Inside you are more than thirty different organs, which include more than two hundred different cell types and about 100 trillion cells. Each organ is patrolled by your immune system, which is constantly performing surveillance for possible threats. Following are some of the major players.
Bone marrow, the reddish-looking material inside nearly every bone in the human body, is where all of the blood cells are produced.
Inside the marrow, on an inner framework, lie the bone marrow stem cells. These cells are constantly dividing, producing huge numbers of cells that turn into red and white blood cells.
The bone marrow stem cells continue to divide, even as they keep making red and white blood cells. And just as a couple of million are being produced every second, the same number are being gobbled up and destroyed in the spleen, which is, in effect, the cells' decommissioning center. The spleen is where blood cells are taken out of circulation once they've completed their useful life cycle (see below).
This process-birth in the bone marrow, death in the spleen- should be evenly balanced. If it isn't, you will be prone to a blood disorder. For example, if your system is destroying more red blood cells than it's making, you'll become anemic, which will make you feel tired and appear pale. If you're making more than you are destroying, you'll become polycythemic, or suffer from blood that is overcrowded with red blood cells.
Once the cells form in the bone marrow, they remain there and mature, at which time they exit and enter the bloodstream. There they circulate, the red cells carrying oxygen and carbon dioxide, and the white cells patrolling for invaders.
The thymus, located in front of your windpipe in the upper chest region, is the most mysterious organ in your homeland.
In infants, the thymus is, relatively speaking, huge. It grows until about puberty and then starts to shrink. By old age, the thymus has almost completely disappeared.
The thymus serves the role of a kind of boot camp for the white blood cells called T cells-it's where these cells go to mature. The thymus is especially active early in life because, during youth, T cells are constantly being exposed to new things, from new proteins in your diet to new germs. These T cells need to have a place to congregate, share information, and learn about threats and attacks.
For example, let's say one of the cells in your immune system comes in contact with a foreign invader. It now has to communicate that information to other cells so they can become aware of the invader too. The thymus is where this information is shared and training occurs.
Why does the thymus later atrophy? Science doesn't know for certain, but as you grow older, your immune system has less to learn. So the current thinking is that there may be less need for a large thymus as we age. But although the thymus is not as big or as active as it was when we were young, the smaller thymus is still able to help train T cells well into our eighties, nineties, and beyond.
Another important training center and meeting ground for the immune system is the spleen. This large organ, about the size of your fist, is located on the left side of your belly, tucked under your ribs.
The spleen is a common meeting place for all the immune system's cells. Blood routes to the spleen, where the cells circulate and mingle, allowing them to tell each other what they've learned, what they've seen, what they've killed, and what antibodies they've made.
The heart pumps the blood around the body about once every minute, which means each blood cell might find itself in the spleen about 1,400 times a day. That's a great many trips.
The spleen is also important because it's where the body decides if a red or a white blood cell has gotten too old. Once the decision is made, the cell is decommissioned and disassembled and its building blocks are recycled.
You can live without a spleen; if it ruptures, the liver can take over its functions. Still, if your spleen is removed or no longer works, your immunity becomes impaired; people without a spleen are more susceptible to infections.
Nearly everyone knows about the bloodstream: it's a road map of blood vessels that allow our blood to circulate through our body. But few people realize that another important, and completely different, circulatory system exists in the body-the lymphatic system, which circulates our lymph.
Lymph is a clear fluid that travels through your body, cleaning your tissues and keeping them nourished. Just as blood circulates back to the heart through our veins, lymph must also be recycled and return to the heart, which it does through the lymphatic system.
The lymphatic system is something of a secondary transportation system for your homeland troops. Like the circulatory system, it is composed of a series of vessels and tubes. The major difference between the circulatory and the lymphatic systems is that the latter lacks a pump to move the fluid it carries. For the blood, that pump is the heart. For lymph, the flow back to the heart is achieved through a more passive process involving muscle contractions and gravity.
You may have noticed that your feet swell during a long journey on an airplane or in a car. This is due to lack of movement; your muscles haven't been able to circulate lymph, so because of gravity, it collects at the lowest part of your body-your feet.
