Immortalized cell lines are cells that have been modified to proliferate indefinitely in laboratory conditions. This process often involves altering the cells' genetic material, allowing them to bypass normal cellular aging and death. These cell lines are crucial for research, as they provide a consistent and renewable source of cells for experiments. Researchers use immortalized cell lines to study various biological processes, test drugs, and develop vaccines. Common examples include the HeLa cell line, derived from cervical cancer cells, and the 293 cell line, originally from human embryonic kidney cells. These cell lines have significantly advanced scientific understanding and medical research.