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

by Philipp Dettmer

The majority of pathogenic viruses enter your body via your respiratory mucosa.

we somehow collected them and laid them end to end, they would stretch for 100 million light-years—as many as 500 Milky Way galaxies put next to each other.

The parts of your lungs that do the actual breathing have an enormous surface area, in excess of 145 square yards (120 square meters)—more than sixty times the surface of your skin.

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

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 and nothing else. Talk about an exposed area. A perfect target for all sorts of pathogens.

To keep the area safe a very special type of Macrophage is stationed here: The Alveolar Macrophage

they downregulate other immune cells like Neutrophils and make them less aggressive. But most importantly, they tone down any sort of inflammation.

Influenza A has specialized in infecting the epithelial cells of the respiratory system in mammals.

They have only a few hours to reach their goal because the environment of the swamp kingdom is slowly but surely destroying them.

Your epithelial cells, the “skin” of your insides, have receptors on their surfaces that the influenza A viruses can connect to and manipulate to enter the cells.

It takes the virus only about an hour to gain control over the cell by conquering its natural processes.

Within ten minutes the influenza tricks the cell into delivering its genetic material directly into the brain of the cell, the nucleus. Viral proteins begin to dismantle the cell’s internal antivirus defenses, and with that, the cell has been conquered.

The influenza A virus is trying to directly take over the nucleus, which is the brain of your cell, if you will. It stores the DNA, which carries the instruction manuals for all the proteins of the cell, but not just blueprints but also their production cycles.

To communicate the information stored in a gene to the rest of the cell, living things use RNA

The influenza virus A, for example, just dumps a number of RNA molecules into the nucleus, where it pretends to be commissioned from your own genes and tricks the cell into building specific viral proteins.

According to some estimates, a single cell infected by influenza A, is, on average, able to produce enough viruses to successfully infect twenty-two new cells before the first victim cell dies from exhaustion after a few hours.

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.

At the height of a virus infection, you can have billions of viruses inside your body. All of these special properties require your immune system to defend against them differently than against most bacteria.

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

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

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 up,

your innate immune system and the infected civilian cells need to fight for the most valuable thing in the universe: Time.

the way your cells do this: chemical warfare.

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

There are a lot of different cytokines that are released in this situation and they do a lot of different things but here we want to highlight a very special class: Interferons

So when cells pick up interferon molecules, it triggers different pathways that make them change their behavior drastically.

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

Interferons will seldomly eradicate an infection by themselves, but they don’t have to. All they have to do is slow down the multiplication of new viruses by making cells in the vicinity much more resistant to the virus infection.

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.

Two very different enemies, a virus and a human cell, and both are struggling with each other for time.

The more cells the virus infects, the more civilians will be able to activate chemical warfare,

chemical warfare triggers the next step on the escalation ladder of the antivirus section of your Innate Immune System: Plasmacytoid Dendritic Cells.

if they do pick up signs of a viral infection, they activate and turn into chemical power plants that ooze out extreme amounts of interferons, alerting not only civilians to turn on their antiviral modes (shut down protein production, etc.) but also the immune system to activate and get ready for a proper fight.

your immune system is often the part that causes the largest amount of damage in an infection and influenza is no different. All the unpleasant things you are about to experience are the result of the desperate attempts to halt the brutal invasion of your lungs.

Neutrophils seem to not really be able to fight viruses well, so their help is mostly passive: Being the unhinged warriors they are, they increase the level of inflammation.

Your soldier cells realize that they are dealing with a virus infection and that they need help on a larger scale, so 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.

there are regions of your brain where this barrier is partially penetrable by pyrogens.

Your brain cranks up the heat in two main ways: For one, it may generate more heat by inducing shivering,

And by making it harder for this heat to escape by contracting the blood vessels close to the surface of your body, which reduces the heat that can escape through your skin.

your metabolic rate increases by about 10% for every two degrees Fahrenheit your body temperature rises, which means that you burn more calories just to stay alive.

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

Certain chemical reactions between proteins have a sort of optimal zone, a temperature range in which they are most efficient. By increasing the temperature in the body during fever, pathogens are forced to operate outside this optimal zone.

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