The Immune System

The immune system is designed to defend you against millions of bacteria, microbes, viruses, toxins and parasites that would normally invade your body. It works around the clock in thousands of different ways, but it does its work largely unnoticed. One thing that causes us to really notice our immune system is when it fails for some reason. We also notice it when it does something that has a side effect we can see or feel. Here are several examples:

• When you get a cut, all sorts of bacteria and viruses enter your body through the break in the skin. When you get a splinter you also have the sliver of wood as a foreign object inside your body. Your immune system responds and eliminates the invaders while the skin heals itself and seals the puncture. In rare cases the immune system misses something and the cut gets infected. It gets inflamed and will often fill with pus. Inflammation and pus are both side-effects of the immune system doing its job.
• When a mosquito bites you, you get a red, itchy bump. That too is a visible sign of your immune system at work.
• Each day you inhale thousands of germs (bacteria and viruses) that are floating in the air. Your immune system deals with all of them without a problem. Occasionally a germ gets past the immune system and you catch a cold, get the flu or worse. A cold or flu is a visible sign that your immune system failed to stop the germ. The fact that you get over the cold or flu is a visible sign that your immune system was able to eliminate the invader after learning about it. If your immune system did nothing, you would never get over a cold or anything else.
• Each day you also eat hundreds of germs, and again most of these die in the saliva or the acid of the stomach. Occasionally, however, one gets through and causes food poisoning. There is normally a very visible effect of this breach of the immune system: vomiting and diarrhoea are two of the most common symptoms.
• There are also all kinds of human ailments that are caused by the immune system working in unexpected or incorrect ways that cause problems. For example, some people have allergies. Allergies are really just the immune system overreacting to certain stimuli that other people don't react to at all. Some people have diabetes, which is caused by the immune system inappropriately attacking cells in the pancreas and destroying them. Some people have rheumatoid arthritis, which is caused by the immune system acting inappropriately in the joints. In many different diseases, the cause is actually an immune system error!

• Thymus
• Spleen
• Lymph system
• Bone marrow
• White blood cells
• Antibodies
• Complement system
• Hormones

Thymus

The thymus lives in your chest, between your breast bone and your heart. It is responsible for producing T-cells, and is especially important in new-born babies - without a thymus a baby's immune system collapses and the baby will die. The thymus seems to be much less important in adults - for example, you can remove it and an adult will live because other parts of the immune system can handle the load. However, the thymus is important, especially to T cell maturation.

Spleen

The spleen filters the blood looking for foreign cells (the spleen is also looking for old red blood cells in need of replacement). A person missing their spleen gets sick much more often than someone with a spleen.

Lymphatic system

The immune system maintains its own system of circulation--the lymphatic vessels-- which permeates every organ in the body except the brain. The lymphatic vessels contain a pale, thick fluid (lymph) consisting of a fat-laden liquid and white blood cells.

Along the lymphatic vessels are special areas--the lymph nodes, tonsils, bone marrow, spleen, liver, lungs, and intestines--where lymphocytes can be recruited, mobilised, and deployed to appropriate sites as part of the immune response. The ingenious design of this system ensures the ready availability and quick assembly of an immune response anywhere it is needed. This system can be seen at work when a wound or an infection in a fingertip leads to an enlarged lymph node at the elbow, or when a throat infection causes the lymph nodes under the jaw to swell. The lymph nodes swell because the lymphatic vessels drain the infection by carrying it to the nearest area where an immune response can be organised

Bone marrow

Bone marrow produces new blood cells, both red and white. In the case of red blood cells the cells are fully formed in the marrow and then enter the bloodstream. In the case of some white blood cells, the cells mature elsewhere. The marrow produces all blood cells from stem cells. They are called "stem cells" because they can branch off and become many different types of cells - they are precursors to different cell types. Stem cells change into actual, specific types of white blood cells.

Antibodies

Antibodies (also referred to as immunoglobulins and gammaglobulins) are produced by white blood cells. They are Y-shaped proteins that each respond to a specific antigen (bacteria, virus or toxin). Each antibody has a special section (at the tips of the two branches of the Y) that is sensitive to a specific antigen and binds to it in some way. When an antibody binds to a toxin it is called an antitoxin (if the toxin comes from some form of venom, it is called an antivenin). The binding generally disables the chemical action of the toxin. When an antibody binds to the outer coat of a virus particle or the cell wall of a bacterium it can stop their movement through cell walls. Or a large number of antibodies can bind to an invader and signal to the complement system that the invader needs to be removed.

Hormones

There are several hormones generated by components of the immune system. These hormones are known generally as lymphokines. It is also known that certain hormones in the body suppress the immune system. Steroids and corticosteroids (components of adrenaline) suppress the immune system. Tymosin (thought to be produced by the thymus) is a hormone that encourages lymphocyte production (a lymphocyte is a form of white blood cell). Interleukins are another type of hormone generated by white blood cells. For example, Interleukin-1 is produced by macrophages after they eat a foreign cell. IL-1 has an interesting side- effect - when it reaches the hypothalamus it produces fever and fatigue. The raised temperature of a fever is known to kill some bacteria.

White blood cells

You are probably aware of the fact that you have "red blood cells" and "white blood cells" in your blood. The white blood cells are probably the most important part of your immune system. And it turns out that "white blood cells" are actually a whole collection of different cells that work together to destroy bacteria and viruses.

All white blood cells are known officially as Leukocytes. White blood cells are not like normal cells in the body - they actually act like independent, living single-cell organisms able to move and capture things on their own. White blood cells behave very much like amoeba in their movements and are able to engulf other cells and bacteria. Many white blood cells cannot divide and reproduce on their own, but instead are produced in the bone marrow.

Because white blood cells are so important to the immune system, they are used as a measure of immune system health. When you hear that someone has a "strong immune system" or a "suppressed immune system", one way it was determined was by counting different types of white blood cells in a blood sample. One important question to ask about white blood cells (and several other parts of the immune system) is, "How does a white blood cell know what to attack and what to leave alone? Why doesn't a white blood cell attack every cell in the body?" There is a system built into all of the cells in your body called the Major Histocompatibility Complex (MHC) (also known as the Human Leukocyte Antigen (HLA)) that marks the cells in your body as "you". Anything that the immune system finds that does not have these markings (or that has the wrong markings) is definitely "not you" and is therefore fair game. The Encyclopaedia Britanica has this to say about the MHC:

"There are two major types of MHC protein molecules--class I and class II--that span the membrane of almost every cell in an organism. In humans these molecules are encoded by several genes all clustered in the same region on chromosome 6. Each gene has an unusual number of alleles (alternate forms of a gene). As a result, it is very rare for two individuals to have the same set of MHC molecules, which are collectively called a tissue type. MHC molecules are important components of the immune response. They allow cells that have been invaded by an infectious organism to be detected by cells of the immune system called T lymphocytes, or T cells. The MHC molecules do this by presenting fragments of proteins (peptides) belonging to the invader on the surface of the cell. The T cell recognises the foreign peptide attached to the MHC molecule and binds to it, an action that stimulates the T cell to either destroy or cure the infected cell. In uninfected healthy cells the MHC molecule presents peptides from its own cell (self peptides), to which T cells do not normally react. However, if the immune mechanism malfunctions and T cells react against self peptides, an auto- immune disease arises."

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