Human Immune System: General Overview
Synopsis: An article about the Human Immune System that is somewhat more user-friendly than other more technical articles available. The human immune system is made up of a number of interdependent cell types which collectively protect the person's body from various parasitic, fungal, bacterial and viral infections, as well as from the growth of tumor cells. The immune system works twenty-four hours a day in thousands of ways, working largely unnoticed. One of the things that may cause us to notice our immune system is when it fails.
The immune system is a protection mechanism, designed to defend a person's against microbes, bacteria, toxins, viruses and parasites. In order to understand the human immune system, it may help to take on the perspective of what happens once it dies. Doing so demonstrates important aspects of the immune system.
When something dies its immune system shuts down and within a few hours it is invaded by a number of various parasites, bacteria and microbes. When the immune system is working these things are unable to invade it; once the immune system stops functioning they can. When a person dies it takes only a few weeks for these organisms to completely break down their body and essentially carry it away, leaving only the person's skeleton. The human immune system is keeping this from happening while a person is still living.
The Immune System in Action
The human immune system is made up of a number of interdependent cell types which collectively protect the person's body from various parasitic, fungal, bacterial and viral infections, as well as from the growth of tumor cells.
A number of these cell types have specialized functions, are able to kill parasites, engulf bacteria, or kill tumor cells or viral-infected cells. Frequently, these cells are dependent upon the, 'T,' helper subset for activation signals in the form of secretions which are more formally referred to as, 'Lymphokines,' 'Cytokines,' or specifically as, 'Interleukins.' An understanding of the T helper subset may assist in comprehension of the root of immune deficiencies, as well as perception of the potential avenues that the human immune system can be modulated in the case of particular diseases.
The immune system works twenty-four hours a day in thousands of ways, working largely unnoticed. One of the things that may cause us to notice our immune system is when it fails. We may also notice our immune system when it presents a side-effect that we can either see or feel. There are several things you may notice that are related to your immune system
When you experience a cut, various forms of viruses and bacteria enter your body through the cut in your skin. Your immune system responds by eliminating the bacteria and viruses and while the skin heals. On rare occasion, the immune system may miss something and the cut could become infected, becoming inflamed and filling with pus. Both the inflammation and pus are side-effects of the immune system at work.
Each day people breathe in thousands of germs that are floating in the air they inhale. Their immune system fights these germs, although on occasion a germ gets past their immune system and they catch a cold, the flu or worse. The presence of a cold or flu is a sign that the person's immune system has failed to fight off a germ. Getting over the flu or a cold is a sign that the person's immune system has been able to fight off the flu or cold after recognizing it. If a person's immune system did nothing at all, they would never recover from the flu or a cold, for example.
Every day a person also consumes hundreds of germs; most of them die in the person's saliva or in their stomach acid. Every now and then some of these germs gets through and causes food poisoning; something that is usually a very visible effect of the breach of the immune system. Both diarrhea and vomiting are common symptoms of food poisoning.
There are a number of ailments caused by an immune system that is working in either incorrect or unexpected ways, resulting in problems. Allergies are an example of an immune system overreaction to certain stimuli which other persons do not react to. Persons with diabetes have an immune system that inappropriately attacks cells in their pancreas and destroys them. Persons with rheumatoid arthritis have an immune system that acts inappropriately in their joints. A number of different diseases have causes that are related to immune system errors.
Sometimes the immune system prevents people from doing things that would otherwise be beneficial. Organ transplants are more difficult because the immune system many times rejects a transplanted organ.
Immune System Basics
When a person gets sick their body is unable to function properly or at its fullest potential. There are a number of ways a person may become sick.
Persons who experience broken bones or torn ligaments may be considered, 'sick,' because their body is unable to perform at its fullest potential. The cause is easily understood and visible.
