As important as your body’s physical barriers are, they can’t repel all invaders. Minor cuts in your skin open a brief, tantalizing entryway to your body’s nutrient-rich interior. Tiny germs can eventually make their way through the thickest mucous coating. And while the acid in your stomach is strong enough to pickle steel, crafty microbes can survive or slip through it to your much more hospitable intestines. (For example, H. pylori, the culprit behind most heartburn-causing stomach ulcers, secretes an acid-neutralizing enzyme that protects the microbe until it gets a chance to worm its way into your stomach walls.)
When a pathogen breaches your body’s first line of defense, the lowly foot soldiers of your immune system meet it within minutes. One of your best defenders is the macrophage (which translates as “big eater”), a swollen blob of a cell that sucks in almost any foreign particle that crosses its path, including dead cells, debris, and pathogens. Once enveloped, a battery of powerful chemicals attacks the foreign particle, destroying it in minutes. A typical macrophage may swallow some hundred bacteria before it dies, finally done in by its own toxic chemicals.
A macrophage is a type of white blood cell. All white blood cells are immune system soldiers—they simply use different tactics.
For its first strike to be successful, your body needs to respond quickly and with overwhelming force. It’s not enough to wait until wandering macrophages stumble across new invaders. Your body needs to summon its defensive forces in a hurry.
Its trick is the inflammation response, your immune system’s call to arms. Your body triggers inflammation when it detects damaged tissue, intense heat, dangerous chemicals, or potential attackers. The first effect of the inflammation response is increased blood flow—your blood vessels dilate and gaps open in your cell walls so the blood can pour into the surrounding tissue. As the blood rushes in, you feel the resulting swelling, as well as pain (because the swollen tissues press on nearby nerves that carry pain signals) and heat (because of the influx of heated blood).
The main goal of the inflammation response is to stock the affected area with your body’s immune system soldiers. In addition, the added blood increases the heat, which spurs your macrophages to work harder and can alter the delicate balance of chemical reactions in the invading pathogens, throwing them off balance. (Your body uses a fever—a sudden spike in body temperature—with much the same effect when battling more stubborn enemies.)
The only time you see your white blood cells is when pus oozes from a wound. This creamy, yellow substance contains the detritus of biological warfare—brokendown tissue cells, living and dead pathogens, and scores of dead white blood cells.
Source of Information : Oreilly - Your Body Missing Manual