MOBILIZING A MICROSCOPIC ARMY
Have you ever noticed your skin begin to swell a bit shortly after you receive a cut? That is your immune system going to work!
Beneath the skin, the immune system has “sentries,” like soldiers standing at the wall of a city. When the city—your body—has been breached by invaders, the soldiers go to work! The immune system sends signals to increase fluid in the affected tissues and draw white blood cells and other defense mechanisms to the site to do battle with the invaders. Your swollen skin means that the enemy has been engaged!
The chemical reactions and transformations of the immune system in this battle are remarkable. For instance, one chain of reactions creates a molecular machine that literally drills a hole into invading cells, causing them to burst and die. Other white blood cells destroy invaders through chemical warfare, such as by producing hydrogen peroxide. Researchers have even discovered that some white blood cells will enclose the unwelcome microbe within a small container and then flood the container with naturally produced bleach!
As with an invaded city, while the initial soldiers may slow the advance by engaging the attackers, that does not prevent more invaders from continuing on through the breach. As the microscopic warfare continues, the immune system escalates its efforts in both power and precision.
IDENTIFYING THE ENEMY
This happens when the immune system recognizes the exact identity of the attacker. Is it the H3N2 influenza virus? Is it the Streptococcus pyogenes bacterium?
By some estimates, the immune system has a functional library of ten billion different types of potential threats. Describing how the system creates this library is beyond the scope of this article, but suffice it to say that this number is believed to be greater than the number of threats that exist on earth! For example, scientists researching the robust nature of the immune system in mammals injected a couple of man-made molecules into some animals, knowing that such molecules did not exist naturally and had never been encountered by the animals before. They learned that the animals’ immune systems already had a response specifically designed to counter those molecules!
So how does the immune system recognize its attackers?
When the “first responder” cells are fighting the beginning of the infection, special cells in that initial wave not only destroy invaders, but also take them apart and then transport the pieces of the invader to the body’s lymph nodes, where they “present” the broken-up pieces of the enemy microbe to other cells in the immune system. Those defensive cells that are specifically prepared to combat that exact invader are then activated, and more cells—as well as special proteins called antibodies—that are all specific to that pathogen can be mass produced.