Mineral or Vitamin Deficiency
If a person does not get enough Vitamin D for example, their body will not be able to metabolize calcium properly and they may develop the rickets. Persons with the rickets have bones which break easily, as well as deformities due to bones that do not grow properly. If people do not get enough Vitamin C they may develop scurvy. Scurvy causes bleeding and swollen gums, swollen joints and bruising. Persons who do not receive enough iron may experience anemia, and so forth.
In some instances, a person's organ may be either weakened or damaged. In one example, heart disease is caused by obstructions in the person's blood vessels leading to their heart muscle, depriving their heart of sufficient quantities of blood. In another example, liver disease caused by damage to liver cells known as, 'Cirrhosis,' causes organ degeneration.
Genetic diseases, caused by coding errors in a person's DNA, may cause too much or little of certain proteins to be made. The result may be problems at a cellular level.
On occasion a cell changes in a way that causes it to reproduce uncontrollably. Cells in the skin, for example, called, 'melanocytes,' may be damaged by ultraviolet radiation in sunlight and change in a characteristic way into cancerous cells. The tumor that appears on the person's skin is referred to as a, 'melanoma.'
Bacterial or Viral Infection
When bacteria or a virus, commonly referred to as, 'germs,' invade a person's body and reproduce, the person may become sick. Strep throat, for example, releases a toxin causing a person's throat to inflame. The polio virus releases toxins that destroy nerve cells, many times leading to paralysis. There are some forms of bacteria that are actually beneficial, such as the millions of bacteria in our intestines that assist us in the digestion of food. Bacterial and viral infections are the most common causes of sickness, causing things such as influenza, colds, mumps, measles, AIDS, malaria and so forth.
The immune system protects people from infections in three different ways. First, it creates a barrier preventing viruses and bacteria from entering the body. Second, should a virus or bacteria enter a person's body, the immune system attempts to both detect and eliminate it before it can reproduce. Third, should a virus or bacteria enter a person's body and reproduce, the immune system works to eliminate it.
There are other jobs that the immune system performs. The immune system may detect cancer in its early stages, for example, and assist in eliminating it in many cases.
Immune System Components
Your skin is an important part of the immune system, believe it or not. Skin acts as a boundary between germs and your body; it is tough and essentially impermeable to both viruses and bacteria. A person's epidermis has cells referred to as, 'Langerhans,' cells that are an early-warning component in the immune system. A person's skin secretes antibacterial substances as well, which explains why people do not wake up in the morning with a layer of mold growing on their skin.
A person's mouth, nose and eyes are points of entry for germs. Both mucus and tears contain an enzyme referred to as, 'Lysozyme,' that breaks down the cell wall of a number of types of bacteria. Saliva also has anti-bacterial properties as well. Because both the lungs and nasal passages are coated with mucus, a number of germs are not killed immediately; they are instead trapped in mucus and swallowed. A person's nasal passages, lungs, throat and skin are lined with Mast cells; any virus or bacteria attempting to enter that person's body has to make it past these defenses.
Germs, once inside a person's body, deal with their immune system in various ways. The major components of a person's immune system include the Thymus, Lymph System, Spleen, White Blood Cells, Bone Marrow, Complement System, Antibodies, and Hormones.
The Lymph System
A person's lymph nodes are one part of a system which extends throughout their body, much like their blood vessels do. Fluids move through the lymph system in a person's body through common body and muscle motion to the lymph nodes. Lymph is a fairly clear liquid which bathes cells with nutrients and water. Lymph is blood plasma, a liquid that makes up blood less the white and red cells. Blood transfers materials to the lymph via the capillary walls. The lymph then carries it to the person's cells. The person's cells produce proteins and waste products which the lymph absorbs and carries away. Bacteria that enter a person's body find their way into the person's lymph fluid, where the lymph system removes the bacteria.
A person's lymph nodes contain both filtering tissue and a large number of lymph cells. When the person is fighting a bacterial infection, their lymph nodes swell with bacteria and the cells fighting the bacteria. Swollen lymph nodes are an indication that a person has an infection. Once lymph has been filtered through the person's lymph nodes, it re-enters their bloodstream.
