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The bodies first line of defense and most powerful antiviral tool.

The body is made up of several systems that each consist of a set of organ and component tissues and which have particular functions.

The circulatory system for instance is designed to circulate nutrients and Oxygen to the cells of the body and to carry away waste products. It is made up of the heart, veins and arteries. The other systems are the digestive and excretory systems, the respiratory system, the nervous system, the reproductive and the immune systems. Most of us have a pretty good idea how we breathe, what happens to food and what the brain does. The immune system is more mysterious and less understood.

The major components of the immune system are the:

Thymus:

The thymus is located near the heart and is the most important part of the lymphatic system. It is most active in the fetus and young children. After puberty it shrinks. It is responsible for the production of t-lymphocytes, white blood cells that protect the body against bacteria and viruses as well as killing cancerous cells.

Spleen:

While the thymus is the most important organ of the lymph system, the spleen is the largest. The spleen is located in the upper left area of the chest and is about the size of a fist. It is supplied by blood from the kidney. The spleen filters the blood, removing damaged cells and pathogens such as bacteria and viruses. The spleen houses also aids in the maturation of white blood cells. It is also a stores red blood cells which are released when bleeding occurs.

Lymph system:

This is made up of vessels that drain fluids from body tissues. The lymph fluid also picks up bacteria and other microbes and traps them in the lymph nodes, where white blood cells can destroy them. Swollen lymph nodes are often a sign of infection.

Bone marrow:

Our marrow is the main site of blood production, both red and white cells. The spongy bone marrow in our long bones releases newly formed blood cells into the blood stream as they are needed. As many as five hundred million red blood cells are produced every day as well as white blood cells.

Antibodies:

Antibodies are Y shaped proteins produced by white blood cells that respond to the presence of specific antigens (bacteria, viruses or toxins). Each antibody has special sections at the tips of the two branches of the Y that are sensitive to a specific antigen and bind to it in a specific way. When an antibody binds to a toxin it is called an antitoxin, unless the toxin comes from some form of venom, in which case it is called an antivenin. By binding to the toxin the antibody disables its chemical actions. When an antibody binds to the outer coat of a virus particle or the cell wall of a bacterium, it can also stop their movement through cell walls. The antibodies then send signals to the proteins of the complement system that the invader needs to be removed.

Antibodies have been classified by their structures and functions into five groups:

Immunoglobulin A (IgA):

Produced in the mucosal linings and present in bodily fluids such as saliva, tears, colostrum, mucus, sweat and stomach fluids, where they prevent invading pathogens from attaching to and penetrating epithelial surfaces.

Immunoglobulin M (IgM):

Predominantly found in the lymph fluid and blood. It is an effective neutralizing agent in the early stages of disease. Elevated levels can be a sign of recent infection or exposure to a pathogen.

Immunoglobulin D (IgD):

This antibody is not well understood but appears to activate B cells. It is found in small amounts in blood serum.

Immunoglobulin E (IgE):

The fundamental role of IgE is in allergic reactions, but it is also active where there are parasitic infections. It occurs in the cells of lungs, skin and mucous membranes and is the least common immunoglobulin in blood serum. When there is an allergic reaction, this antibody releases amines such as histamines and heparin into the surrounding tissues.

Immunoglobulin G (IgG):

IgG is the major immunoglobulin in blood, lymph fluid, peritoneal fluid and fluids of the brain and spine. It can activate the complement system, and plays an important role in the destruction (phagocytosis) of invading microorganisms.

Complement system:

The complement system is made up of a series of proteins which are manufactured in the liver and float in the blood. Compared to the millions of different antibodies in your blood stream, there are only a few types of complement proteins.. The complement proteins are activated by and work with the antibodies. When the antibodies send out chemical signals to alert them, complement proteins attack the invaders and cause lysing (bursting) of cells. This then signals cells in the body called phagocytes to remove the broken cells.

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, which is produced by the thymus is a hormone that encourages white blood cell production.

The white blood cells then produce another type of hormone called Interleukins after they attack and destroy a foreign cell. A side effect of interleukins is that when they reach the hypothalamus they produce the typical fever and fatigue associated with illness. However the raised temperature of a fever is known to kill some bacteria, so this is another way the body actually fights the infection.

Interferon is a hormone that specifically interferes with viruses and is produced by most cells in the body. Interferons are proteins whose job is to help cells communicate with one another. When a cell detects interferon from other cells, it produces proteins that help prevent viral replication in the cell.

