Before delving into the astounding details of the war of defence fought in the innermost recesses of our bodies, we must first have a general look at the defence system and its elements.

Briefly, the defence system may be defined as "an extremely disciplined, hard-working and organised army that protects the body from the clutches of external enemies." In this multi-faceted war, the main duty of the elements fighting in the front line is to prevent the enemy cells, such as bacteria or viruses, from entering the body.

Although it is not easy for the enemy organisms to enter the body, they exert themselves to the utmost to reach their ultimate goal of invading the body. When they successfully do so, after overcoming various obstacles such as the skin, and the respiratory and digestive tracts, they will find tough warriors waiting for them. These tough warriors are produced and trained in specialised centres such as the bone marrow, spleen, thymus, and lymph nodes. These warriors are "the defence cells" referred to as the macrophages and lymphocytes.

First, various types of phagocytes, which are called the "the eater cells" will swing into action. Then the macrophages, another specific type of phagocytes, take their turn. They all destroy the enemy by engulfing it. Macrophages also perform other duties such as summoning other defence cells to the battleground, and raising the body temperature. The rise in temperature at the onset of a sickness is very important, for the afflicted the person will feel fatigued by it and need to rest, thus reserving the energy needed to fight against the enemies.

If these elements of the immune system prove insufficient against the enemies penetrating the body, then lymphocytes, the champions of the system, come into play. Lymphocytes are of two types; B cells and T cells. These are again further divided into sub groups.

The helper T cells are next in reaching the battleground after the macrophages. They may be considered the administrative agents of the system. After the helper T cells identify the enemy, they warn other cells in order to initiate a war against it.

Thus alerted, the killer T cells come into play to destroy the besieged enemy.

The B cells are the armaments factory of the human body. Following their stimulation by the helper T cells, they immediately start to produce a sort of weapon called the "antibody".

After the alarm is over, suppressor T cells stop the activity of all defence cells, and therefore prevent the war from lasting any longer than is necessary.

However, the mission of the defence army has not yet ended. The warrior cells, called the memory cells, store necessary information about the enemy in their memories and keep it for years. This will enable the immune system to mount a quick defence against the same enemy at later meetings with it.

There are many more incredible factors hidden in the details of the defence system, which we have very briefly outlined above. As mentioned before, in this book, these extraordinary events are told in an easy-to-understand way.