The Immune System: What happens when you get a small cut?
We know that our blood contains white blood cells. And we probably heard of macrophages and T-Cells. But the immune system has so much more. The book Immune from Philipp Dettmer, the author of Kurzgesagt channel, will provide insights into the beautiful mechanisms of our immune system. This article is my summary of our immune system, what they are and how they protect our body.
The immune system can be divided into the Innate immune system and the Adaptive immune system.
You get a small cut
Getting a cut may be a bit annoying to you. With just a small bandage and you can continue doing your things. But inside you, a war has started! Cells got killed. bacteria pour into the wound. Alarm everywhere! Immune cells immediately move from blood to the battleground. They are fighting depressively, trying their best to win the war. To protect you!
Innate immune system
Macrophages and neutrophils, belonging to the innate immune system, are the first guys getting from the blood to the wound. Macrophages swallow bacteria as a whole and break them into smaller parts to be recycled by our body. While Neutrophils are like crazy suicide warriors. They eat the enemy, throw acid at them and blow themself to create traps - a net of their DNA to prevent the enemy from moving farther.
From the blood, how do they know which way to go?
How do cells communicate that they need help? By using cytokines. It’s a chemistry to alert immune cells and gets the first immune cells to come. As the battle continues, more cytokines are released by cells, macrophages, and neutrophils to call for more help. And how can they tell which way to move? As cytokines are flooding the wound and spread, the farther immune cells can feel the difference in the concentration of cytokines from different directions - which may be just 1% higher. But that’s enough to communicate the direction. This way! Go! Go!
How do immune cells know which are enemies and which are you?
By having receptors on their surface. The innate immune cells have receptors that can bind to some recognized parts, or antigens, of the bacteria. This way, our immune cells can recognize enemies, pull them, and kill them. But there are a lot of bacteria out there. They can evolve to have trillion-trillions of different parts. Actually, there are some common and necessary parts that they can’t get rid of. Similar to how a car is built - whatever type of car you intend to build, you will always need wheels. The same for bacteria. Many bacteria have flagellum, long proteins that attach to the bacteria and help them move forward. Our innate immune cells have receptors for these kinds of parts, immediately recognize them, and kill the enemy.
The complement system
While blood is pouring into the wound, our immune system has another weapon: the complement system. They consist of over 30 proteins, which work together to stop the enemy. They cripple the enemy, rip holes in them, and activate the immune system. While macrophages are fighting their best, the bacteria run away for survival too. They are fast, slippery, and hard to catch. But now thousands of the complement system proteins attach to them, slow them down, and make them tastier for the macrophages to eat.
If the innate immune system wins, most bacteria get killed. No more signals from the enemy’s antigen, the exhausted macrophages and neutrophils will start killing themself. To save energy for your body and leave the battleground for other cells to repair. You recover.
But things are not that easy. Among these bacteria, there is a pathogen sneaking into your body.
As a pathogen, it has evolved to evade the complement system. With the abundant resources from your body, they start multiplying quickly. More and more immune cells have died, but they still can not stop the invader. The adaptive immune system must come into play.
Adaptive immune system
The process starts with dendritic cells. Dendritic cells are highly specialized antigens representing cells. They are the bride between the innate immune system and the adaptive immune system. When the battle is happening, they move from the blood to the battleground and start collecting samples from bacteria for a few hours. These are parts from bacteria, broken into individual proteins. Then these antigens are represented on the dendritic cells’ surface.
Each T-Cell has a specific receptor to recognize a specific antigen. There are many classes of T-Cell, the most important one is helper T-Cell. They constantly move through the whole body, looking for something that looks like the antigens presented by dendritic cells. When they find an antigen that matches their specific receptors, they activate and get to work. They now multiply and make a giant army. They split into 2 groups. One group of helper T-Cell move to the battleground. They meet macrophages and tell them to be stronger, and fight harder. With the appearance of helper T-Cells, these exhausted macrophages start fighting with even more anger. The second group of helper T-Cell moves to the lymph nodes and tells the B-Cells to start making antibodies.
to be continue