,​
is only one player in the complex and beautiful organization that is our immune system.​
A beautiful example of how many mindless things can do smart things together.​