For the immune system, life is hard. It is a 24-hours-a-day, 7-days-a-week, 52-weeks-a-year battle against a well-equipped and persistent army trying to harm your health. The immune system never rests and must always be on red alert. It takes no furloughs.

The soldiers who make up the immune system come from and are found in a diverse collection of organs. Although the components, when taken together, weigh only about two pounds (900 g), these two pounds are integral in keeping the scale of good health balanced.

The sentries

The largest and most easily seen component of the immune system is the skin. The skin is a physical barrier against pathogens—harmful bacteria, viruses, and fungi—and also a chemical barrier: the skin’s natural acidity is a poor environment for invaders.

The mucous membranes are the sentries at the gates of our body: the openings of the eyes, sinuses, mouth, and so on. They secrete mucus, which both cleans away and traps pathogens. If bacteria should progress through the mouth or nose and into the stomach, it is still difficult for them to get into the blood. They must survive the stomach, which to them is a poisonous torture chamber of acids and digestive enzymes.

Within the gates

When a pathogen breaches a gate and enters the body, other components of the immune system—white blood cells—go to work. One type of white blood cell, a phagocyte, is like the skin in that it counters all invaders. Other types of white blood cells, in the class known as lymphocytes, are programmed to go after only certain pathogens. The various types of white blood cells all work in different ways, but they all need each other to complete the job of protecting the body.

Phagocytes

Phagocytes ("cell eaters") are large white blood cells that engulf and digest pathogens. An important type of phagocyte is monocytes, which circulate throughout the body looking for trouble—looking for pathogens. When monocytes get into tissue, they develop into macrophages ("big eaters"). Macrophages are positioned throughout body tissue and are often specialized: you might say some have an appetite for pathogens found in the lungs, while others prefer the taste of those found in the kidneys. Macrophages also are the "sweepers" of the body, as they dispose of worn out cells.

Other phagocytes are granulocytes. Of these, mast cells are found in the tissue, and neutrophils, eosinophils, and basophils are found in the blood.

Lymphocytes

Lymphocytes are small white blood cells that travel through the lymph system. They are not able to attack just any pathogen, as phagocytes are. They are programmed to go after specific pathogens. They also bear the major responsibility for the actions of the immune system.

The two major classes of lymphocytes are B cells, which reach maturity in the bone, and T cells, which reach maturity in the thymus. Both of these recognize specific pathogens.

B cells work by producing, transporting, and secreting antibodies. Upon meeting a pathogen, B cells begin dividing and releasing antibodies, which seek out and destroy the pathogen. Immune system components known as "complements" also aid the antibodies in destroying pathogens.

Each B cell makes one specific antibody for one specific pathogen. If a B cell meets another pathogen, nothing can be done. One type of T cell, a helper T cell, must be involved for a B cell to destroy pathogens.

T cells can help B cells or other T cells, or directly attack pathogens. When they directly attack the pathogen, they are known as cytotoxic T cells. Another important T cell is the natural killer (NK). These T cells are similar to phagocytes in that they do not need to recognize a pathogen to swing into action. They are important in targeting tumor cells.

There are also suppressor T cells, which act as the referee in the battle. When the immune system has won a battle, the suppressor T cells call off the troops.

Other Players

Bone marrow: The soft tissue in the center of bones cells, including white blood cells.

Thymus: The thymus fosters development of  T cells.

Lymphatic vessels: The lymphatic vessels are arteries that carry white blood cells throughout the body.

Lymphatic nodes and the spleen: The nodes and spleen are both "stopping pints" for white

Lymphatic System