The immune system
From the point of view of an immunologist, the body can be considered as a large community of cells all working together to keep the system functioning. Some cells have jobs that keep them in the same place all the time; nerve cells, for instance. Other cells, such as blood cells, are constanly on the move, carrying nutrients, and taking away waste. Some cells, such as brain cells, are very long-lived, while other cells, such as skin cells, have a relatively short life.