An immortal cell line is a group of cells that can divide and grow indefinitely in a laboratory, unlike normal cells, which stop dividing after roughly 50 rounds of replication. Normal human cells have a built-in expiration date. Immortal cell lines have bypassed it, making them one of the most important tools in modern biomedical research.
Why Normal Cells Stop Dividing
Every time a cell divides, the protective caps on the ends of its chromosomes, called telomeres, get a little shorter. Think of them like the plastic tips on shoelaces: they keep the important material from fraying. After about 50 divisions, those caps wear down so much that the cell can no longer copy itself safely. It enters a permanent retirement state called senescence and eventually dies.
This built-in limit was first described by biologist Leonard Hayflick in the early 1960s and is still called the Hayflick limit. It applies to most human somatic cells (the everyday cells that make up your tissues, as opposed to reproductive cells). For researchers who need large, consistent supplies of cells for experiments, this limit creates a serious practical problem: primary cells harvested from a donor run out quickly and vary from person to person.
How Cells Become Immortal
Immortalization is the process of overriding the Hayflick limit so cells can keep dividing without entering senescence. There are two main ways this happens.
The first is reactivating telomerase, the enzyme that rebuilds telomere caps. Most adult cells have the gene for telomerase but keep it switched off. Scientists can insert an active copy of the telomerase gene (called hTERT) into cells, which restores the enzyme’s function and prevents telomere shortening. This single change is often enough to immortalize skin cells, muscle stem cells, blood vessel cells, and several other cell types.
The second approach uses viral proteins or other genetic tools to disable the cell’s internal brakes on division. Certain tumor-suppressor pathways normally force a cell to stop growing when something goes wrong. Introducing specific viral genes can shut down those pathways, allowing the cell to keep replicating. In practice, researchers sometimes combine both methods, pairing telomerase activation with one or more additional genetic changes, depending on the cell type.
Some cells become immortal naturally. Cancer cells, for instance, often reactivate telomerase on their own or develop alternative mechanisms to maintain their telomeres, which is part of what makes tumors so difficult to stop.
HeLa: The First Human Immortal Cell Line
The most famous immortal cell line traces back to a woman named Henrietta Lacks. In 1951, doctors at Johns Hopkins Hospital in Baltimore took a sample from her aggressive cervical tumor during treatment. Researcher George Otto Gey noticed something unusual: unlike every other human cell sample his lab had tried to grow, these cells thrived and kept multiplying. He isolated a particularly vigorous cell, grew it into a self-sustaining line, and named it HeLa after the first two letters of her first and last names.
Lacks died of her cancer later that year at age 31. Her cells, however, are still alive and growing in laboratories around the world more than 70 years later. HeLa cells were the first human cells that could be kept alive in a lab indefinitely, and they enabled breakthroughs in virology, genetics, and cancer biology. Critically, the cells were taken without Lacks’ knowledge or consent, which was standard practice at the time but has since become a landmark case in research ethics.
How Immortal Cell Lines Are Used
Immortal cell lines solve several problems at once. They provide an unlimited supply of genetically identical cells, they eliminate the variability that comes from using tissue from different donors, and they are easier to genetically modify than freshly harvested cells. This makes them indispensable across several fields.
In vaccine production, Vero cells (originally derived from African green monkey kidney tissue) are one of the most widely used substrates for growing viruses. HEK293T cells, derived from human embryonic kidney tissue, are a workhorse for producing viral vectors used in gene therapy. As gene therapies move from small clinical trials to large-scale manufacturing, the ability to grow these cells in massive bioreactors has become a bottleneck that researchers are actively working to overcome.
In drug development, immortal cell lines serve as standardized test subjects for screening thousands of compounds for toxicity or therapeutic effect. Because every flask of a given cell line is genetically near-identical, results from one lab can be compared meaningfully with results from another. And in basic research, cell lines allow scientists to study disease mechanisms, test gene functions, and model human biology without constantly needing fresh donor tissue.
The Genetic Drift Problem
Immortal cell lines are not frozen in time. They continue to mutate and evolve with every division, and over years of passage through different laboratories, a single cell line can diverge into genetically distinct versions of itself. This is a well-documented issue with HeLa cells in particular.
When researchers in the 1950s first examined HeLa chromosomes, they found the cells carried between 75 and 82 chromosomes per cell, far more than the normal 46. By now, HeLa cells have accumulated so many duplications, deletions, and rearrangements that some scientists have argued the genome is no longer recognizably human. In 1991, one researcher even proposed classifying HeLa as a new species.
A study comparing four batches of HeLa cells that had been cultured in different Italian laboratories for 8 to 12 years found deep variability between them. Not only did the batches differ in their DNA gains and losses, but they also responded differently to the same experimental conditions. One batch showed dramatic gene expression changes under low-oxygen conditions, while another barely responded at all. Nearly 2,900 genes were affected in one clone versus just 145 in another from the same experiment. These differences mean that a study performed with HeLa cells in one lab may not be directly comparable to a study using HeLa cells from another, which has real implications for reproducibility.
Ethics and Informed Consent
The story of Henrietta Lacks reshaped how the research community thinks about consent. Her cells were taken during a routine biopsy without any discussion about their use in research, and her family did not learn about the HeLa line’s existence for decades. Today, ethical standards require voluntary informed consent from anyone whose biological material will be used for research purposes. NIH guidelines, for instance, specify that donors must give written consent freely, without financial inducement, and must be told they can withdraw that consent up until the point when their identity can no longer be linked to the material.
These protections reflect a broader shift toward transparency that has developed over the past several decades. The creation of new immortal cell lines from human tissue now involves institutional review and clear documentation of consent, a direct legacy of the ethical questions raised by HeLa.
Limitations Compared to Primary Cells
For all their usefulness, immortal cell lines are not perfect stand-ins for the cells inside your body. The very genetic changes that make them immortal can also alter their behavior. Immortalized cells may differ from their source tissue in how they metabolize drugs, respond to signals, or express genes. This is why results obtained in cell lines always need further validation, typically in animal models and eventually in human clinical trials, before they can inform medical decisions. Researchers choose immortal cell lines when consistency and scale matter most, and primary cells when staying close to natural biology is the priority.