What do your skin, your thymus and tears in your eyes have in common?

They are each part of your ‘immune system.’

The immune system is a complex but essential part of your body function.  Every moment of every day, we are exposed to a vast array of bacteria, microbes, viruses, toxins and parasites waiting to invade our bodies and attack our well-being.

The immune system provides three different levels of protection:

1.  It creates a barrier that prevents germs entering your body.

2.  If germs do get into your body, the immune system tries to detect and eliminate them before they can make themselves at home and reproduce.

3.  If the germs are able to reproduce and start causing problems the immune system is in charge of depending the body and eliminating the invaders.

But, let’s go back a step.  What does it mean when you say,  ‘I’m sick?’

When you are ‘sick’ it mean that your body is not working properly or to its full potential.  There are many different ways for you to get sick.

1.  Physical damage.  If you break a bone you will not be able to perform at your full potential 💯 until the break is healed.

2. Vitamin or mineral deficiency.  A lack of vitamin C may cause scurvy, and a lack of vitamin D can cause rickets.

3.  Organ degradation.  Drinking excessive amounts of alcohol can damage the liver and lead to cirrhosis.

4.  Genetic disease.  This is brought about by an error in the DNA coding which causes too much or too little of certain proteins to be manufactured.  This can lead to problems at the cellular level.

5.  Cancers.  Sometimes a cell will change in a way that causes it to reproduce in an uncontrollable way, creating tumours.

6.  Viral and bacterial infection.  When viruses or bacteria (more commonly called grrms) invade your body and reproduce they normally cause problems.  The germ’s presence produces some effect that makes you feel ‘sick.’

Viral and bacterial infections are by far the most common cause of ‘sickness’  for most people.  They cause illnesses such as colds, influenza, measles, mumps, AIDS and so on.  It is the work of your immune system to keep these infections at bay.  The size of the task can be illustrated by the rate of reproduction of bacteria.  Bacteria are single-called organisms that are capable of dividing into two separate bacteria about every 20 to 30 minutes.  So one bacterium entering your body and finding the right conditions to reproduce can become millions in just a few hours.

How does it work?

Your immune system has been working away in your body from the moment you were born, yet most of us are completely unaware that we have such a productive mechanism.

The most visible part of the immune system is the skin.  It acts as primary barrier between germs and the body.  Skin is tough and generally impermeable to germs unless the barrier has been penetrated in some ways, as with a cut.  The skin also excretes antibacterial substances that prevent bacteria building up on the skin.

The nose, mouth and eyes are also obvious points of invasion for germs.  Tears and mucus contain an enzyme (lysozyme) that breaks down the cell wall of many bacteria.  Saliva is also antibacterial.  Since the nasal passage and lungs are coated in mucus, many germs not killed immediately are trapped in the mucus and soon swallowed.  Most cells (which release histamine during inflammatory and allergic reactions) also line the nasal passages, throat, lungs, and skin.   Germs attempting to invade the body must first make it past these defenses.

One germs enter the body the immune system deals with them at a different levels.  The major components of the internal immune system are:

●  Thymus

●  Spleen

●  Lymph system

●  Bone marrow

●  White blood cells

●  Antibodies

●  Complement system

●  Hormones

Let’s look briefly at each of these components.


The thymus is in your chest, between the breastbone and the heart.  It produces T-cells (of which more later) and is vital to newborn babies.  Without a thymus a baby’s immune system collapses and the baby will die.  The thymus is less important in adults.  It can be removed and the adult will live because other parts of the immune system are capable of taking up the load.


The spleen, located in the abdomen, filters the blood looking  for foreign cells and for old red blood cells in need of replacement.  A person without a spleen will get sick much more often than someone with a spleen.

Lymph system.

The lymph system extends throughout the body in much the same way that your blood vessels do.  The main difference is that the blood flow is pressurized by the heart while fluids ooze into the lymph system and are pushed along by normal body and muscle motion to the lymph nodes.

Lymph is a clearish liquid that bathes the cells in water and nutrients.  Each cell does not have its own blood supply yet it has to get food, water and oxygen to survive.  Blood transfers these materials to the lymph carries it to the cells.  The cells also produce proteins and waste products and the lymph absorbs them and carries them away.  Any random bacteria that enter the body also find their way into this fluid.

The job of the lymph system is to drain and filter these fluids in the lymph nodes to detect and remove bacteria.  Lymph nodes contain filtering tissue and a large number of lymph cells.  When fighting certain bacterial infections the lymph nodes swell with bacteria and the cells fighting the bacteria, growing to the point where the enlarged nodes can be felt.  Swollen lymph nodes are therefore a good indication that you are fighting an infection of some sort.

Bone marrow.