A person's thymus is located in their chest, between their breast bone and their heart. The thymus is responsible for the production of T-cells and is particularly important in newborn babies. Without a thymus, a baby's immune system would collapse. An adult who has their thymus removed will still live because other parts of their immune system can still handle the load. The thymus is still and important thing, especially concerning T-cell maturation.
A person's spleen filters their blood in search of foreign cells; it also looks for old red blood cells which need replacing. Persons who are missing their spleen tend to become sick more often than people who have a spleen.
A person's bone marrow produces both new white and red blood cells. New red blood cells are fully-formed in their marrow, after which they enter the person's bloodstream. White blood cells mature elsewhere in the person's body. Bone marrow produces both types of blood cells from stem cells, referred to as stem cells because they are able to branch off and become a number of different forms of cells. Stem cells are essentially precursors of a variety of cell types. Stem cells are able to change into specific types of white blood cells as well.
Antibodies, which are also referred to as, 'Gammaglobulins,' or, 'Immuno-globulins,' are produced by white blood cells. Antibodies are, 'Y,' shaped proteins which each respond to a specific antigen. Each antibody has a section at the tips of the branches of the, 'Y,' that is sensitive to a specific antigen and binds to it. When an antigen binds to a toxin it is referred to as an, 'antitoxin.' The process of binding commonly 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. A large number of antibodies may bind to an invader and signal to the person's compliment system that the invader needs to be removed.
There are five classes of antibodies:
- Immunoglobulin A
- Immunoglobulin D
- Immunoglobulin E
- Immunoglobulin G
- Immunoglobulin M
The Complement System
The Complement System consists of a series of proteins; there are only a handful of proteins in the compliment system, floating freely in the blood. Compliments are created in a person's liver, and are activated by and work with antibodies. Compliments cause, 'Lysing,' or bursting of cells and signal to phagocytes that a cell needs to be removed.
There are a number of hormones that are created by components of a person's immune system, referred to as, 'Lymphokines.' There are certain hormones in a person's body which suppress the immune system as well, known as, 'Corticosteroids,' or, 'Steroids,' which are components of adrenaline. Tymosin is a hormone, thought to be produced by a person's thymus, which encourages lymphocyte production generated by white blood cells. Interleukin-1, for example, is produced by macrophages once they have eaten a foreign cell. Interleukin-1 has a notable side-effect; when it reaches the hypothalamus, it produces both fatigue and fever. The increase in fever that a person experiences is known to kill some bacteria.
The Tumor Necrosis Factor
Tumor Necrosis Factor (TNF) is produced by macrophages as well, and has the ability to kill tumor cells; it is also able to promote the creation of new blood vessels which are important to the process of healing.
Interferon, to be simple, interferes with viruses. Interferon is produced by most of the cells in a person's body. Interferons are proteins whose job is to allow cells to signal to one another. Should a cell detect interferon from other cells, it produces proteins which assist in preventing viral replication in the cell.
White Blood Cells
White blood cells are perhaps the most important part of a person's immune system. White blood cells are actually a collection of various cells which work together in order to destroy both viruses and bacteria. There are different types, names and classifications of white blood cells that are at work inside your body this very moment. These include Lymphocytes, Leukocytes, Monocytes, Granulocytes, Plasma cells, B-cells, T-cells, Helper T-cells, Killer T-cells, Natural Killer cells, Suppressor T-cells, Eosinophils, Neutrophils, Basophils, Macrophages, and Phagocytes.
Every white blood cell is officially known as a, 'Leukocyte.' White blood cells are not like common cells in a person's body, they perform like independent, living, single-cell organisms that have the ability to both move and capture things on their own. White blood cells have the ability to engulf other types of cells and bacteria. A number of white blood cells are unable to either reproduce or divide on their own; instead they have a factory somewhere in the body which produces them, such as bone marrow. There are three classes of Leukocytes:
Granulocytes make up between fifty and sixty-percent of all Leukocytes. Granulocytes are comprised of three different classes themselves; Eosinophils, Neutrophils, and Basophils. Granulocytes received their name because they contain, 'granules;' these granules contain different chemicals, depending on the type of cell.
