Most of the 300 trillion cells that make up the human body are fully specialised for particular functions in organs such as the heart or the brain, or in tissues like muscle, fat and bone. Others play a supply or defensive role in the blood or immune system.
In fact, each cell type has a specific lifespan and function, which is dependent on the activity that cell is required to perform. Some cells are replaced, others live for the duration of a person’s life. For example, blood cells only live for up to a few months, and are replaced at a rate of several billion each day, whereas brain cells may last a lifetime.