HeLa cells are immortal because a cascade of biological accidents, driven by an aggressive strain of human papillomavirus, rewired nearly every safeguard a normal cell uses to limit its own lifespan. Henrietta Lacks’ cervical cancer cells, taken in 1951 at Johns Hopkins Hospital, carried HPV 18 DNA that had integrated into the cellular genome and shut down the two most important tumor-suppressing proteins in human biology. That alone would have produced a dangerous cancer, but the same viral disruption also unlocked telomerase, the enzyme that rebuilds the protective caps on chromosomes, solving the problem that causes ordinary cells to stop dividing after a set number of generations. The result was a cell line that could, in principle, divide forever, and more than seven decades later, it still does.
How HPV Knocked Out the Cell’s Brakes
Every human cell carries built-in mechanisms to stop itself from dividing uncontrollably. The two most critical are the proteins p53 and Rb (retinoblastoma protein). Think of p53 as a damage inspector: when DNA gets corrupted, p53 halts the cell cycle so repairs can be made, or triggers the cell to self-destruct if the damage is too severe. Rb acts as a gatekeeper for cell division, preventing the cell from copying itself until it gets the right signals. Together, they form a powerful two-layered defense against cancer.
In Henrietta Lacks’ tumor cells, HPV 18 produced two viral proteins, E6 and E7, that neutralized both defenses simultaneously. E6 targeted p53 for destruction, while E7 bound to and inactivated Rb. Researchers confirmed how dependent HeLa cells are on this arrangement by engineering a way to shut off E6 and E7 expression. When those viral proteins were repressed, p53 levels surged dramatically, the Rb protein reappeared in its active form, and DNA replication ground to a halt within about 12 hours. Several genes downstream of p53, including ones that block cell-cycle progression, switched on in sequence, essentially restoring the tumor suppressor network that had been dormant the entire time the viral proteins were active.1PubMed Central. Repression of human papillomavirus oncogenes in HeLa cervical carcinoma cells causes the orderly reactivation of dormant tumor suppressor pathways
The experiment revealed something striking: the tumor suppressor genes were never actually deleted or permanently broken. They were simply silenced by the virus. Remove the virus’s influence and the cell remembers how to die. This is a key part of why HeLa cells are immortal: their normal death program is intact but perpetually suppressed. The viral proteins act like a finger permanently pressing the “override” button.
Telomerase and the Problem Normal Cells Cannot Solve
Even with tumor suppressors knocked out, a cancer cell faces another barrier to true immortality. Every time a cell divides, the protective caps at the ends of its chromosomes, called telomeres, get a little shorter. In most human cells, this shortening acts as a biological countdown clock. After a certain number of divisions, the telomeres become critically short, the cell receives a signal to stop dividing permanently, and it enters a state of irreversible arrest. This process evolved in part as a defense against cancer, because a cell that cannot divide indefinitely is a cell that cannot form a tumor indefinitely.2PubMed Central. Cellular senescence, rejuvenation and potential immortality
HeLa cells bypass this countdown entirely. They express telomerase, an enzyme that rebuilds telomeres after each division, resetting the clock before it ever runs out. Measurements of HeLa cells growing in the lab showed that their average telomere length remained stable over 60 rounds of population doubling, meaning the telomerase was keeping pace with the shortening perfectly.3PubMed. Telomere length dynamics in telomerase-positive immortal human cell populations Most adult human cells have telomerase switched off. Embryonic cells and certain stem cells keep it active, which is why those cells can continue dividing. Many cancer cells, including HeLa, reactivate it.4PubMed Central. Cellular senescence, rejuvenation and potential immortality
This is the second pillar of HeLa immortality. Disabling tumor suppressors allows the cells to keep dividing when they should stop. Activating telomerase allows them to keep dividing without their chromosomes falling apart. Neither trick alone would be enough. A cell with active telomerase but intact p53 would still detect its own abnormalities and shut down. A cell with disabled p53 but no telomerase would eventually hit telomere crisis and die. HeLa cells have both advantages working in concert, which is why they have persisted in labs around the world for over 70 years.5PubMed. Telomerase activity in HeLa cervical carcinoma cell line proliferation
A Metabolism Built for Constant Growth
Immortality requires fuel. Normal cells rely heavily on oxygen-based energy production, which is efficient but relatively slow. HeLa cells, like many cancer cells, lean heavily on a different metabolic strategy: they burn through glucose at a high rate even when plenty of oxygen is available, converting it to lactate rather than running it through the full oxygen-dependent pathway. This phenomenon, observed across many tumor types, generates energy and raw building materials fast enough to support rapid, continuous division.
