A cell in the kidney, although it contains the same genetic information as a brain cell, performs a completely separate role.
Cancer happens when a tiny part of the cell's mechanism goes wrong.
What is a cancer cell?
And just as there are hundreds of types of cell, there are hundreds of types of cancer, few of which can be treated in the same way.
Every cell's life is mapped out in advance by coded instructions, called genes, held in its nucleus.
These tell it how to behave, when to reproduce by dividing - and when to die.
When the instructions relating to cell multiplication and dying are wrong, the cell may start dividing uncontrollably, and not die when it should.
In addition, the cancer may not follow the usual instructions that keep cells spaced out properly. �3C/p>
Every time the cell divides, the "bad" instruction is reproduced, so the out-of-control multiplication carries on.
As these cells can be multiplying more rapidly than healthy cells, the cancer cells can form a growing lump in the body called a tumour or a lesion.
As this gets larger, it can even grow its own vessels to keep it supplied with blood.
A benign, or non-cancerous tumour shares this uncontrolled growth, but will not generally invade neighbouring tissues and damage them.
Tumours which do this are "malignant", or "cancerous".
The type of cell in which the cancer starts will generally determine the speed at which it grows, and its resistance to treatment, although there are many variations.
Cancers harm health in a number of ways. The very size of the tumour can interfere with nearby organs, or ducts which carry important chemicals, causing pain or other symptoms.
For example, a tumour on the pancreas can grow to block the bile duct, leading to the patient developing obstructive jaundice.
And a brain tumour can push on important parts of the brain, causing blackouts, fits and other problems.
Even benign tumours can cause these problems if located in the wrong place.
When a cancer invade nearby tissues, they can cause bleeding from damaged blood vessels, and stop the organ which they are invading from working properly.
What happens if it spreads?
As a tumour grows, cells can break off and start growing on adjacent tissues and organs.
For example, if a bowel cancer has spread through the wall of the bowel itself, it can start growing on the bladder.
Cells can also enter the bloodstream and travel to distant organs, such as the lungs or brain.
The technical term for this is "metastasis".
When new tumours form on distant organs, they behave like the original tumour - so a bowel cancer cell growing in the lung will not be lung cancer.
Once other organs are involved, then any symptoms of the cancer can get worse.
However, it may be some time before a growing cancer in certain parts of the body produces symptoms that the patient can notice.
Once a cancer has started to spread beyond its original site, then the chances of a cure often begin to fall, as it becomes more difficult to treat.
How is it treated?
There are three principal ways of treating cancer.
The first is surgery, normally an operation to remove the cancerous growth, and, depending on its type, nearby tissues and organs.
A cancer patient may first undergo a minor operation called a biopsy to take a small sample of the cancer for analysis.
The surgeon will try to remove as much of the cancer as possible, but sometimes extra treatment will be needed.
This could either take the form of radiotherapy or chemotherapy, or a combination of treatments.
http://news.bbc.co.uk/go/pr/fr/-/2/hi/health/medical_notes/3243569.stm
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