Bone marrow produces new blood cells, both red and white.  Red blood cells are fully formed in the marrow and then enter the blood stream.  In the case of some white blood cells, the cells mature elsewhere.  The marrow produces all blood cells from ‘stem’ cells, so-called because they can branch off and become many different types of cells.

White blood cells.

White blood cells are probably the most important part of the immune system.  White blood cells are actually a whole range of different cells that work together to destroy bacteria and viruses.

All white blood cells are  called, generally, Leukocytes.  They are not like normal body cells.  They act like independent, living, single-called organism, able to move and engulf things on their own.  Many white blood cells cannot reproduce on their own and rely upon a factory within the body to produce them.  That factory is the bone marrow.

Leukocytes are divided into three classes;

1.  Granulocytes.  These make up 50% to 60% of all Leukocytes and are themselves divided into three classes; neutrophils, Eosinophil and Basophils.  Granulocytes contain granules of different chemicals, depending on the type of cell.

2.  Lymphocytes.  30% to 40%.  There are two classes:  B cells which mature in bone marrow and  T cells which mature in the thymus.

3.  Monocytes make up about 7% of all leukocytes and evolve into macrophage.

All white blood cells start in bone marrow as stem cells, genetic cells that can form into the many different types of leukocytes as they mature.  (A ‘bone marrow transplant’ is accomplished by injecting stem cells from a donor into the blood stream.  The stem cells find their way into the bone marrow of the recipient and make their home there).

Each of the different types of leukocytes has a special role to play in the immune system.  The following will help you to understand the different roles.

Neutrophils are the most common form of white blood cells.  They have only a short life, generally less than a day, so the bone marrow produces them in huge numbers every day and releases them into the blood stream.  They are attracted to foreign material, inflammation and bacteria.  When they find a foreign particle or bacteria they engulf and kill it.  At a site of serious infection pus will form.  Pus is simply dead neutrophils and other dead cell matter.

Eosinophils and Basophils are far less common.  Eosinophils are focused on parasites in the skin and lungs while Basophils carry histamine and are therefore important, together with Mast cells, in causing inflammation.  From the immune system’s view inflammation is a good thing.  It brings in more blood to the site and dilates capillary walls so that more immune system cells can attack the site of infection.

Of all blood cells, Macrophages are the largest (hence the name ‘macro’).  Monocytes released from the bone marrow, enter tissue and turn into macrophages.  Most boundary tissue has its own particular Macrophages.  For example, alveolar Macrophages live in the lungs which they keep clean (by ingesting smoke and dust) and disease free (by ingesting germs).  One of their main jobs is to clean up dead neutrophils.  Macrophages clean up pus, for example, as part of the healing process.

Lymphocytes handle most of the bacterial and viral infections.  They start in the bone marrow and those that become B cells mature there before entering the blood stream.  T cells start in the marrow but migrate through the blood stream to the thymus where they mature.  Both types are found in the blood stream but tend to concentrate in lymph tissue such as the lymph nodes, thymus and spleen.  B cells, when stimulated, mature into plasma cells that produce antibodies.  A specific B cells is tuned to a specific germ, and when the germ is present in the body the B cell produces millions of antibodies designed to eliminate the germ.  T cells, on the other hand, actually bump against other cells and destroy them.  ‘Killer’ T cells detect body cells harboring a virus and kill them.  Two other types, known as ‘Helper and ‘Suppressor’ T cells, help sensitise killer T cells and control the immune response.

How does a white blood cell know what to attack and what to leave alone? Why doesn’t a white blood cell attack every other type of cell in the body?

There is a system built into every cell in your body called the Major Histocompatabiliy Complex (MHC) which marks the cells in your body as ‘you’.  Anything  the immune system finds which does not have these markings, or has the wrong markings, is fair game.


Antibodies, also known as immunoglobulins or 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 section at the tips of the two branches of the Y which is sensitive to a specific antigen and which binds to it in some ways.  The binding generally disables the action of the antigen.  When an antibody binds to the outer coat of a virus particles or the cells wall of a bacterium it can stop their movement through cell walls.  A large number of antibodies can bind to an invader and signal to the complement system that the invader needs to be removed.

Protection against specific diseases can be induced by inoculation or vaccination,  where a weakened strain of the antigen that causes the disease is introduced into the body to stimulate the production of antibodies against that disease.

Complement system.

The complement system, like antibodies, is a series of proteins.  Unlike antibodies, which are present in millions in the blood stream, there are only a handful of proteins in the complement system, floating freely in the blood.  The complement proteins are activated by the work with (complement) the antibodies.


There are several hormones generated by components of the immune system, known generally as lymphokines.  It is also known that the presence of certain hormones, such as steroids and corticosteroids, suppresses the immune system.

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