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

by Philipp Dettmer

But what even is the immune system and how does it actually work?

You become less susceptible to snake oil salesmen who offer wonder drugs that are entirely devoid of logic.

The immune system is the most complex biological system known to humanity, other than the human brain.

All of this makes the immune system horrible to explain.

So my aim for this book is to try to carefully dance around all these problems. It will use human language and use complicated words only when necessary.

So humoral immunity is very tiny stuff, made out of proteins, that floats through the bodily fluids outside of the cells of an animal. These proteins hurt and kill microorganisms that have no business being there.

Which finally brings us to humanity. And to you. You get to enjoy the fruits of hundreds of millions of years of immune system refinement.

At its very core, the immune system is a tool to distinguish the other from the self. It does not matter if the other means to harm you or not.

Whatever you do in life, it is not possible to exist and function without the world and the things it offers.

So your body can’t be a closed system—it

As we mentioned at the beginning, your lungs, your guts, mouth, and respiratory and reproductive tracts are really just outsides wrapped up in the insides. These insides are lined with what you could call your “inside skin.” Unfortunately the correct name is Mucosa. But to make it a bit more badass we will call it the Swamp Kingdom of the Mucosa.

In contrast, in most parts of your body, your immune system is not tolerant at all.

Imagine it reacting violently to every little flake of dust that you breathed in. No, the immune system of the swamp kingdom can’t be as aggressive as the immune system in other parts of the body

If the skin is like a vast desert and a nearly impassable wall protecting the border of the continent of flesh, the mucosa is like a vast swampland, with deadly traps and groups of defenders patrolling.

The first line of defense

The mucus layer. Mucus is a slippery and viscous substance that behaves a little bit like watery gel.

It covers all of your surfaces that interact with the outside that are wrapped inside you. Mucus is continuously produced by Goblet Cells,

This slimy mucus serves multiple purposes—in the simplest sense it is just a physical barrier so intruders have a harder time reaching the cells that it covers.

And mucus is not just a sticky barrier but also filled with unpleasant surprises similar to the desert kingdom: salts, weaponized enzymes that can dissolve the outsides of microbes, and special substances that sort of sponge up crucial nutrients that bacteria need to survive, so they starve to death inside the mucus.

Epithelial Cells. These cells are the equivalent of your skin cells,

And one of their jobs is to move the slime with the hairlike cilia that cover their membranes.

The direction depends on their location. In your respiratory tract, your nose, and lungs, the slime is moved either directly out of the body through your mouth or nose or via a slight detour it is swallowed and ends up in your stomach.

the mucosa in your lungs has a very different job from the mucosa in your gut, which has a completely different job from the mucosa in the female reproductive tract.

Your intestines are a very special place for your immune system because a lot of complicated challenges need to be managed here to keep the body healthy and functioning.

There are a lot of things about the interactions and functions of your Immune System and the gut microbiome we don’t understand yet. We know that many diseases and disorders are associated with these interactions being out of balance, but a lot of research needs

the immune system of your intestines is a semi-closed system that tries not to mix too much with the immune system in the rest of the body.

After the stomach the journey continues through your intestine, which

Over 90% of the nutrients you need to survive are absorbed here.

constant stream of stuff they can eat, and in turn they break down carbohydrates that we can’t digest and produce certain vitamins that we can’t make ourselves. The bacteria of the microbiome are tenants of sorts and

because if they were to enter the bloodstream and enter your actual insides, they could do horrible damage or even kill you. So the mucosa of your intestines is built in a way to prevent that.

First a layer of mucus filled with antibodies, defensins (we met these before on the skin, the tiny needles that can kill microorganisms),

Below the mucus layer, the intestinal epithelial cells are the actual barrier between the inside and the outside.

Your immune system monitors this area and is especially annoyed with any kind of microorganism that is trying to attach itself to the epithelial cells.

the third layer of the gut mucosa, the Lamina Propria, which is the home of most of the immune system of your gut.

How do Dendritic Cells know if the bacteria they sample in the gut are dangerous pathogens or just harmless commensal bacteria? Well, right now we don’t know but we know that when Dendritic Cells sample commensals, they order the local immune system to chill out and not be too annoyed by their antigens.

your immune system keeps the mucus free of overly ambitious friendly bacteria but also makes sure that it does not cause damage by overreacting. Your gut immune system really is a peacekeeping force.

To catch these serious enemies as early as possible, your gut has a type of special lymph node called Peyer’s patches that are directly integrated into your intestines. Microfold Cells

This way your intestines have a superfast immune screening that constantly monitors the population of bacteria in your gut mucosa very closely.

So let’s explore your (arguably) most sinister enemy, the virus.

Poop transplants have been shown to have a high chance of restoring the natural balance and help the patients to get rid of the invaders on their

Bacteria are the same, protein robots able to do amazing things, although, in a sense, they could be considered a bit less sophisticated.

Whatever the truth is, viruses turned out to be incredibly successful. In fact, viruses are arguably the most successful entity on the planet.

a virus needs to be able to do to thrive is to get inside cells. And for that they abuse a weak point of all cells that living things will never be able to completely protect against: They attack receptors.

This means that viruses can’t attach to just any cell—only to the ones that have a receptor they can

attach to. In a sense every virus has a lot of puzzle-piece proteins that can only connect to a cell if it happens to have the correct puzzle-piece receptor.

Once a virus gets in contact with the kind of cell it is looking for, it quietly takes it

they have a superpower: Nothing multiplies as fast as they do. And that also means that nothing mutates or changes as fast as viruses.

Viruses are so basic that they lack most of the intricate safeguards your cells have to prevent mutations, so they mutate all the time.

It’s the old evolution, brute-force, throw-shit-at-the-wall-until-something-sticks approach. And it’s quite effective.*2

Pathogenic viruses are terrifyingly dangerous enemies.