Body Story

• Lymphocytes

• Phagocytes

• Mast cells

• Support

• Communication

• The lymphatic system

The human body's rich and varied biology provides an ideal breeding ground for all sorts of unwanted outsiders. Bacteria, viruses, fungi and other parasites seek it out to set up home, eat well and multiply. When they move in, they bring with them infection and disease. But first they must overcome the body's army of defences: the cells and organs of the immune system, which are dedicated to protecting the body from just such unwelcome intruders.

All an individual's cells carry ID declaring their right to be present. The ID takes the form of intricate shapes on the surface of cells, like the protrusions on the pieces of a jigsaw. Having the wrong shape labels a cell an intruder. Only the cells of an identical twin are indistinguishable.

The body's immune system is constantly on the lookout for intruders. There are guards at all entry points, backed up by battalions stationed close by. Squads patrol constantly, some members attaching themselves to intruders until they can get help, others killing on contact. A captured intruder can be blown apart or eaten. It is a remarkable system of defence.

• Lymphocytes
The foot soldiers of this army are the white blood cells, which are produced, like all blood cells, in the bone marrow. There are two major groups of white blood cells: lymphocytes and phagocytes.

Lymphocytes fall into two divisions. On encountering an intruder, B-cells, which make up the first division, earmark it as an enemy. Each B-cell does this by producing small pieces of jigsaw that fit the intruder's own shapes. These attaching jigsaw pieces are called antibodies, and they work in different ways.

Some disable bacteria directly, just by locking onto them. Others prevent viruses from invading cells. And others act to make intruders more palatable to scavenging cells, which will later engulf and destroy them. Another antibody method is to trigger a domino reaction in a group of chemicals that circulate in the blood. The chain of events eventually forces an entry channel into the intruder's cells. As fluids flood in, the intruder bursts. Yet another class of antibodies can make intruders more vulnerable to attack from different types of white blood cells.

B-cells are specialists. Each one is programmed to make a specific antibody to fit a particular intruder. For example, there is a B-cell that produces antibodies to block the virus that causes the common cold, and another to fight the bacteria that causes pneumonia. Each B-cell can create millions of its specialised antibodies.

The second division of lymphocytes are the T-cells. Some of these are 'helpers' or 'inducers' which alert other white blood cells to the presence of intruders. They are partnered by suppressor T-cells, which turn the helpers off when they have done their job. Together these are the regulatory T-cells. Cytotoxic T-cells are more dramatic. They are assassins loaded with chemical grenades with which they blow holes in intruder cells, allowing fluids to force their way in, causing the intruder cell to explode. Cytotoxic T-cells, like B-cells, are programmed to fight specific intruders.

There are so many potential intruders that it is impossible for the body to maintain a strong force of cells specialised to fight each and every enemy. Instead, the immune system stores just a few of each kind. When a particular intruder is encountered, these cells multiply to form a full-scale army. Then, once the threat has passed, they are suppressed, leaving a just a few in readiness for a renewed attack.

Natural killer cells, on the other hand, are capable of reacting to all intruders. They are also lymphocytes, and like cytotoxic T-cells they explode their targets by piercing them. Because they are non-specific, they are effective against a wide range of infections.

• Phagocytes
The second major group of white blood cells are the cell-devouring phagocytes.

Most phagocytes begin life as monocytes, which are sent out from the bone marrow to circulate in the blood. After about a day they move into tissue where they develop into macrophages — the white army's big eaters. In different tissues, macrophages specialise into tissue-specific cells, for example, the alveolar macrophages of the lungs, or the microglial macrophages in the brain.

Macrophages act as scavengers, consuming the body's worn out cells and other debris. They also engulf and digest intruders (antigens). The antigen fragments then attract cytotoxic T-cells and lock onto the macrophage, destroying it and the broken-down antigen that it contains. It's a form of suicide attack.

Another important type of phagocyte is the neutrophil. This also ingests intruders, but kills them with its own stock of granule-packed potent chemicals. Because of these granules neutrophils are also known granulocytes. Other granulocytes include eosinophils, which fight parasites, and basophils, whose function is still unclear.

• Mast cells
Mast cells are another type of white blood cell that swollen with chemicals vital to the body's defence. They are stationed around the body, particularly where the body comes into contact with the outside world, for example, in the lungs, the skin, the tongue and the linings of the nose. The chemicals they contain include some controlling the action of eosinophils and basophils, and others that create inflammatory responses, particularly histamine, which is significant in creating allergic reactions.

• Support
These major players in the battle against infection are supported by other cells: dendritic lymphocytes, which use their thread-like tentacles to trap intruders; and blood platelets, which can release chemicals to activate immune responses in lymphocytes.

• Communication
Communication between the different divisions of the immune system is carried out through diverse chemical messengers, secreted by some cells to target others. B-cells and T-cells produce communicating lymphokines, while monocytes and macrophages produce monokines. These messengers can activate other cells and chemicals, and they are also important in creating an inflammatory response.

• The lymphatic system
The trenches in the war against infection are the lymph vessels, a parallel transport system to the arteries, veins and capillaries which circulate the blood. These vessels carry lymph, a clear fluid which contains, amongst other things, lymphocytes and phagocytes. Fluids can move easily between the lymph vessels, the tissues and the blood. Along them, immune stations called lymph nodes contain clusters of waiting white blood cells. Some intruders are brought along the lymph vessels to stations to be destroyed.

The lymphatic system also includes: the bone marrow and the thymus, which are important in the production of white blood cells; the spleen, which filters both lymph fluid and blood; and other smaller organs including the tonsils and the appendix. Both the tonsils and the appendix are formed from lymphatic tissue made up of white blood cells, acting as a cache of cells that ensure the availability and quick assembly of an immune response anywhere it is needed.

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