How important is this metabolic shift to HeLa’s survival? When researchers silenced a key enzyme involved in maintaining this fast glycolysis in HeLa cells, the consequences were severe. Glycolytic activity dropped, the cells’ energy currency (ATP) and key metabolic intermediates fell, and the cells began dying at higher rates. They also lost the ability to grow without being anchored to a surface, a hallmark of cancer cells that allows them to spread and survive in unnatural environments.6PubMed. PFKFB3 gene silencing decreases glycolysis, induces cell-cycle delay and inhibits anchorage-independent growth in HeLa cells
The metabolic rewiring is not the primary reason HeLa cells are immortal, but it enables their immortality to function in practice. Cells that cannot fuel endless division will stop dividing regardless of what their genes are doing. HeLa’s aggressive glucose consumption keeps the engine running at a pace that matches its deregulated growth signals.
A Genome in Constant Flux
One of the more unsettling aspects of HeLa cells is that they no longer carry a normal human genome. Normal human cells have 46 chromosomes. HeLa cells are far from that number, typically carrying between 60 and 80 chromosomes depending on the particular laboratory clone, with some individual cells carrying well over 100. Analysis of four different HeLa clones found chromosome counts ranging from 56 to 79 in some, with roughly 10 to 30 percent of cells in certain clones carrying between 101 and 198 chromosomes.7Scientific Reports. High variability of genomic instability and gene expression profiling in different HeLa clones
This instability is not a one-time event frozen in 1951. HeLa cells continue to gain and lose chunks of chromosomal material as they divide. Even clones that were recently split from the same parent batch developed their own unique gains and losses that were then stably passed on to daughter cells. Pairs of clones that shared a more recent common ancestor were more similar to each other than to clones with a more distant shared origin, much like branches on a family tree.8Scientific Reports. High variability of genomic instability and gene expression profiling in different HeLa clones
This ongoing genomic reshuffling means that “HeLa cells” in one lab may differ meaningfully from “HeLa cells” in another. Some researchers have gone so far as to describe HeLa as its own evolving microbial-like population rather than a frozen snapshot of a human tumor. The practical consequence for science is real: experiments done on one HeLa clone may not replicate perfectly when repeated with a different clone, because the underlying genome has drifted. It also contributes to immortality indirectly, because genomic instability gives the cell population enormous adaptability. If growth conditions change, some cells in the population are likely to carry random chromosome changes that happen to confer a survival advantage, allowing the lineage to persist through conditions that would kill a genetically uniform population.
Why HeLa Cells Were So Revolutionary in 1951
Before HeLa, scientists had been trying for decades to grow human cells in the laboratory, with limited success. Cells taken from human tissues would divide a handful of times and then die, making long-term experiments almost impossible. George Otto Gey, a tissue culture pioneer at Johns Hopkins who had developed specialized equipment like the roller drum for keeping cells alive, had been working on this problem for years.9PubMed. The Tissue Culture Laboratory of Dr. George Otto Gey 60 yrs ago as recalled by a former student When a sample from Henrietta Lacks’ tumor arrived in his lab, it behaved unlike anything his team had seen. The cells divided aggressively and showed no signs of slowing down.
Gey recognized the significance immediately and began sharing the cells with other researchers. Within a few years, HeLa cells were being used in laboratories worldwide. They played a central role in developing the polio vaccine, because researchers needed large quantities of human cells to grow the virus for testing, and HeLa provided an essentially unlimited supply. Over the following decades, HeLa became the workhorse of biomedical research, used in studies of cancer biology, virology, drug testing, radiation effects, and genetics. Estimates of the total number of HeLa cells ever grown reach into the tens of tons.
The Contamination Problem That Almost Undermined Cell Biology
The very qualities that make HeLa cells useful also make them dangerous to other cell cultures. HeLa is the most frequent cause of cross-contamination in cell biology laboratories.10PubMed Central. Discovery of HeLa Cell Contamination in HES Cells: Call for Cell Line Authentication in Reproductive Biology Research Because HeLa cells grow faster and more aggressively than almost any other human cell line, even a tiny number of stray HeLa cells introduced into another culture through shared equipment, contaminated reagents, or simple human error can quickly outcompete and replace the intended cells. A researcher might believe they are studying liver cells or breast cancer cells when their culture has quietly been taken over by HeLa.
The scale of this problem became apparent starting in the late 1960s, when scientists developed methods to detect when one human cell line had been replaced by another. Stanley Gartler first demonstrated that many supposedly independent cell lines were actually HeLa in disguise. Through the 1970s, Walter Nelson-Rees catalogued contaminated lines and published a damning list in 1981 showing that HeLa contamination was not an occasional mishap but a systemic problem across the entire field.11PLoS ONE. The ghosts of HeLa: How cell line misidentification contaminates the scientific literature The scientific community was slow to respond. Researchers whose decades of work had been done on the wrong cells understandably resisted the implications. Papers built on contaminated cultures continued to be cited long after the contamination was documented.
