Dr. Martin Philpott from Auckland Medical School explains thatlines are made from cells that are , or don't die. An example of an immortal cell is a cell. It keeps growing and dividing uncontrollably. It does not have the triggers that cause it to die.
Dr Martin PhilpottMost cells from your body won’t grow indefinitely in tissue . If you put a single one of your cells, some of them will divide a few times and keep growing and then realise that they are not really supposed to be doing that. All cells in your body have a set lifetime, and they will just over time slowly die out. So all of our lines have had to be immortalised in one way or another.