White blood cells:

These are the soldiers of the immune system. They roam through the body, attacking and destroying bacteria, viruses and damaged cells. They come in many different forms, each with particular functions. Names are confusing because they are often used interchangeably.

Leukocytes

– This is the proper scientific name for white blood cells

Lymphocytes

– Lmphocytes are the leukocytes that determine the type of immune response to infectious microbes or other foreign substances that threaten the body. About 20 to 40 percent of the white blood cells in a human body are lymphocytes. They are found in the blood as well as in the main lymph organs and tissues, such as the spleen and lymph nodes, where the initial immune response to an invader is most likely to occur.

Lymphocytes come in several forms: The two primary types of lymphocytes are B lymphocytes, or B cells and T lymphocytes or T cells. Both originate from stem cells in the bone marrow. Some lymphocytes migrate to the thymus, where they mature into T cells; while others remain in the bone marrow, where—in humans—they develop into B cells. Most lymphocytes are short-lived, with an average life span of a week to a few months, but a few live for years, providing a pool of long-lived T and B cells. These cells account for so-called immunological “memory,” when the immune system generates a more rapid response to a second encounter with the same microbe or foreign substance (collectively called antigens).

T and B cells have receptor molecules on their surfaces which enable them to bind on to antigens and then help remove them from the body. Each lymphocyte bears unique receptors that bind to a specific antigen. The ability to respond to virtually any antigen in nature comes from the enormous variety of lymphocytes in the body. Each lymphocyte has a receptor capable of recognizing one type of unique microbe or foreign substance.

T cells come in three basic types depending on their functions. Killer T cells attack the invaders with the assistance of Helper T-cells. Suppressor T-cells then work to prevent the invaders from multiplying by creating adverse conditions for their survival.

Macrophages and their precursors, monocytes, are also white blood cells which attack and eat invading antigens. They are found in virtually every tissue of the body. Macrophages can change their form and function depending on the environmental signals they receive. They help defend the body but are also implicated in autoimmune diseases where they get their signals crossed and attack their own body cells.

Granulocytes are another type of white blood cell that distinguished by the presence of small granules, which contain proteins. The types of these cells are neutrophils, eosinophils, and basophils. Granulocytes help the body fight bacterial infections.

All of the above can also be known as phagocytes, which basically means cell-eating cells. Any white blood cell (leukocyte) that destroys invading cells or viruses, whether it be a monocyte, macrophage, granulocyte or lymphocyte, is a phagocyte.

Now that we have a basic knowledge of the types of cells and organs that make up our immune system we need to look at how it works to defend our bodies against disease. Once we understand what should happen, it is easier to understand how things can go wrong and how we can assist our immune system in protecting us.


 

Basically the body and the immune system have three levels of defense against infection and disease:

1. The first line of defense consists of non-specific barriers that are designed to keep all forms of disease organisms out.

Our skin is the most important part of the first line. As long as it is not damaged, it can keep out most bacteria and other parasites. Our bodily orifices are more vulnerable so mucous, saliva and other bodily fluids work to protect those surfaces. Tears, spit and snot all trap and carry away bacteria and other invaders. If they make it to our stomachs, strong acids destroy them.

2. The second line of defense consists of our white blood cells.

If the skin is damaged our bodies react with secondary measures to attack the invading bacteria, viruses and other organisms that we collectively call ‘germs’ and science calls antigens. The body sends white blood cells to the cut or the infection to gobble up the invaders. Infection is actually the body fighting against the germs and pus is made up of the dead and dying cells, both yours and theirs. The white blood cells are our front line soldiers in the second line of defense.

3. The third line of defense involves specific immune responses to specific invaders.

At this point the body declares war on a particular invader, producing the specific antibodies needed to fight it.

The world is full of pathogens whose purpose is to invade and infect a host and then produce more of the pathogens and transmit them to new hosts. Our immune systems have evolved to prevent this happening. As in any war, the enemy is constantly finding new and better ways to invade and the hosts are evolving new and better defense mechanisms. The better we understand how our immune systems work the better we can assist them in the never ending battle against infection and disease.


 

References:
http://health.howstuffworks.com/human-body/systems/immune
http://biology.about.com/od/anatomy/ss/thymus.htm
http://www.britannica.com/EBchecked/topic/352799/lymphocyte
http://www.ebioscience.com/knowledge-center/antigen/immunoglobulin.htm
http://tle.westone.wa.gov.au/content/file/969144ed-0d3b-fa04-2e88-8b23de2a630c/1/human_bio_science_3b.zip/content/004_internal_defence/page_02.htm

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