Inside the Immune System: Cells, Soldiers, and Strategies

Every second of your life, you are under attack. Billions of bacteria, viruses and fungi are trying to make you their home. So our bodies have developed a super complex little army with guards, soldiers, intelligence, weapons factories and communicators to protect you from, well, dying.

Infection Scenario: The Rusty Nail

It's a beautiful day when suddenly a wild, rusty nail appears and you cut yourself. The first barrier of the immune system is breached, your skin. Nearby, bacteria seize on the opportunity and enter your wound. They start using up the body's resources and double their numbers about every 20 minutes. At first, they fly under the radar. But when a certain bacteria population is reached, they change their behavior and start to damage the body by changing the environment around them.

Macrophages Respond and Initiate Inflammation

The immune system has to stop them as fast as possible. First of all, your guard cells, known as macrophages, intervene. They are huge cells that guard every broader region of the body. Most of the time, they alone can suffocate an attack because they can devour up to 100 intruders. They swallow the intruder hole and trap it inside a membrane. Then the enemy gets broken down by enzymes and is killed. On top of that, they cause inflammation by ordering the blood vessels to release water into the battlefield, so fighting becomes easier. You notice this as a very mild swelling. When the macrophages fight for too long, they call in heavy backup.

Neutrophiles and Their Aggressive Defense

By releasing messenger proteins that communicate location and urgency. Neutrophiles leave their patrol routes in the blood and move to the battlefield. The neutrophiles fight so furiously that they kill health healthy cells in the process. On top of that, they generate barriers that trap and kill the bacteria. They are indeed so deadly that they evolved to commit suicide after five days to prevent them from causing too much damage.

Dendritic Cells and T Cell Activation

If this is not enough to stop the invasion, the brain of the immune system kicks in. The dendritic cell gets active. It reacts to the signals of the soldiers and starts collecting samples from the enemies. They rip them into pieces and present the parts on their outer layer. Now the dendritic cell makes a crucial decision. Should they call for antivirus forces that eradicate infected body cells or an army of bacteria killers, in this case, antibacterial forces are necessary. It then travels to the closest lymph node in about a day. Here, billions of helper and killer T cells are waiting to be activated. When T cells are born, they go through a difficult and complicated training process, and only a quarter survives. The surviving cells are equipped with a specific setup, and the dendritic cell is on its way. Looking for a helper T cell with a setup thats just right. Its looking for a helper t cell that can bind to the parts of the intruders which the dendritic cell has presented on its membrane. When it finally finds one, a chain reaction takes place.

Helper T Cells and B Cell Activation

The helper T cell is activated. It quickly duplicates thousands of times. Some become memory T cells that stay in the lymph node and will make you practically immune against this enemy. Some travel to the field of battle to help out. And the third group goes on to travel to the center of the lymph node to activate a very powerful weapons factory. Like the T cells, they are born with a specific setup. And when a B cell and a T cell with the same setup meet, hell breaks loose. The B cell duplicates rapidly and starts producing millions of little weapons. They work so hard that they would literally die from exhaustion very fast. Here, helper T cells play another important role. They stimulate the hard working factories and tell them, don't die yet. Still need you. Keep going. This also ensures that the factories die if the infection is over, so the body doesn't waste energy or hurt itself. But what is produced by the B cells? You've heard of them, of course. Antibodies, little proteins that are engineered to bind to the surface of the specific intruder. There are even different kinds of antibodies that have slightly different jobs. The helper t cells tell the plasma cells which type is needed the most. In this particular invasion, millions of them flood the blood and saturate the body. Meanwhile, at the site of infection, the situation is getting dire. The intruders have multiplied in number and start hurting the body. Guard and attack. Cells fight hard but also die in the process. Helper T cells support them by ordering them to be more aggressive and to stay alive longer. But without help, they cant overwhelm the bacteria. But now the second line of defense arrives. Billions of antibodies flood the battlefield and disable lots of the intruders, rendering them helpless or killing them. In the process, they also stun the bacteria and make them an easy target. Their back is built to connect to killer cells, so they can connect and kill the enemy more easily. Macrophages are especially good at gnoming up the bacteria, which antibodies have attached to. Now the balance shifts.

Conclusion 

In a team effort, the infection is wiped out. At this point, millions of body cells have already died. No big deal. The losses are quickly replenished. Most immune cells are now useless, and without the constant signals, they commit suicide so as not to waste any resources. But some stay behind the memory cells. If this enemy is encountered ever again in the future, they will be ready for it and probably kill it before you even notice. This was a very, very simplified explanation of parts of the immune system at work. Can you imagine how complex this system is, even at this level, when we ignore so many players and all the chemistry? Life is awfully complicated. But if we take the time to understand it, we always encounter endless wonders and great beauty.