Do you remember that cold you had when you were 10?

Or that shot you got before kindergarten?

Or that guy who sneezed next to you while you were watching Speed 2: Cruise Control.

No?

Well, your body does.

Each of these is a time when you faced some germy invader, and after you fought them off, your body stored a memory so it would recognize these threats if they showed up again.

Not in your brain – a cellular memory… in your immune system.

We’re learning that the immune system is so adaptable and long-lasting that we might be able to engineer our bodies and become immune to germs we’ve never met, or even give ourselves universal immunity.

But how does this incredible army of cellular guardians do its job?

Are we so powerful, that we could ever be immune to *everything*?

[open] Our first level of defense against would-be invaders is physical: like your skin.

But something inevitably slips through.

Then your second line of defense springs into action.

Hungry white blood cells patrol your body looking for anything that isn’t you.

They attack these biohazards with chemical weapons and call in reinforcements to engulf the invaders, and trigger an alarm system called inflammation.

You see all this as a swollen red bump.

This is your body’s innate immune system at work.

Billions of cellular soldiers that have been protecting you since you were born.

But it isn’t always enough… especially if a more serious threat comes along.

Some bacteria and viruses can double in your body every 20 minutes.

This can quickly overwhelm that *innate* immune system, but luckily, we’ve evolved a more sophisticated line of defense to stop these formidable foes: The adaptive immune response.

This cellular army is like your body’s special forces, and its MVPs are antibodies.

Each of these proteins carries a unique region with a special shape that allows it to latch onto one particular enemy, like a lock that fits a single key.

When one finds its match, it can coordinate all your body’s defenses to fight this one nasty intruder.

And your immune system learns from this fight so it can attack even stronger if you ever meet that germ again in the future.

That’s called being immune to something.

Antibodies stick like velcro to tiny shapes or handles on the surface of the enemy.

Your body is full of different shaped antibodies, all waiting to run into their perfect target.

But how does your immune system make the right antibodies for germs you’ve never met?

Look in your DNA and you won’t find specific blueprints for the flu or a cold virus.

The unique shape of an antibody is built from random shuffling.

Your immune system’s strategy?

Make enough lock combinations, there’s a good chance that one will fit that key.

Deep in your bone marrow, B-cells destined to carry antibody armor go through a rigorous training program.

Each of these cells has the power to make a mind-boggling number of unique antibody shapes by shuffling just three sets of genetic instructions.

You can think of it like a card game: B cells have three gene decks to work with, V, D, and J, to start building an antibody.

By picking one card, from each deck, you can make about 10,000 genetic hands.

Next, random DNA letters are inserted in between these genes.

This adds another layer of variety, up to 10 million times more possible combinations.

After they’re sent out to patrol your body and fight germs, these antibodies continue to mutate and tweak their shape to stay a step ahead of the enemy.

With the shuffling of genes, random spacers, and extra mutations combined, each of us has the potential to make a quintillion possible antibodies.

That’s five million-trillion possible antibody shapes… 10 million times more than there are stars in our galaxy, so could you be immune to anything?

Hypothetically, these aren't big enough to hold a galaxy's worth of antibodies This fine-tuned system is incredibly powerful.

When a B-cell locks onto its match, it clones itself.

Some of those new soldiers become little factories, shooting thousands of identical antibodies at the invader every minute.

They can neutralize a threat on their own, or wait for the rest of your body’s immune army to arrive.

After the battle is over, a few of these clones hang back and become your body’s immune system memory.

They carry a copy of the unique antibody, spending the rest of their lives on patrol, ready to jump into action if that enemy returns.

This is what vaccines do: Show your immune system what a germ looks like, to create a memory, all without getting you sick.

That memory can last for a lifetime.

In 1918, the Spanish flu infected 500 *million* people worldwide.

Today, in the blood of flu survivors, scientists can still find antibodies to the virus.

But what if we could fix the deck… could we start with a key, from a germ we already know, and mold the perfect lock to fight it?

Actually, yes… and scientists are close.

We can take an antibody from one person that’s fought a disease, copy it, and put it in another person – so they can become immune to enemy #1.

We’ve actually done this: When the spanish flu came back in 2009, antibodies from 1918 survivors, who still carried their original immunity, kept healthy people safe.

Unfortunately, germs are always mutating and evolving as well, coming up with new disguises to slip past your immune memory.

This game of cat and mouse is why we face new versions of germs like influenza each year, and why we constantly need new vaccines to help fight them.

But, scientists have found that some rare antibodies bind to every version of a virus.

Using these, we may be getting closer to a universal flu treatment, which may work even better than vaccines and be put in doctors’ hands faster.

Your adaptive immune system is a powerful and versatile tool that stands guard every minute of every day to keep your body safe in a world of invisible enemies.

So maybe you don’t remember every time you’ve ever gotten sick.

But your immune system does.

Stay curious.