By Georgia Purdom ♦ Illustrated by Chad Frye
Your body is a war zone. Every time your skin is cut, millions of invaders can invade the tissues and cause disease. To combat these threats, a horde of defenders patrol your bloodstream.
When you cut your skin, millions of bacteria and other hostile invaders may take residence in your body. To fight these intruders, your body unleashes a horde of first-line defenders -- billions and billions of white blood cells -- which patrol your bloodstream looking for these invaders.
Each patrol is equipped with ultra-sophisticated technology to stay in constant contact with your body’s other cells. Once alerted to a threat, a patrol will anchor to a spot near the danger, then slip into the tissue beneath the blood vessel lining and eliminate any foreign bacteria hiding there.
The complexity of the cells’ mission is beyond imagination, yet each cell is fully armed to perform its varied duties. Consider just one type of white blood cell, called a neutrophil -- the most common foot soldier in the body’s vast army.
Unlike the red blood cells that rush through the bloodstream, neutrophils "roll" methodically along the cells lining the blood vessel walls. They "cling" despite the violent rush of the flowing blood, constantly patrolling for signs of trouble.
Zooming in to the Surface of the Neutrophil Cell . . .
The neutrophils are covered with strings of molecules that latch like Velcro onto molecular hooks that line the blood vessel walls. Each cell rolls along slowly, snagging and ripping free of the hooks as it goes.
The neutrophils are specifically looking for chemical signals on the blood vessel walls that were sent from the underlying body tissues. These chemicals (called chemical attractants) warn that bacteria have infected the tissue. When the netutrophil encounters a chemical attractant, it binds to a receptor on the neutrophil’s surface, like a key in a lock.
"Stop and help us!" The infected tissue transmits a desperate plea to the neutrophil. Then, like a row of dominoes falling, a chain reaction occurs inside the neutrophil, sending a message to the cell’s nucleus.
Inside the Neutrophil Cell . . .
On its way to the nucleus, the chemical message passes an assortment of factories, machinery, and structures inside the neutrophil. Once it reaches the nucleus, where DNA is stored, the cell goes to work.
The nucleus sends out information for the cell factories to manufacture a protein that will bind the neutrophil tightly to the trouble spot. Like a high-speed copy machine, the nucleus makes a copy of the DNA instructions to make a protein, called integrin.
The copy of DNA instructions is called messenger RNA (mRNA). The nucleus spits this string of information out through holes, or pores, in the nucleus. The mRNA contains information for the cell’s factories to manufacture the binding protein integrin.
Outside the nucleus are miniature machines, called ribosomes. One of these machines latches onto the mRNA and reads part of the mRNA instructions to produce a small portion of the integrin protein.
Next, this portion of the integrin binds to another protein, which transports the whole ribosome to a cell factory, called the endoplasmic reticulum or ER. There it docks and completes production of the integrin. Like a skilled machinist, the ER then adds final modifications to this high-tech product.
The integrin proteins then gather into cargo vessels, or "vesicles." These cargo vessels are "pinched off" at the surface of the ER, ready for transport to the surface.
A motor protein is then attached to each newly made cargo vessel. These molecular motors (such as kinesins) literally "walk" their cargo to the neutrophil cell’s surface.
The motors travel on sophisticated superhighways, called microtubules, which crisscross the interior of the cell.
Back on the Surface of the Neutrophil . . .
Once at the surface (called the cell membrane), the cargo vessel fuses with the membrane surrounding the surface of the cell, emptying its contents at the outside of the cell.
The integrin proteins remain on the outer surface of the cell membrane and cluster together on rafts, ready to bind with the blood vessel.
The integrin proteins now change shape and grab proteins, called I-Cam, which extend down from the blood vessel wall. This causes the neutrophil to bind tightly to the blood vessel.
The Neutrophil Now Leaves the Blood Vessel to Fight Infection . . .
Once it binds tightly, another chain reaction occurs inside the neutrophil. To reach the infected tissue, the cell must become very flat and slip between the cells lining the blood vessel (called extravasation).
The cell sends out a flurry of instructions to disassemble its many interior parts. Like Mr. Fantastic, the neutrophil changes shape. First the cell quickly modifies its inner skeleton and forms a foot-like structure (called a lamellopodium).
Then the rest of the skeleton begins collapsing, like a tent pulled down at the end of a circus. The trailing end follows the foot through the space between the cells.
Once the migration is complete, the cell restores its full, three-dimensional shape. Now it’s ready for battle.
The neutrophil is the most common type of white blood cell. It kills bacteria by "eating" them. First, the cell sends out extensions, known as pseudopods, which surround and engulf the bacterium (a process called phagocytosis). Once a bacterium is trapped inside, the cell produces enzymes to digest it.
Neutrophils can eat several bacteria before they die. The pus that you find at the site of an infected wound is composed of dead neutrophils, which have sacrificed themselves to protect the rest of the body.
Dr. Georgia Purdom is a speaker and researcher for Answers in Genesis. She earned her doctorate from Ohio State University in molecular genetics and spent six years as a professor of biology at Mt. Vernon Nazarene University. Dr. Purdom is also a member of the American Society for Microbiology and the American Society for Cell Biology.