Immortalized cell lines are cells that have been genetically altered to bypass the normal cellular aging process, allowing them to divide indefinitely in culture. This ability makes them invaluable tools in cell biology research, enabling scientists to conduct experiments over prolonged periods without the limitations associated with primary cells, such as senescence and limited lifespan. These cell lines facilitate reproducibility and consistency in experiments, playing a critical role in various research applications, including drug development, cancer research, and genetic studies.