Somewhere in your body, your immune system just quietly killed one of your own cells,
stopping it from becoming cancer, and saving your life. It does that all the time.
The vast majority of cancer cells you develop will be killed without you ever
noticing. Which is an incredibly hard job because of what cancer cells are:
parts of yourself that start to behave as individuals even if it hurts you.
What is cancer and how does your body kill it all the time?
INTRO
Cancer is when corrupted cells multiply uncontrollably.
It can emerge from basically every type of cell in your body,
so there is not just a single type of cancer but hundreds.
Some grow slowly, others are aggressive, some can be treated effectively, others are deadly.
In a sense, a cell that becomes cancer turns into something ancient and something new. Over
billions of years, evolution has molded cells to survive and thrive in a hostile environment,
fighting for space and resources. Until a new and exciting way of life emerged: Cooperation,
a division of labour that allowed cells to specialize and become more successful together.
But cooperation requires sacrifices. For a multicellular being to stay healthy,
the wellbeing of the collective has to matter more than the survival of the individual cell.
Cancer cells stop being part of the collective and become individuals again.
Your body can handle a few rogue cells but some cancer cells divide, again, and again,
becoming a sort of new organism within you. Taking resources you need to survive,
competing for the space you inhabit, destroying the organs they were part of in the process.
Despite the harm they cause, cancer cells are not evil. They don’t want to hurt you.
They don’t want anything. Cells are protein robots that just follow their programming,
which unfortunately has been corrupted.
The Soul of the Cell
In a nutshell, your cells have a nucleus filled with DNA. It consists of genes – instructions
for how to build proteins and when to make each one.These building instructions are
copied and transferred to ribosomes, where they are used to make proteins. What kind
of proteins your cells make determine what they can do. The important thing here is
that a corrupt gene means you get a corrupt protein, which will get important later.
Your DNA gets a tiny bit corrupted – it mutates – tens of thousands of times each day.
Most of the time without any special cause, just by being alive.
Almost all of these mutations are fixed very quickly or are not problematic.
Still, over time as your cells make copies of themselves,
damage is accumulating. Imagine having to make copies from copies from copies,
for decades. Maybe one day a hair got on the scanner or a corner got frayed.
Each new mistake becomes part of the new copies and all the copies that follow.
You can increase DNA damage by doing things like smoking, drinking alcohol,
by being obese, breathing in asbestos, by not using sunscreen or contracting a virus
like HPV. But the simplest way to damage DNA and get cancer is to be alive long enough.
For many cancer cases, there is no cause other than bad luck.
The Damage that leads to Cancer
We are simplifying, but roughly,
there are three categories of genes that need to be corrupted so cancer can arise.
The first key mutation is in the appropriately named tumor suppressor genes, or TSGs.
These genes are a bunch of things. For one, they produce control mechanisms that
continuously scan your DNA for mistakes and copying errors and fix them right
away. And then they keep normal cells from multiplying recklessly. If TSGs become damaged,
your cells basically forget how to repair themselves and can reproduce unchecked.
The second crucial mutation can happen in your oncogenes. When oncogenes are turned on the
cell is told to multiply rapidly.They were super active when you were inside your mother’s womb.
To turn a single original cell into trillions in months, it needs to divide
and grow rapidly. These rapid growth genes are turned off when there is enough of you.
When your oncogenes get corrupted, they basically turn on again.
The third crucial mutation is in your cells’ suicide switch.
Most cells are constantly recycled and refreshed. When cells amass too much damage, they usually
notice and special genes trigger a controlled suicide called apoptosis. If the genes that
control this process get damaged, cells are free to live on despite being dangerously corrupted.
So if a cell becomes unable to fix the mistakes in its genetic code,
loses the ability to destroy itself when it notices the damage, and begins
to grow rapidly without restraint, it turns into a young cancer cell.
These cells have to be killed as quickly as possible: while they are bad at this stage,
they are still pretty weak and easy to kill. But if they continue to mutate and
increase in number, they can learn to avoid your defenses and become a real threat.
At any moment of your life, your immune system is hunting these cells. But how do
you identify and kill corrupted cells that seem indistinguishable from healthy ones?
How to Find Cancer
Well, here we come back to the proteins your cells produce and the story they tell. So if
for example, your oncogenes switch back on, they make oncogene proteins. Your
immune system knows that they should not be present if you are an adult.
So to know which cells are corrupt and which are healthy,
your immune system needs to know what proteins they are making inside. To solve this evolution
came up with MHC class I molecules, a sort of display window that makes cells transparent.
Cells constantly take little samples of the proteins they make and put them into
thousands of these MHC molecules, to showcase what they are doing.
The selection is constantly refreshed, always giving an up to date picture.
There is a whole library of proteins that are highly dangerous and should not be made
by healthy cells, and your immune system has them all on file. It has billions of specialized cells,
called T Cells, made to recognize specific proteins. If a T Cell sees a forbidden protein
in an MHC display window, it knows that the cell is corrupted and kills it immediately.
But there is a flaw in this system.
What if a cancer cell mutates and finds a way to circumvent this process? All it
needs to do is to stop making MHC Class I molecules, and boom, it’s invisible.
Without display windows, the immune system is blind and can’t identify cancer anymore.
Fortunately evolution found an ingenious solution:
The Natural Killer Cell. A judge, jury and executioner.
The Killer
At this very second, hundreds of millions of Natural Killer Cells are patrolling your body
looking for cells that have already turned into cancer or are corrupted by a virus.
Natural Killer Cells go from cell to cell to check for one thing: Does a cell have MHC class
I molecules? Does it have a display window and is it doing its duty of showing off what is going on
inside itself? This is so amazing because it covers all of your bases: While T Cells look
for the presence of the unexpected, something that should not be here, Natural Killer Cells
look for the absence of the expected, the absence of something that should be here.
The logic is: if a cell does not have display windows,
it wants to hide something. And a cell that hides something must be killed.
What makes the Natural Killer Cell even more metal, is that it is always in murder mode. It
patrols your body, checking cell after cell with the intention of killing it.
Your healthy cells have to convince it that they
should not die today. And a way to do that is to have MHC class I molecules.
So in summary, almost all young cancer cells you will ever develop in your life will be killed
by your immune system. Ok, but if your body is this prepared, why do we still get cancer? Well,
sometimes cancer cells mutate more and get much better at fighting back.
Cancer is a story of an arms race. An arms race that we will win eventually, maybe with the help
of Natural Killer cells! Right now a number of therapies are beginning to show amazing promise,
from cancer fighting vaccines, to engineered T Cells and even Natural Killer cells – we will
look at these therapies in future videos. So the war is not won yet, but we are on
to cancer and eventually it will be eliminated once and for all – maybe sooner than we think.