Today, cell line authentication using genetic fingerprinting is standard practice in well-run laboratories, and major journals require it before publishing cell-based research. But contamination remains an ongoing concern, particularly in labs that do not follow strict protocols. The irony is hard to miss: the qualities that make HeLa scientifically valuable, its aggressive growth, its resilience, its refusal to die, are the same qualities that allow it to invade and destroy other cell cultures.
What Made Lacks’ Cancer Unusually Aggressive
Not every cervical cancer produces cells that can grow indefinitely in the lab. Researchers have tried to establish immortal cell lines from countless tumors over the years, and the success rate is low. Henrietta Lacks’ cancer was exceptional for several reasons that went beyond just carrying HPV 18.
Her tumor was an adenocarcinoma, a particularly aggressive subtype. The HPV DNA had integrated into the host genome in a way that produced unusually high levels of the E6 and E7 viral proteins, keeping the tumor suppressors more thoroughly suppressed than in many HPV-positive cancers. The combination of complete p53 and Rb inactivation with robust telomerase expression and a fast-burning metabolism created a perfect storm for laboratory survival. Many other cancer cell lines carry some of these features, but HeLa appears to have them all, and all of them functioning at a high level simultaneously.
The genomic instability described earlier also plays a role in this exceptional hardiness. A cell line with a fluid, rapidly evolving genome can adapt to laboratory conditions much faster than one with a more stable genetic makeup. HeLa cells essentially underwent a kind of natural selection in the dish, with variants that grew best in culture media, at room oxygen levels, and on glass or plastic surfaces being the ones that survived and proliferated. The cells growing in labs today are not genetically identical to the cells in Henrietta Lacks’ body in 1951; they are the descendants of seven decades of evolution under artificial conditions.
The Ethics That Changed How Science Handles Tissue
Henrietta Lacks never consented to having her cells used for research. She was a 31-year-old Black woman treated at a segregated ward at Johns Hopkins, and the tissue sample was taken as part of her cancer diagnosis without any discussion of its potential use beyond her own care. This was standard practice at the time; the concept of informed consent for tissue use barely existed. Her family was not told about HeLa’s existence for more than two decades, and they never received any compensation even as the cells became one of the most commercially valuable biological materials in the world.
The story, brought to wide public attention by Rebecca Skloot’s 2010 book, became a catalyst for significant policy changes. It highlighted uncomfortable questions about who owns biological material once it leaves a person’s body, whether donors and their families have a right to know how their tissues are used, and how racial and economic disparities in healthcare intersect with scientific exploitation. The case directly influenced regulatory changes proposed in the United States around informed consent for biospecimens.12PubMed Central. Lessons from HeLa Cells: The Ethics and Policy of Biospecimens
In 2013, the situation came to a head again when researchers published the full HeLa genome without consulting the Lacks family, effectively making Henrietta’s genetic information, and by extension partial genetic information about her living descendants, publicly available. The National Institutes of Health subsequently reached an agreement with the family: researchers seeking access to the HeLa genome sequence must apply to a committee that includes family members. It was a modest step, but it marked the first time the family had any formal say over how the cells were used.
Why Other Cancer Cells Usually Cannot Do What HeLa Does
A common misconception is that any cancer cell is immortal. In reality, most primary tumor cells taken from patients and placed into a culture dish will die within days or weeks. Establishing a stable, continuously dividing cell line from a tumor is difficult and fails far more often than it succeeds. The cells need to survive the shock of being removed from the body, adapt to artificial growth medium, and have the right combination of genetic and metabolic features to thrive outside a living organism.
Many cancers do activate telomerase, and many do carry mutations in p53 or Rb. But the level of activation matters, the specific combination matters, and the ability to adapt to lab conditions matters. Some tumors carry p53 mutations that only partially disable the protein, leaving residual tumor-suppressing activity. Others activate telomerase weakly, slowing telomere loss without fully preventing it. Still others rely on blood supply from the body to deliver nutrients and oxygen in ways that cannot be replicated in a dish.
HeLa succeeded because everything aligned. The viral suppression of both major tumor suppressors was thorough and sustained. Telomerase was highly active and kept telomeres at a stable length indefinitely. The metabolic machinery was already configured for rapid glucose consumption. And the inherent genomic instability provided a deep well of genetic variation for natural selection to act on in culture. Researchers sometimes describe it as a “perfect storm” for laboratory immortality, which is why, even after decades of trying, relatively few human cell lines have matched HeLa’s combination of growth speed, resilience, and adaptability.