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

by Philipp Dettmer

the mucosa of your lung is a very inviting entry point for a virus. This does not mean it is easy to enter here—just like the skin, the body created a powerful defensive kingdom here.

Let us just agree that there are a whole lot of viruses on earth and they seem to be doing quite all right. The fact that some apes with pants are discussing if they are alive or not could not be more irrelevant to them.

Actually this is the only trick evolution has. It tries a lot of things and whatever does not die before it makes a few offspring gets another attempt at making offspring before dying. Repeat this often enough and you get the amazing variety of living things on earth. And new strains of cold viruses every season. So basically it’s a mixed bag.

Aside from this artificial pollution, air can carry a large number of allergens like the pollen from various plants or the dust in our homes, spiked with the droppings of mites.

Bacteria, viruses, and the spores of fungi are also riding on these particles or fine droplets of water,

A balanced system had to evolve in your lungs—able to fend off intruders and clear up the pollution while still allowing for the exchange of gases.

The defenses of your respiratory system begin in the nose,

any mucosal environment, the mucus is covering the surfaces and in your respiratory system can be rapidly expelled by the explosive sneezing reflex.

The mucus is being moved constantly either outside or swallowed.

at your deepest and most vulnerable places in your lungs, there is literally only a single layer of epithelial cells between the inside and the outside

The Alveolar Macrophage. Its main job is patrolling the surface of your lungs and picking up trash.

But most importantly, they tone down any sort of inflammation. Because the thing you really don’t want in your lungs is fluid.

There is evidence that your lungs might have a microbiome (which means a collective of microbes that live in your lungs), or at least some sort of transient community of organisms that live in your lungs and is tolerated. But in contrast to the gut microbiome, we still know very

But one of the biggest problems holding us back is that it is pretty hard to collect samples

What we know for sure, though, is that a lot of the most common and dangerous pathogenic viruses that infect humans use the respiratory system as an entry point.

The influenza A virus that you breathed in so casually belongs to one of the most powerful and consistently dangerous

strains of the very annoying family of Orthomyxoviridae. Influenza A has specialized in infecting the epithelial cells of the respiratory system in mammals.

Viruses don’t want attention. The beginnings of an influenza A infection are less of a full frontal attack and more like an invasion by a bunch of commandos trying to stay undetected and silently taking out your defenses.

In a very real way, this makes viral infections much crueler and more insidious than bacteria barging into an open wound.

At this point, maybe a few dozen cells have been infected, but the first countermeasures are already booting up. This early on in the infection there is a struggle between your infected cells that want to alert the immune system and the virus that tries to silence them.

The first real defense of your body against viruses is Chemical Warfare!

So yeah, quorum sensing is pretty cool and bacteria have more than one strategy.

The first step in this struggle for your cells is to be able to realize that they have been invaded.

Your epithelial cells have a bunch of different receptors that scan their own insides for red flags.

something is massively wrong, which triggers an immediate emergency response.

your body desperately needs help from the adaptive immune system to stand a chance of clearing an invasion.

the adaptive immune system is slow and needs a few days to wake

They need to slow down the infection and make it as hard as possible for the virus to spread further to more civilians.

In short, cytokines are the molecules that activate and guide your immune system.

If one of your cells realizes that it is infected by a virus, it immediately releases a number of different emergency cytokines to the cells surrounding it and to the immune system.

Interferons are the ultimate “get ready for a virus” signal.

So one of the first changes is for the cells to temporarily shut down protein production.

So basically just by ordering cells to slow down, interferon slows down the production of viruses considerably.

What is important for you to take away here is that interferons interfere with every step of viral replication.

The influenza virus has adapted to the human immune system and comes prepared. When it unloaded its genetic information to take over the cell, it also came prepacked with a bunch of different viral “attack” proteins. These weapons are able to destroy and block the internal defense mechanism of the infected cells.

even the sneakiest virus will be detected sooner or later.

Plasmacytoid Dendritic Cells.*1 These special cells spend their lives moving through your blood or camp out in the lymphatic network, scanning specifically for signs of viruses—panic interferons from civilian cells or just straight up viruses that float around in your fluids. In any case, if they do pick up signs of a viral infection, they

Plasmacytoid Dendritic Cells are able to detect even subtle signs of their presence and amplify them considerably to sound

the influenza A virus is rapidly gaining ground, spreading throughout your lungs. It now has become a proper infection that is dangerous and that is still not contained.

if they stumble over them, while they release cytokines to call backup and cause more inflammation.

Neutrophils join the fight

Being the unhinged warriors they are, they increase the level of inflammation.

they release another set of cytokines: Pyrogens.

pyrogens are chemicals that cause fever. Fever is a systemic, body wide response that creates an environment that is unpleasant for pathogens and enables your immune cells to fight harder. It also is a strong motivator to lie down and rest, to save energy, and to give your own body and immune system the time they need to heal or to fight the infection.*3

But there are regions of your brain where this barrier is partially penetrable by pyrogens. If they enter and interact with your brain they trigger a complex chain of events that basically cranks up the temperature by changing the internal thermostat of your body.

Still, fever is a serious investment for your body as it costs a lot of energy to heat up the whole system by a few degrees, depending on how harsh your fever is. On average your metabolic rate increases by about 10% for every two degrees Fahrenheit your body temperature

So the increased body heat actually directly slows down the reproduction of viruses and bacteria and makes them more susceptible to your immune defenses.*4

Just overall fever seems to activate the immune system to improve the ability to fight pathogens.

So the complexity of your cells, in contrast to many microorganisms, makes them not suffer from fever but instead work more efficiently.

A proper influenza A infection comes with a whole trove of nasty symptoms. Aside from a high fever, you feel extremely tired and weak, your head hurts, which makes thinking or reading hard, your throat is sore and you have to cough intensely.