How does human immune system work against a virus or bacteria?

The human immune system is the most complex biological system we know, after the human brain and yet most of us never learn how it works. Or, what it is.

Your immune system consists of hundreds of tiny-and two large-organs. It has its own transport network spread throughout your body. Everyday, it makes hundreds of billions of fresh cells organized like an army. With soldiers, captains, intelligence officers, heavy weapons, and crazy suicide bombers. It’s not some sort of abstract entity. Your immune system is you. Your biology protecting you from the billions of microorganisms that want to consume you, and from your own perverted cells that turn into cancer.

In this article: What happens when your body is invaded and your first lines of defenses are engaged in a fight for life and death?

It’s been a normal day, when suddenly the world explodes and an asteroid rips the sky open. Countless aliens life forms invade, ready to destroy cities and infrastructure, and eat civilians… Or, this is what your cells experience. You look at you bleeding thumb that you just cut on a dirty twig in the park. How annoying! But inside the wound, a horrible catastrophe has happened. There are dead cells, blood and dirt everywhere. Even worse, countless bacteria invade the warm caverns between your helpless cells to explode their new home, steal your resources, and poop everywhere.

Immediately, the first stage of your defense kicks in. The cells that survived the impact, or are hurt or dying, scream in panic releasing an onslaught of chemical alarm signals that awaken your immune system.

The First Line of Defense

The first cells to show up are macrophages. If an average cell were the size of a human, a microphage would be the size of a black rhino. A stoic cell in principle but you wouldn’t want to annoy it. Bacteria DO annoy them. Within seconds, the large cells attack and begin killing them without mercy. They stretch out parts like the arms of an octopus, and grab the bacteria to swallow them whole and digest them alive. A macrophage can eat 100 bacteria before it’s exhausted. But there are too many enemies, so the macrophages call for reinforcements.

Suicide Warriors

In your blood, hundreds of thousands of neutrophils pick up their signals and move to the battlefield. Neutrophils are intense suicide warriors that only live to kill. They are so enthusiastic about killing that they kill themselves a few days after birth, so they don’t have time to accidentally destroy your body from the inside. As soon as neutrophils arrive, they begin vomiting deadly chemicals at bacteria, or devour them. They are so careless in their attacks that they are causing real damage to your own cells. But collateral damage is not their concern now, or ever. Some neutrophils go so far to push their suicide button and explode, casting a wide and toxic net made from their own DNA filled with deadly chemicals that trap and kill bacteria. Sometimes they can continue fighting after that, even though they are sort of dead already. This is how much fun they have killing!

While the battle rages, your blood vessels let fluid stream into the battlefield like a dam opening up towards a valley. You notice this as inflammation. Your thumb swells up a little and gets red and warm. The fluid bring a silent killer into the battle zone, millions of complement proteins. A sort of automated liquid weapon that stuns and kills bacteria by ripping holes into them.

We are reaching a crossroad now. If things go well, your first line of defense kills the invaders quickly. But sometimes, the enemies are too strong and would overwhelm your defenses eventually, which means certain death for you, the human.

Intelligence Officers

This is the hour of the dendritic cell, your immune system’s intelligence officer. While your soldiers were bashing in heads, it was collecting samples by ripping bacteria into tiny parts and covering itself in it. Like a soldier decorating itself in the guts of a dead enemy.

The cell leaves the battlefield and enters the superhighway of your immune system that connects all your tissues with your immune headquarters.

Your Lymph Nodes

The dendritic cell coming from the battlefield is looking for a helper T-cell, which is a sort of all-purpose commander cell within your immune army. But not any helper T-cell. One that happens to have just the right weapon for the bacteria that infected your wound. So it goes around and rubs itself, still covered in bacteria parts, against every helper T-cell it meets. Most T-cells are a bit disgusted and not interested. But after a few hours, something clicks.

A helper T-cell recognizes the bacteria parts. This cell is the weapon that’s needed right now. The dendritic cell is overjoyed and activates the helper T-cell.

Okay, wait. How come your immune system has a cell that has a weapon against the specific bacteria that infected you?

Well your immune system has a perfect weapon against every possible disease in the universe. Against the Black Death, the Coronavirus, or an infection that will emerge in 100 years on Mars.

How you have billions of unique helper T-cells that each have weapons against every possible disease?

After the right T-cell is activated, your second line of defense awakes and rises like a teenager that needs to get up on a school day…very slowly.

The Second Line of Defense

Your heavy weapons are incredibly effective but they aren’t fast. The activated helper T-cell begins to clone itself over and over again. One becomes two, two becomes four, until there are thousands of them. Now they split into two groups.

1. The first group quickly moves to help out your soldiers. At the battlefield, things are getting out of hands. A tired macrophage is ready to give up. After fighting for days, it just wants to go to sleep like many of its buddies have done already. But now the helper T-cell arrives. One of them comes to the tired macrophage and whispers something using special chemical signals. In a heartbeat, the demoralized soldier feels fresh again. But there’s something else. A hot white anger.The macrophage knows what it needs to do… kill. Invigorated, it throws itself against the enemies once again. All over the battlefield, this begins to happen.
2. Meanwhile, the second group of helper T-cells was working on activating another line of defense : B-cells, your antibody factories. Antibodies are protein super weapons that look like tiny crabs with two pincers to grab enemies. Just like the helper T-cells, there are B-cells in your body that are able to make just the right antibodies for every possible enemy. And the helper T-cell is looking for exactly these B-cells. After a day or two, the right B-cell is found, and begins to clone itself. As soon as enough clones have been made, each B-cell begins pumping out up to 2,000 antibodies per second! About a week after you injured yourself and bacteria invaded, your second line of defense finally arrives in full force.

The tiny army begins to saturate the battlefield, pinching and stunning desperate bacteria. The antibodies clump them together and make them unable to move or fight, while your soldiers massacre the defenseless victims. The tide is turning fast. As the last enemies are cleaned up, your soldiers realize that they are no longer needed, and begin to kill themselves to save resources. But not all of them. A few helper T-cells remain and turn into memory cells. They will guard the tissue for years, making sure the same bacteria will never again gain a foothold here. Similarly a few B-cells will stay alive and keep producing a low amount of antibodies, making you immune against the bacteria, maybe for the rest of your life.

One day you’ll wake up and notice that wound has grown over and left noting but a faint red mark. You were completely unaware of the drama your cells had to deal with. For you, the whole ordeal was a slight annoyance. While for millions of cells, it was a desperate fight for life and death.