You also may be aware of a condition known as edema, which is swelling that results from a collection of lymph. Edema occurs when excess lymph fluid cannot be returned back into circulation.
The bloodstream is extensive, branching out from its main trunk, the aorta, as well as smaller arteries, arterioles, capillaries, veins, and venules, but there is still a large portion of our tissues the capillaries can't reach.
Here is where the lymph comes into play. Nutrients such as glucose (blood sugar) must be helped so they can reach and nourish each and every cell, including those the bloodstream can't reach. That happens via the lymph fluid, which bathes and nourishes all of the body's tissues. And once those nutrients have been used, the fluid must be recycled or your body would swell up like the Michelin tire man.
Along the course of the lymphatic system are way stations known as lymph nodes. These are outposts whose sentries make sure nothing passes through the lymphatic channels that shouldn't. The lymphatic system could provide easy and direct access for a germ or microbe to our heart and bloodstream, so to prevent that from happening, lymphocytes (T cells and B cells) aggregate in lymph nodes, waiting for something bad to pass by. When they spot that something, the node cells attack before it can venture into the heart and bloodstream.
When a problem is stirring in your body, the lymph nodes become enlarged. For example, if you have swollen glands in the neck, your nodes may have found some virus that landed in the back of the throat and is trying to gain access to the lungs or bloodstream. Lymph nodes responding to some infection can become swollen almost anywhere: in the groin, neck, chest, abdomen, and so on.
WHITE BLOOD CELLS
Probably the most important cells of our immune system, as well as the best-known and the most numerous, are the white blood cells. This term distinguishes them from the red blood cells, the disk-like cells responsible for carrying oxygen and carbon dioxide from our lungs throughout the rest of the body.
Most people probably think of white blood cells simply as formless globules floating through our bloodstream, randomly patrolling for microbes. But our white blood cells are very purposeful and deliberate in their surveillance, and there are actually many different types of white blood cells, each with specialized functions. And these white blood cells are found not just in the blood, but throughout our bodies-in each of our organs, from the brain to the liver to the lungs, as well as throughout the lymphatic system.
The first line of immune cells are the lymphocytes. Cyte means "cell," so lymphocytes are the lymph or lymphatic cells. The ones you most need to know about are the B lymphocytes (better known as B cells) and the T lymphocytes (better known as T cells).
B cells were so named because they were first studied in the bursa of Fabricius, an organ unique to birds. In humans, B cells actually originate in the bone marrow.
The B cells have two major jobs: they maintain a memory database, and they create complex protein structures that are used as weapons against threats and invaders. These complex structures are called antibodies, about which more is coming.
B cells keep a record of every single interaction your immune system has ever had. This means that within your body, a record of every germ and virus you've ever encountered, every protein you've ever eaten, every piece of pollen you've ever inhaled, has been stored in a memory bank-not in your brain, but in your immune system. Think about it: for each of these interactions, there is a B cell floating around inside your body that has retained a memory of the encounter.
Your immune system's memory is, in some ways, more impressive than your brain's. Most of us can only evoke faint memories from early childhood. Your immune system, however, remembers your first vaccination, which probably occurred in the earliest days of your life. Research now suggests that your immune system even stores memories from when you were still developing in your mother's womb.
Shortly after birth, you were probably immunized with vaccines for diphtheria, tetanus, and pertussis (whooping cough). Although the memory of that immunization may fade somewhat, and a booster may be needed to remind your immune system, some remnant of that memory lasts a lifetime.
These stored memories are critically important for your survival; they are what make you immune to becoming sick more than once from certain illnesses.
For example, after a bout of chicken pox in childhood, you become immune-you usually can't catch chicken pox again as an adult. Likewise, after being immunized with a shot for tetanus, you won't succumb to the bacterial infection that causes tetanus. Your B cells now have the memory stored away and prevent you from coming down with the disease.
A memory of exposure to prior threats is crucial because it allows your immune system to respond more quickly and effectively to serious threats if you are re-exposed. Without such a memory, and a rapid response, exposure to ailments such as tetanus or diphtheria could be fatal.
It's also important for your immune system to remember prior contacts and exposures even if they're not potentially lethal, as it makes your immune system less likely to cause an overwhelming reaction when encountering nonlethal microbes. If, for example, your immune system overreacted every time you ate a particular food, or breathed in a particular pollen, you would forever exist in a state of immune hyperactivity and unnecessary battle.
