Immune: A Journey Into the Mysterious System That Keeps You Alive

by Philipp Dettmer

Step 3: The B Cell breaks the big chunk of antigen (turkey drumstick) down into dozens or hundreds of small antigens (wiener sized) and begins presenting them in MHC class II molecules (hot dog buns).

Step 4: An activated B Cell that is presenting hundreds of different antigens (wiener-sized sausage pieces) needs to meet a T Cell that can recognize one of these antigens with its specific T Cell...

Only if this exact sequence of events occurs does a B Cell get...

If this sequence of events happens, the last and most powerful stage of the Adaptive Immune System finally starts up in earnest and awakens.

Your B Cell

begins to swell, to almost double its size, and transforms into its final form: the Plasma Cell.

It can release up to 2,000 antibodies per second that saturate the lymph and blood and the fluids between your tissue.

Now your immune system truly begins beating microbes in their own game as a beautiful dance begins, a dance that makes your defenses even better and stronger.

this stuff is mind-bendingly unintuitive and hard even in a strongly simplified form. If you manage to remember that B Cells activate by picking up stuff themselves and then get activated a second time by T Cells, that is already amazing.

So B Cells can get activated if their receptors are good enough at recognizing an antigen.

having Antibodies with a really good shape that fits really well on an antigen is an incredible advantage that can mean the difference between life and death. And your immune system wants it all, a quick response and then a perfect defense.

We said before that B Cells need to be activated by Helper T Cells (that were activated themselves by Dendritic Cells) to turn into Plasma Cells but actually this process is a tad more amazing and sophisticated. Your immune system makes sure that only B Cells that are able to make amazing Antibodies turn into Plasma Cells. OK, so how does this work?

Every time a B Cell receives a positive signal from a Helper T Cell, it begins a round of purposeful mutation.

The B Cell mutates the gene fragments that make its receptors, and therefore its antibodies.

But if the mutation improved the B Cell receptor it will now be even better at recognizing the antigen and the B Cell will get an activation signal once again!

Over time a natural selection takes place. The better B Cell receptors become at recognizing the antigen that flows through the lymph node, the more stimulation and encouragement they get. While at the same time, the B Cells that get worse or don’t improve kill themselves.

you have probably heard the term “Antibody” a bunch of times from medical professionals. They are your superweapons, the main reason you can survive serious infections.

This mechanism makes the Adaptive Immune System actually adapt to the enemy in real time. We asked before how you could keep up with the billions of different enemies that are also able to change themselves. This is one way. A system that can replicate very quickly, that has a defined target and can adapt to it fast, that fine-tunes and improves its weapons until they are perfect. What a beautiful and ingenious solution that shows that the Adaptive Immune System really deserves its name—it truly can beat microbes at their own game.

The most common Antibodies are shaped like little crabs with two pincers

they are sort of comparable to the proteins of the complement system—which

Complement proteins are generalists, while Antibodies, as we just learned, are specific.

Antibodies are B Cell receptors.

With these tools Antibodies do multiple things: First, similar to complement, they can opsonize enemies. In this context it means that Antibodies swarm an enemy and grab them,

Or in the case of viruses antibodies can directly neutralize them and make them unable to infect cells.

If millions of Antibodies flood a battlefield, they can clump up large piles of pathogens together that are now even more helpless and even more unhappy and scared, as a big pile of victims are even easier to detect for your Macrophages and Neutrophils,

Antibodies act as a special superglue—your immune cells, specifically your phagocytes, the cells that eat enemies alive, can grab the butts of antibodies very easily.

As soon as an Antibody has grabbed a victim with its tiny pincers, its butt changes its shape and is now able to bind to immune cells.

Another thing Antibodies can do with their cute butts is to activate the complement system.

Antibodies are able to activate the complement system and sort of attract it to bacteria, increasing its efficiency wildly. Again we see the principle of our two immune systems: The innate part does the actual fighting, but the adaptive part makes it more efficient with deadly precision.

Antibodies are not just tiny crabs though. There are multiple classes that actually do very different things and are used for different situations.

IgM Antibodies are usually the majority of Antibodies B Cells produce when they get activated.

IgM is basically five Antibodies merged together at the hips, which has the advantage that they have five butt regions. Two of these butt regions together can activate an additional complement pathway.

their most important job is to buy time until better Antibodies are available.

IgG Antibodies come in a few different types.

is really good at opsonizing a target

In general IgG is not nearly as good at activating complement as IgM is, but it is still pretty solid at it.

special IgG Antibodies are specifically made to be unable to activate the complement system late in an infection, which limits inflammation.

only Antibodies that are able to pass from the blood of a mother into the blood of an unborn fetus via the placenta.

gives a newborn human a passive defense against virus infections that protects it in the first few months

IgA is the most abundant antibody in your body and its main job is to serve as a cleanup mechanism for your mucosa.

by neutralizing pathogens early on before they have the chance to get in and establish a foothold.

So if you have a nasty cold, your snot is full of IgA giving viruses and bacteria a hard time.

IgA is different from other antibodies in one major way: IgA have their little butts merged together, which means that IgA can’t activate the complement system at all.

And since IgA Antibodies are constantly produced in your gut, if they could activate complement this would mean that your gut would be constantly inflamed.

When mothers are breastfeeding they provide their offspring with a large amount of IgA Antibodies through their breast milk. These antibodies then cover the gut of the newborn and protect its still-fragile intestinal tract from infections.

The original purpose IgE Antibodies serve is to protect you against infections by huge enemies: Parasites. Especially worms.

Now you may ask yourself: If there are so many different types and variants, how do B Cells know what sort of antibody is needed?

they always start with IgM but can switch the Antibody type if the Helper T Cell asks and encourages them to!

(Once an Antibody class has been switched there is no going back though.)

We mentioned before that your spleen is a sort of lymph node for your blood but there is more! This tiny organ is the main source for superfast responding IgM Antibodies in your blood. A sort of emergency base that can react fast if pathogens like bacteria make it into your blood flow,