Lymphocytes comprise between thirty and forty-percent of all leukocytes. Lymphocytes have two classes; B-cells, which mature in a person's bone marrow, and T-cells, which mature in a person's thymus.
Monocytes represent about seven-percent of all leukocytes, and evolve into macrophages.
Every white blood cell begins in a person's bone marrow as a stem cell. Stem cells divide and differentiate into every other type of white blood cell. Each of the types of white blood cells has a different role in a person's immune system; many have the ability to transform themselves in different ways. For example:
Neutrophils are the most common form of white blood cells in a person's body. Bone marrow produces trillions of them each day, releasing them into the bloodstream. The lifespan of a neutrophil is short, commonly less than half of a day. Once neutrophils are in a person's bloodstream, they have the ability to move through capillary walls and into tissue. They are attracted to foreign material, bacteria and inflammation. Should a person get a splinter or a cut, for example, neutrophils will be attracted through a process referred to as, 'Chemotaxis. A number of single-celled organisms use the same process. Chemotaxis allows motile cells to move towards larger concentrations of a chemical. Once a neutrophil discovers a foreign particle or bacteria, it engulfs it, releases enzymes, hydrogen peroxide or other chemicals, and attempts to kill the bacteria. If the site is seriously infected, pus will form; pus is merely dead neutrophils combined with additional cellular debris.
Basophils and Eosinophils
Eosinophils and Basophils are much less common than neutrophils. Eosinophils tend to focus on parasites which are in a person's lungs and skin, while Basophils carry histamine and are therefore important to the promotion of inflammation. From the perspective of the immune system, inflammation is a positive thing; it brings additional blood while dilating capillary walls so more immune system cells may access the site of infection.
Macrophages are the largest of all blood cells. Monocytes are released by a person's bone marrow; they then float in the person's bloodstream, enter tissues and turn into macrophages. The majority of a person's boundary tissue has its own devoted macrophages. Alveolar macrophages, for example, live in a person's lungs and keep their lungs clean through ingestion of foreign particles such as dust and smoke. They also keep the person's lungs disease-free by ingesting microbes and bacteria. When macrophages live in a person's skin, they are referred to as, 'Langerhans,' cells. Macrophages may also swim freely in a person's body. One of the duties that macrophages perform is to clean up dead neutrophils.
Lymphocytes deal with the majority of the viral and bacterial infections people experience. Lymphocytes begin in a person's bone marrow. T-cells begin in marrow, but migrate through the person's bloodstream to their thymus where they mature. T-cells and B-cells are many times found in a person's bloodstream, but often concentrate in their lymph tissues; lymph nodes, the thymus, and the spleen. People have an amount of lymph tissue in their digestive system as well. Both T-cells and B-cells have different functions.
B-cells, upon stimulations, mature into plasma cells; plasma cells produce antibodies. A specific B-cell is tuned to a particular germ; when the germ is in the person's body, the B-cell clones itself producing millions of antibodies that are designed to kill the germ.
T-cells, 'bump up,' against cells and kill them. T-cells referred to as, 'Killer T-cells,' are able to detect cells in a person's body which are harboring viruses and kill them. Helper and Suppressor T-cells help to sensitize Killer T-cells and control the immune response. Helper T-cells are activated by Interleukin-1, which is produced by macrophages. Once they have been activated, Helper T-cells produce Interleukin-2, then Interferon and additional chemicals. The chemicals produced then activate B-cells, which then go on to produce antibodies.
White blood cells are very important to the immune system and are used to measure the health of a person's immune system. A normal white blood cell count is considered to be in the range of 4,000 to 11,000 cells per micro-liter of blood. 1.8 to 2.0 Helper T-cells per Suppressor T-cell is also considered to be within a normal range. A normal Absolute Neutrophil Count (ANC) is considered to be within a range of 1,500 to 8,000 cells per micro-liter.
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