The Immune System: An Army Inside You
Editor's note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that's because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, "The Designed Body." For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.
The body is made up of matter organized into trillions of cells that make up its tissues and organs. Since all matter must follow the laws of nature, this means that the body must do the same. In earlier articles I have shown that the body must overcome the laws of nature to survive. The sodium-potassium pumps, for example, are needed to allow each cell to control its volume and chemical content by resisting diffusion and osmosis. There must be enough albumin in the blood to resist the natural force of hydrostatic pressure and maintain blood volume and flow to the tissues. The sympathetic nervous system must increase the cardiac output and peripheral vascular resistance to elevate the blood pressure sufficiently to counteract gravity when we stand up.
With the emergence of life, not only did the cellular and organic make-up of the body require specific innovations to overcome the laws of nature and survive, it also had to learn how to deal with what it encountered in its environment. Life does not take place in a vacuum or in the imaginations of evolutionary biologists. Hemostasis and the clots it forms allow the body to prevent itself from bleeding to death when it is bumped, scraped, or cut. And the bones, muscles, and nerves work together so the body can detect danger and avoid or defend itself from it.
In my last article I showed that the body is always being exposed to microorganisms, such as bacteria, viruses, and fungi, which are present in nature but are too small to see with the naked eye. If these microbes invade the body and become widespread, they can cause a lot of damage. We saw that the first line of defense against infection is the skin and the epithelial tissues that line the respiratory, gastrointestinal, and genitourinary tracts.
If microbes breach these passive barriers and enter the tissues, the second line of defense swings into action. This is called the immune system, and it consists of numerous different cells and proteins that work together to fight and usually defeat the invading force. For our earliest ancestors to survive long enough to reproduce, they would have needed this two-pronged defense. Neither the passive barriers that protect the underlying tissues, nor the immune system, is capable, on its own, of protecting the body from life-threatening infection. They both have to be present and in working order .
In ancient times, when invaders penetrated the surrounding protective wall of a town, the defenders generally had four important tasks to perform very quickly. The first was to detect and positively identify the enemy. The second was to sound the alarm so others could help join in the defense. The third was to provide information on the enemy to those in reserve. And the fourth was to repel, wound, or kill the intruders to protect the residents. Similarly, once microbes get past the epithelium and penetrate into the tissues below, the body's immune defense must have the ability to perform these same four important tasks as well.
The first requires that the cells and proteins of the immune system have a way of detecting the presence of the microbes and be able to identify them as an invading force that needs to be destroyed. In other words, are these cells host cells (self), or foreign cells (not self)? The job of the immune system is to kill invading microorganisms, so it had better be sure that what it's encountering is indeed foreign and in need of destruction, otherwise it may end up killing its own cells by friendly fire. As with hemostasis, it's important that the immune system only turn on when it's needed and turn off and stay off when it's not.
After determining that there is a microbial invasion going on, the second task of the immune system is to send out messages so that it can bring other forces to the field. This involves releasing chemicals that not only increase the blood flow to the site of infection, allowing immune cells and proteins to leak out of the blood through the capillaries, but attracts them to the battlefield as well. This causes the area around the infection to swell up and become red -- what we call inflammation.
In addition to rallying the troops, the third task of the immune system is to provide information about the whereabouts and nature of the enemy to those in reserve. This is accomplished by some of the first responder immune cells snipping off pieces of the dead microbes and sending them to the forces in reserve so that they can better prepare for what's awaiting them.
Finally, once the weapons of the immune system have been brought to the site of infection, it's up to them to wound or kill the invading force to prevent the infection from spreading further. The immune cells and proteins involved have many different weapons at their disposal to accomplish this task.
As with most military operations, the body's immune system has regular and specialized forces. The regular forces make up what is called the innate (natural) immune system. It's the microbial defense system with which everyone is born and it is the first to encounter the enemy, reacting within minutes. But this system, on its own, is usually not able to protect the body from overwhelming infection. Many pathogenic microorganisms have the ability to remain invisible and resistant to its strategies, allowing them to proliferate and spread throughout the body.
The specialized forces are usually needed to bolster and improve the effects of the innate immune system. Together, they are called the adaptive (acquired) immune system. This system usually requires a few days to adjust to the idiosyncrasies of the invading microbes. But when it swings into action, it provides extra intelligence, firepower, and precision accuracy that usually allow it and the innate immune system to get the job done. In contrast to the innate immune system, which is present at birth, the adaptive immune system develops over time as the body is exposed to more and more different microbes in its environment.
Now that you have a general idea of how the immune system works, we will press on. Next time, we'll look at the first responders of the innate immune system and how they do their jobs. Comparing how our immune system works to a military exercise in which an enemy must be tracked down, identified, and destroyed is a very accurate analogy.
Evolutionary biologists usually point to the ability of microorganisms to develop resistance to the body's immune system and medical therapies through genetic modification as evidence that life came about by chance and the laws of nature alone. However, this assumes the presence of the hardware needed not only to survive, but also to reproduce. Once you have the system in place, it's obvious that life can change over time, which is all that the word evolution denotes. However, the ability of life to change over time doesn't necessarily mean, as evolutionary biologists suggest, that it came about by chance and the forces of nature alone. One need only consider what it takes for the body to stay alive to recognize that important truth.
Image credit: � kaninstudio / Dollar Photo Club.