How does your immune system work?

In your body, the first soldier on the ground you need to know is a type of white blood cell called the macrophage. When your body spots some type of intruder such as a bacterium or virus, the macrophage moves in, finds the offender, engulfs, and digests it - sort of the way a soldier would take a prisoner of war hostage. But the macrophage isn't equipped with all the tools necessary to finish the job, so it radios for backup; that's when other helper cells arrive to assist in the operation.

While the macrophage waits for the other immune cells to speed their way through the bloodstream, it takes notes about the intruder, or antigen: who it is, what it is, and any identifying characteristics. That's important, right? Your body needs to be able to recognize this bad boy the next time he invades your territory, so it can respond quickly and flush it out of your system.

Now, back to the action. Thankfully, reinforcements arrive in the form of other white blood cells known as T cells and B cells. Both play a role in actually killing the invader. The T cells, which in infants mature in an organ called the thymus, directly attack the invader (think hand-to-hand combat). The B cells create chemicals called immunoglobulin antibodies that act like bullets against the foreign substance. Die, antigen! After a successful battle, the foreign cells do, in fact, die. But the interesting part here is that the T and B cells die as well, in a cellular process that's called apoptosis, or programmed cell death. The reason? If they didn't die, the T and B cells could attack healthy cells after they've finished their primary job - sort of like soldiers suffering from post-traumatic stress disorder who may confuse wartime with peacetime.

This process typically works just fine, especially in the case of very simple and straightforward bacterial and viral infections. When the immune system tackles an invading agent such as a virus, the invader triggers body reactions that cause symptoms, then the immune system fights off the invader and ends the symptoms. Say, for instance, that a cold virus attaches to respiratory cilia: little hairs that serve as mini street cleaners in the respiratory system. When the immune cells are alerted to the problem, they start an inflammatory process that forces the nose faucets to turn on in the form of a runny nose. Then, as the immune cells do their job and kill the offender and then kill themselves, we see the remnants of the battle: more snot.

Your body's cells are a little like your taste buds in that they know exactly what they like and what they don't. For example, when introduced with staphylococcus—a common and quick growing bacteria that causes pimples—your body recognizes it as a foreign substance, just like a guard would notice an unwanted intruder on a surveillance camera.

When it spots the intruder, a type of white blood cell—called a macrophage—finds the bacteria, engulfs it, and digests it. Assuming there are other intruders, the macrophage gets on its walkie-talkie and calls for back-up. The message—the chemical equivalent of "Help! Help! Intruder! Pimple forming on tip of nose! Prom's tomorrow night!"—is an SOS to other cells so they'll immediately respond to the area by traveling through the blood stream (that's why scabs are red—it's the additional blood supply).

At the same time, the macrophage records info about the foreign cells so that the immune system can recognize it. As the back-up immune cells arrive on the scene of the infection, they'll look for the code—that mug shot—to identify and obliterate harmful bacteria.

The immune cells attack in different ways. The "T" cells directly attack and digest the offending bacteria, while the "B" cells create immunoglobulins that act like bullets and blast through the covering of the bacteria.

Finally, that battle leaves the bacteria dead, but it also kills the T and B cells, leaving a pool of waste behind (it's called apoptosis—they self-destruct so they don't destroy healthy cells).

This pool of waste builds up pressure under the skin, creating redness from new blood supply lines coming to battle and pus from the white blood cells responding to the scene. Like a volcano, the waste material rises to the surface to form the pimple.