One example of an overreaction by our immune system is an allergy, which can trigger serious problems like asthma or even anaphylaxis, which can be lethal. We'll talk more about this later. B cells possess another important function. They make antibodies, whose role in your internal homeland security is similar to that of the U.S. Army Corps of Engineers. Here B cells work closely with T cells to build complex mechanical and chemical structures that act as deadly weapons to neutralize invaders.
Antibodies are complex proteins manufactured to exact specifications. Each antibody is built by the B cells to neutralize one specific invader. Medical science does not yet fully understand the way in which these antibodies are made. What is known is that B cells team up with macrophages in order to create them.
First, what is a macrophage? Phage comes from the Greek phagon, meaning "to eat." Macro, from the Greek macron, means "big." Thus a macrophage is a "big eater."
Macrophages constantly patrol the body, which means they can be considered your body's military police, or MPs. The MPs are constantly on the lookout, in every organ in your body. Always moving, they crawl between the trillions of cells in all of your organs like an amoeba might crawl across a petri dish.
These bloblike, voracious eaters are searching for any invaders that might have sneaked into your body. When an MP encounters one, it will capture it, cut it up into a thousand tiny pieces, and then take the bits and show them around the body to let the other troops, such as the B cells, know exactly what the invader looks like.
Sound overly dramatic? Actually, it's quite close to the literal truth. When macrophages encounter an invader, they grab it with amoebalike fingers and engulf it, swallowing the invader whole so that it becomes a captive. The macrophage then releases enzymes to digest it, breaking it down into tiny bits. These little sections become pieces of proteins-small enough to be moved around but large enough to provide a unique "fingerprint" identifying the invader the MP just gobbled up and digested.
The MP next spits up these little digested bits of protein fingerprints and displays them on its own surface, similar to placing a "Wanted" poster for the rest of your immune system to see. Now the B cells move in, going up to that "Wanted" poster, or that piece of the invader, and learning its shape. They then build an antibody that suits the shape of the invader perfectly. This antibody can now recognize and attach itself to the invader the next time it comes into contact with it.
The B cells next start making millions and millions of copies of this antibody, releasing them into the bloodstream. They fl oat through our blood, and if they come into contact with one of these microbial invaders, the antibodies immediately attach themselves to it, triggering a series of events that will ultimately kill it.
Our B cells manufacture antibodies to exceptionally tight specifications so that the antibodies are specific to one particular germ. And the antibodies must be a perfect fit; otherwise, critical problems would result, the most important being that the antibody might not be properly able to recognize and neutralize the threat.
If, for instance, the antibody mistakenly grabbed onto your eye, you would go blind. If it grasped your brain, you'd suffer brain damage. As soon as antibodies take hold of something, a chain reaction is produced, and that thing will either be neutralized or killed.
Once a B cell learns to produce an antibody with the help of a macrophage, it retains the memory to produce that antibody forever. So in effect, that B cell is forever programmed to recognize a specific invader. If it ever encounters that invader again, the B cell will immediately begin producing millions of copies of its antibody, and will also reproduce itself thousands of times to create a force of so-called daughter cells. Each daughter cell also inherits the know-how to recognize that specific invader and to produce its specific antibody.
It is thought that B cells live anywhere from about a year to about six years. B cells reproduce themselves (by cell division) to maintain their collective memory, which is crucial for your very survival. And B cells can also multiply quickly in response to the recognition of a known threat. This ability to multiply helps build a formidable antibody response in the event a known invader gains entry into your body again.
There's a lot of brainpower in the B cells.
The other major group of lymphocytes is the T cells (T stands for thymus, the body organ in which they mature).
T cells look like B cells under a microscope-they are spherical, with a big nucleus and not much cytoplasm (the stuff that surrounds the nucleus). These T cells are the marines of your homeland security team. Just as every marine has a job classification, or a military occupational specialty (MOS), every T cell also has an MOS.
Excerpted from UltraLongevity by Mark Liponis Copyright © 2007 by Mark Liponis, MD. Excerpted by permission.
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