The Immune System: The Body's First Responders
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.
In the last two articles in this series, I pointed out that besides following the laws of nature, the body must also protect itself from its environment. In particular, the body is constantly exposed to microorganisms that can cause serious infection.
The body has a two-pronged defense strategy to deal with invading microbes. The first is the skin and the epithelial tissue, which lines the respiratory, gastrointestinal, and genitourinary tracts and acts as a passive barrier to entry. The second is the immune system, which consists of many different cells and proteins that work together to identify and destroy the invading microbes. My last article showed that the immune system can be divided into two categories; the innate immune system that each of us is born with and the adaptive immune system which develops over time as we are exposed to the environment. Each of these systems has its unique cells and proteins, which are needed for the body to defend against microbial invasion. Now let's look at the first responders of the innate immune system and the role they play in helping to keep the body alive.
In the old days, the first responders for a walled town defending itself from invasion were the sentries. They basically had four jobs to do; identify the enemy, sound the alarm to bring more soldiers to the battle, provide strategic information about the invading force, and repel, incapacitate, or kill the intruders. Together, the first responders of the innate immune system perform these same four important tasks.
The sentries had to be able to listen for intruders and see them clearly enough to positively identify them as being an enemy. Moreover, their fellow defenders had to be able to listen for a call to arms and see where to go to engage the enemy. Furthermore, the defenders of a city ultimately needed to use all of their wits and motor skills to preserve their lives, their freedom, and their property.
Rather than having eyes to see with and ears to hear, the cells of the body interact with chemicals and other cells, both foreign and domestic, through the plasma membrane. The surface of the plasma membrane contains thousands of different molecules that relate to its structure and function. Moreover, to detect a specific set of chemicals on the surface of another cell the plasma membrane must have a specific receptor. These receptors consist of protein molecules that contain a specific grouping of chemicals, resulting in a specific, three-dimensional shape that allows them to chemically bind, like Velcro, to the molecule being detected.
Pathogenic (disease causing) microbes have specific chemicals on their surface that relate to their structure and function, called pathogen-associated molecular patterns (PAMPs). The first responder cells of the innate immune system have specific molecular structures on the plasma membrane that are able to detect these PAMPs, called pattern-recognition receptors (PRRs). It is estimated that the immune cells of the innate system can identify about one thousand different PAMPs. Since these PAMPs only occur on microbes and not on human cells, these specific receptors (PRRs) allow the cells of the innate immune system to identify invading microorganisms as foreign and the ones to be destroyed.
As noted previously, when target cells, like the ones lining the tubules in the kidney or the muscles surrounding the arterioles, lock on to a specific hormone or neurohormone, this triggers some action. Similarly, when the first responder immune cells of the innate system use their specific receptors (PRRs) to lock on to the specific chemicals on the surface of the invading microbes (PAMPs), they activate. This activation triggers them to identify the enemy, sound the alarm to bring more defenders to the battle, provide strategic information about the invading force, and repel, incapacitate, or kill the intruders. Here's the starting line-up and how they do it.
Mast cells derive their name from the mistaken impression that the granules they contain were used to feed the cells around them (mast is German for food). Mast cells are located in many tissues of the body, but especially in the skin and the mucous membranes of the respiratory and gastrointestinal tracts. They are usually the first cells that respond to microbial invasion. When mast cells' specific receptors attach to the specific chemicals of an invading microbe, this activates them to release their granules into the surrounding tissue. The granules contain many chemically active molecules, one of which is histamine.
Histamine causes inflammation by increasing the local blood flow and the leakage of fluid out of the capillaries. This not only increases mucous production, but also allows immune cells and proteins within the blood to sneak out into the tissue to help fight against infection. In addition, the granules of the mast cells also contain chemicals called cytokines, chemical messengers, much like hormones and neurohormones. When released, cytokines attach to specific receptors on specific cells, promoting inflammation and recruiting other immune cells to the battlefield. In addition, they cause fever and increase the body's metabolism to fight infection.
Macrophages (large eaters) are immune cells that are located in all of the tissues and organs of the body. Like mast cells, they activate when their specific receptors attach to specific chemicals on foreign microbes. This triggers them to engulf and literally swallow up the invader. Then, by using specific enzymes, they chemically digest and kill them in a process called phagocytosis (phagein is Greek for "to eat"). Macrophages not only kill invading microbes, but also act as scavengers to rid the body of worn-out and abnormal cells and debris from cellular death. They are part of the first responders of the innate immune system, but by processing the chemicals of the killed microbe, they also provide important information to the cells of the adaptive immune system. Like mast cells, activated macrophages release cytokines which cause further inflammation, help to bring other immune cells and proteins to the field of battle, and increase the metabolism to fight infection.
Dendritic cells are so named because of their tree-like projections (dendron is Greek for tree), and resemble the dendrites of nerve cells. The dendritic cells are located in the skin, the epithelium of the respiratory, gastrointestinal and genitourinary tracts, and the lymphatic system. The main job of activated dendritic cells, once their specific receptors have attached to specific chemicals on the surface of the microbe, is to kill it by phagocytosis and process some of its chemicals to provide important information to the cells of the adaptive immune system. Activated dendritic cells also release cytokines that help to improve and regulate the immune response to foreign microbial invasion.
In world history, the element of surprise has played a major role in military operations, causing the defeat and surrender of an enemy. Without sentries to quickly identify invaders, sound the alarm, provide strategic information, and begin to repel, incapacitate, or kill them, most defensive military operations fail. So too, to win the battle for survival, the body needs not just the skin and epithelial tissues of the various organ tracts to provide passive resistance to microbial invasion, but the first responders of the innate immune system as well. Without sufficiently effective mast cell, or macrophage, or dendritic cell function, our earliest ancestors could never have defended themselves adequately from the invisible might of the microbial world.
Evolutionary biologists claim that human life has come about from chance and the laws of nature alone. After all, they point out that the first responders of the innate immune system are present in some of the most ancient forms of life. However, this only verifies their absolute necessity for survival within the world of microbes. It say nothing about how they actually came into existence. It also doesn't explain how the body just happens to have enough of them located precisely where they are needed to get the job done.
William Dembski has said that there are only two options on the table for how life came into being: either chance and the laws of nature alone, or intelligent design in some form. But we have yet to look at the roles played by the immune cells and proteins of the innate system present within the blood. After all, the inflammation brought on by the activated first responders works to allow them to come to the field of battle. So what is it that they do and how important is it for survival? That's what we'll begin to look at next.