Understanding the Immune System

Your immune system does most of its work largely unnoticed, behind the scenes. It is only when it fails to work, that you realize the importance of having one.

When you cut yourself, large amounts of bacteria and viruses enter into your body. Your immune system immediately responds, killing these foreign invaders as the skin heals itself, sealing the open wound.

Sometimes, it fails to do its job, and the cut will become infected, inflamed and full of pus. This is a sign that your immune system is actually working.

Each day, thousands of bacteria enter your body, most of them not making it past the saliva in your mouth, or the acid in your stomach. They are so small, that 1000 of them could fit across the edge of a pencil eraser. However, occasionally, some of them do and cause food poisoning.

There are many human ailments caused by the immune system working inappropriately.

One example would be allergies. Allergies are caused when the immune system over-responds to stimuli that most people don't respond to at all.

Another example would be diabetes. This is caused when the body incorrectly identifies cells in the pancreas, and subsequently, destroys them.

Whenever viruses and bacteria enter your body, and reproduce, their presence can make you sick. Bacterial infections can be caused by a wide range of bacteria, which result in mild to life-threatening illnesses,

A virus is a completely different animal. It is not actually alive, but is a fragment of DNA in a protective coat. It does its damage by coming into contact with a cell, and subsequently injects its DNA into the cell. Once inside the cell, the virus uses the existing cellular components, to help create new virus particles.

Normally, when your immune system is working correctly, it is able to properly eliminate these bacteria and viruses, collectively known as antigens. It does this through a series of steps called the immune response, which involves a network of cells, tissues and organs, whose purpose is to provide protection against these organisms.

In order for an immune response to antigens to occur, there must be an antigen-presenting cell (APC) present.

This is usually in the form of a macrophage, a specific type of white blood cell whose job is to devour certain types of invading pathogens.

Or it may be dendritic cells, specific immune cells containing threadlike tentacles called dendrites, which envelops the invader, and then present it to other white blood cells called T cells or B cells.

Whenever an APC presents an antigen to a B cell, this will cause the B cell to grow rapidly, producing antibodies that bind to the specific antigen. Once it does this, this functions to signal the macrophages to devour and kill them.

Another important function of antibodies is to initiate the "complement destruction cascade."

Whenever antibodies bind to bacteria, specialized proteins called complement, bind to the antibodies, and destroy the bacteria by creating holes in them.

The above-mentioned T cells are a particular type of white blood cell that belongs to a division of the immune system called the acquired immune system.

With acquired immunity, your body learns how to combat foreign invaders, remembering them the next time they are encountered. T cells are so named because they mature in an organ called the thymus.

There are several different types of T cells, each one performing a different function when detecting an antigen. These are the killer T cells, the helper T cells and the suppressor T cells.

On the surface of each T cell, you will find T cell receptors (TCR) that allow each T cell to recognize only a small group of antigens. This means they are only effective against certain antigens while ineffective against others.

Almost all cells in the body express a protein called the Major Histocompatibility Complex (MHC) protein. The function of MHC is to present antigens to the various T cells.

Each MHC has a small slit, shaped like a letterbox, and the cell will then push the antigen through this slit. Then, the T-cell receptors will try to bind with the particular antigen.

MHC comes in two varieties, MHC class I and MHC class II.

Whenever their work is done, the supressor T cells come into play, sending chemical messages to all the WBCs that their work is done, thus ending the immune response.

Your immune system has been working behind the scenes most of your life, but you probably don't know much about it. Although there are many organs that are part of this system, perhaps the most obvious place to start is your skin.

Your skin functions as a protective barrier, preventing germs from entering your body. If this protective barrier is compromised, such as when you cut yourself, your body attempts to heal the open area, while attacking invading germs.

Next, we have the lymphatic system. The lymphatic system is probably familiar to most people, as mothers and doctors typically check for swollen lymph nodes in the neck, to help determine if there is an infection present.

The lymphatic system is part of a circulatory system that runs throughout your entire body. Unlike blood, however, which relies on your heart to circulate it, lymphatic fluid is moved throughout the body because of normal body movement. This is why paraplegics, for example, typically have fluid retention in their extremities.

