How the immune system fights bacteria infection

December 20, 2016

Every second of our life we are under attack, millions of virus, bacteria and fungus are trying to make us their home. Our bodies have developed a super complex army with intelligence, soldiers, guards, weapons, and communications to protect us from attacks that can lead to death.

There are many functions and cells involved in the entire process of the immune system, but to better understand its work when fighting bacteria only ten different cell types and their function will be explained.

Whenever bacteria find their way inside our body, they use the body resources and double their numbers on average every 20 minutes. At first, they are undetected, but when a certain bacteria population is reached, they change their behavior and start damaging the body by altering the environment around them. When this happens, the immune system becomes aware of the invasion and starts to activate the first guard cells, the macrophages. These cells are big 21 micrometers long cells that are scattered across the body and are ready to attack any invasion they can detect. Macrophages are able to devour up to 100 intruders each and most of the times they are able to stop an attack by their own. Also, they control inflammation by ordering the blood vessels to release water in the infected area so fighting becomes easier.

When macrophages are fighting for too long they call in backup by releasing message proteins that will communicate urgency in that location. Upon activation, neutrophiles, leave their patrol routes in the blood and move to the site of infection. Neutrophiles fight so hard that during the process of killing bacteria, they also attack and kill healthy cells. Moreover, they are able to create barriers that can trap and kill the bacteria. They are so deadly that they evolve to commit suicide after five days, to prevent causing too much damage to the host.

If the invasion has not been stopped by now, then another type of cell gets activated, the dendritic cells. These cells collect dead bacteria, rip them in pieces and present their parts on the outer layer. At this point the dendritic cell makes a crucial choice, activating the defenses against virus, by recruiting cells that kill infected body cells, or activate against bacteria, by recruiting cells that are expert in fighting bacteria. In this infection, bacteria killers are necessary.

Dendritic cells will travel to the closest lymph node, which will take around a day, where billions of helper and killer-T cells are waiting to be activated. T cells are trained after birth in a very difficult and complicated process that only a quarter of T-cells are able to survive. Those surviving cells are equipped with a specific setup. The dendritic cell is looking for T-cells with a setup that is able to combine perfectly the parts of the intruders that are being presented on their membrane. As soon as one is found, a chain reaction takes place. Upon getting activated, the T-cell quickly duplicates thousands and thousands of times. Some of those cells become memory cells and stay in the lymph node allowing us to become particularly immune to that enemy in case it attacks in the future, others will travel to the site of infection and help with the fight and the remainers ones travel to the center of the lymph node to activate the B cells. The activated B cells not only duplicate very fast, but also produce millions of weapons called antibodies to fight infection. They produce so many that it would kill them out of exhaustion. The helper T cell stimulates the B cell to prevent it from dying of exhaustion while the infection is still active. Once the infection is over the B cells will also die to prevent the body to waste unnecessary energy.

The antibodies produced by the B cells are small proteins engineered to bind to the surface of the intruder. Millions of antibodies flood the blood and saturate the body until reach the site of infection, disabling lots of bacteria rendering them helpless or killing them in the process. Antibodies also stun the bacteria making them an easy target for killer cells, like macrophages, to attach and eliminate.

The combined actions of these cells are able to wipe out the infection. All the body cells that were killed in the process are rapidly replaced and most immune cells that are now useless commit suicide so resources are not wasted.