In vitro, normal cells taken directly from an animal (referred to as primary cells) can only divide a certain number of times (cells will either enter a senescence stage, or die).
Transformed cells are cancerous cells. They possess all six hallmarks of cancerous cells :
1- Growth factor independency
2- No response to growth inhibitors
3- Evasion of apoptosis
4- Can promote angiogenesis
5- Unlimited proliferation
6- Invasive
Immortalized cells have sufficient mutations to be able to divide indefinitely in culture. However, they are not cancerous seen their dependence of growth factors and their sensitivity to growth inhibitors. Cells can be immortalized spontaneously or established in the lab (by using adenovirus for example).