Lymphatic fluid is a clear liquid that provide cells with water and nutrients. Essentially, it is blood plasma, the liquid that constitutes blood, minus the red and white blood cells.

Any random bacteria entering the body, will eventually find its way into the lymphatic system, whose job is to filter the fluid, thereby eliminating bacteria. When battling bacterial infections, the lymph nodes become swollen with bacteria and the cells fighting them. Consequently, swollen lymph nodes are a good sign that you have an infection.

Next, we have the thymus gland. The thymus gland lies between your chest and your heart, and is responsible for the production of a particular type of white blood cell called the T cell.

The thymus gland is especially important in newborn babies. In newborn babies that are born without a thymus gland, their immune system collapses and they will subsequently die.

Concerning immunity, of all blood cell types, the white blood cells are probably the most important. Many white blood cells work together to destroy bacteria and viruses. Here's a list of all the different white blood cells:

• Leukocytes
• Lymphocyte
• Monocytes
• Granulocytes
• B-cells
• Plasma cells
• T-cells
• Helper T-cells
• Killer T-cells
• Suppressor T-cells
• Natural killer cells
• Neutrophils
• Eosinophils
• Basophils
• Phagocytes
• Macrophages

All white blood cells are officially classified as leukocytes. They function as independent, living, single cell organisms, able to take action on their own. They resemble amoeba, by virtue of their movement capabilities, and their ability to engulf other cells and bacteria.

Furthermore, leukocytes are subdivided into 3 categories:

Granulocytes - Granulocytes comprise 50 to 60% of all leukocytes. There are three types of granulocytes: neutrophils, eosinophils, and basophils.

They are called granulocytes because of their granular texture, with different granules containing different chemicals depending on the type of cell.

Neutrophils are the most common type of white blood cell you have in your body. Your bone marrow literally produces trillions of them each day. However, their life span is short, typically less than a day.

Upon entering the bloodstream, neutrophils can move through capillary walls into tissues. Once a neutrophil encounters any foreign material, it will surround it, releasing enzymes, hydrogen peroxide and other chemicals from the granules to kill it.

The largest of all white blood cells are the macrophages. Most border tissues in many organs contain macrophages, however, they are found mainly in the liver and the lymph nodes. Their primary function is to clear the blood of foreign materials.

Fragments of what they have eaten are presented to the T cells, which release chemical messengers that signal the B cells.

Subsequently, the B cells will produce antibodies which will specifically target and bind to that particular antigen. Once this happens, the macrophages come into play once again, as macrophages have a particular attraction for cells with antibodies attached..

Lymphocytes - Lymphocytes comprise 30 to 40% of all leukocytes. There are two types of leukocytes: B cells, those that mature in the bone marrow, and T cells, those that mature in the thymus

Monocytes - Monocytes comprise approximately 7% of all leukocytes, and are subsequently converted into macrophages.

An antibody is a specific protein produced by plasma cells. Whenever a foreign substance, or antigen is encountered, these antibodies bind to the antigen, which functions to signal other immune system cells that they have been tagged. There can be millions of unique antibodies.

On the surface of each antigen is a specific part called the epitope that the antibody will bind to.

Each epitope has a specific shape that allows only one specific antibody to attach to it. The tip of each antibody also has a unique shape, specific only to that particular antibody. Once this connection has been made, the antigen becomes immediately recognizable to other defensive cells as a foreign substance.

At this point, other specific cells, including the killer T cells, attack the antigen

There are several immune system hormones that are collectively known as lymphokines.

The thymus produces a hormone called tymosin. Tymosin encourages production of lymphocytes, a type of immune system white blood cell.

The interleukins are another immune system hormone. Interleukins are also produced by various white blood cells. They function as signaling molecules, providing direction to certain types of white blood cells.

They are the primary directors of the immune system. At this time, there are 35 known interleukins.

This concludes my overview of the immune system. As you can see, it is a complex system of anatomical components providing protection against foreign invaders such as bacteria and viruses.

There are literally thousands of antigens, which would quickly overcome our bodies if not for the intricate, behind the scenes, working of our immune system. I never cease to be amazed by the intelligence that is built in to the human body.