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Henrietta Lacks (born Loretta Pleasant; August 1, 1920 - October 4, 1951) was an African-American woman whose cancer cells are the source of the HeLa cell line, the first immortalized cell line and one of the most important cell lines in medical research. An immortalized cell line will reproduce indefinitely under specific conditions, and the HeLa cell line continues to be a source of invaluable medical data to the present day. Lacks was the unwitting source of these cells from a tumor biopsied during treatment for cervical cancer at Johns Hopkins Hospital in Baltimore, Maryland, U.S., in 1951. These cells were then cultured by George Otto Gey who created the cell line known as HeLa, which is still used for medical research.

George Otto Gey, the first researcher to study Lacks's cancerous cells, observed that her cells were unique in that they reproduced at a very high rate and could be kept alive long enough to allow more in-depth examination. Until then, cells cultured for laboratory studies survived for only a few days at most, which wasn't long enough to perform a variety of different tests on the same sample. Lacks's cells were the first to be observed that could be divided multiple times without dying, which is why they became known as "immortal." After Lacks' death, Gey had Mary Kubicek, his lab assistant, took further HeLa samples while Henrietta's body was at Johns Hopkins' autopsy facility. The roller-tube technique was the method used to culture the cells obtained from the samples that Kubicek collected. Gey was able to start a cell line from Lacks's sample by isolating one specific cell and repeatedly dividing it, meaning that the same cell could then be used for conducting many experiments. They became known as HeLa cells, because Gey's standard method for labeling samples was to use the first two letters of the patient's first and last names.  The ability to rapidly reproduce HeLa cells in a laboratory setting has led to many important breakthroughs in biomedical research. For example, by 1954, Jonas Salk was using HeLa cells in his research to develop the polio vaccine. To test his new vaccine, the cells were mass-produced in the first-ever cell production factory. Additionally, Chester M. Southam, a leading virologist, injected HeLa cells into cancer patients, prison inmates, and healthy individuals in order to observe whether cancer could be transmitted as well as to examine if one could become immune to cancer by developing an acquired immune response.  HeLa cells were in high demand and put into mass production. They were mailed to scientists around the globe for "research into cancer, AIDS, the effects of radiation and toxic substances, gene mapping, and countless other scientific pursuits". HeLa cells were the first human cells successfully cloned in 1955, and have since been used to test human sensitivity to tape, glue, cosmetics, and many other products. Since the 1950s, scientists have grown 20 tons of her cells, and there are almost 11,000 patents involving HeLa cells.  In the early 1970s, a large portion of other cell cultures became contaminated by HeLa cells. As a result, members of Henrietta Lacks's family received solicitations for blood samples from researchers hoping to learn about the family's genetics in order to differentiate between HeLa cells and other cell lines.  Alarmed and confused, several family members began questioning why they were receiving so many telephone calls requesting blood samples. In 1975, the family also learned through a chance dinner-party conversation that material originating in Henrietta Lacks was continuing to be used for medical research. The family had never discussed Henrietta's illness and death among themselves in the intervening years but with the increased curiosity about their mother and her genetics, they now began to ask questions.
Henrietta Lacks