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Biological immortality refers to a stable or decreasing rate of mortality from cellular senescence as a function of chronological age. Various unicellular and multicellular species may achieve this state either throughout their existence or after living long enough. A biologically immortal living thing can still die from means other than senescence, such as through injury or disease.
This definition of immortality has been challenged in the new Handbook of the Biology of Aging, because the increase in rate of mortality as a function of chronological age may be negligible at extremely old ages, an idea referred to as the late-life mortality plateau. The rate of mortality may cease to increase in old age, but in most cases that rate is typically very high. As a hypothetical example, there is only a 50% chance of a human surviving another year at age 110 or greater.
The term is also used by biologists to describe cells that are not subject to the Hayflick limit.
Cell lines 
Biologists chose the word "immortal" to designate cells that are not subject to the Hayflick limit, the point at which cells can no longer divide due to DNA damage or shortened telomeres. Prior to Leonard Hayflick's theory, Alexis Carrel hypothesized that all normal somatic cells were immortal.
The term "immortalization" was first applied to cancer cells that expressed the telomere-lengthening enzyme telomerase, and thereby avoided apoptosis—i.e. cell death caused by intracellular mechanisms. Among the most commonly used cell lines are HeLa and Jurkat, both of which are immortalized cancer cell lines. HeLa cells originated from a sample of cervical cancer taken from Henrietta Lacks in 1951. These cells have been and still are widely used in biological research such as creation of the polio vaccine, sex hormone steroid research, and cell metabolism. Normal stem cells and germ cells can also be said to be immortal (when humans refer to the cell line).
Immortal cell lines of cancer cells can be created by induction of oncogenes or loss of tumor suppressor genes. One way to induce immortality is through viral-mediated induction of the large T‑antigen, commonly introduced through simian virus 40 (SV-40).
Tardigrades, also known as "water bears", are highly resilient microscopic animals. They are capable of surviving extremes such as heat, radiation, and drought by going into suspended animation, where their metabolism slows to near zero and they simply wait out the harsh conditions until the environment is more favorable.
Bacteria are said to be biologically immortal, but only at the level of the colony. An individual bacterium can easily die. The two daughter bacteria resulting from cell division of a parent bacterium can be regarded as unique individuals or as members of a biologically "immortal" colony. The two daughter cells can be regarded as "rejuvenated" copies of the parent cell because damaged macromolecules have been split between the two cells and diluted. In the same way stem cells and gametes can be regarded as "immortal".
Hydras are a genus of simple, freshwater animals possessing radial symmetry and no post-mitotic cells. The fact that all cells continually divide allows defects and toxins to be diluted. It has been suggested that hydras do not undergo senescence, and, as such, are biologically immortal. However, this does not explain how hydras are consequently able to maintain telomere lengths.
Turritopsis nutricula is a small (5 millimeters (0.20 in)) species of jellyfish that uses transdifferentiation to replenish cells after sexual reproduction. This cycle can repeat indefinitely, potentially rendering it biologically immortal. It originated from the Caribbean sea, but has now spread around the world.
Older lobsters are more fertile than younger lobsters. Some scientists have claimed that they could effectively live indefinitely, barring injury, disease, or capture.
Planarian flatworms 
Planarian flatworms (both sexual and asexual) appear to exhibit an ability to live indefinitely and have an "apparently limitless [telomere] regenerative capacity fueled by a population of highly proliferative adult stem cells".
Political struggle for immortalism and immortality 
Though some scientists state that radical life extension and delaying or stopping aging are achievable, there are still no international or national programs focused on stopping aging or on radical life extension. There are political forces advocating for and against immortality. In 2012 in Russia, and then in the United States, Israel, and the Netherlands, the pro-immortality "longevity" political parties were launched. They aim to provide political support to anti-aging and radical life extension research and technologies and want to ensure the fastest possible—and at the same time, as least disruptive as possible—societal transition to radical life extension, life without aging, and ultimately, immortality. They aim to make it possible to provide access to such technologies to the majority of people alive today.
Life extensionists 
Some life extensionists, such as those who practice cryonics, have the hope that humans may someday become immortal through technological developments that may be used to correct the cause of "death" of the person held in cryonic suspension.
Biogerontologist Marios Kyriazis suggested that biological immortality in humans is an inevitable consequence of natural evolution. His extreme lifespans through perpetual-equalising interventions (ELPIs) theory proposes that the ability to attain indefinite lifespans is inherent in human biology, and that there will come a time when humans will continue to develop their intelligence by living indefinitely, rather than through evolution by natural selection.
See also 
- Aging brain
- American Academy of Anti-Aging Medicine
- DNA damage theory of aging
- Immortality Institute
- Life extension
- Maximum lifespan
- Methuselah Foundation
- Negligible senescence
- Reliability theory of aging and longevity
- Strategies for Engineered Negligible Senescence (SENS)
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- Thomas C. J. Tan, Ruman Rahman, Farah Jaber-Hijazi, Daniel A. Felix, Chen Chen, Edward J. Louis, and Aziz Aboobaker (February 2012). "Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms". PNAS 109 (9). doi:10.1073/pnas.1118885109.
- "Scientists' Open Letter on Aging Research". Immortality Institute. Retrieved January 31, 2013.
- "A Single-Issue Political Party for Longevity Science". Fight Aging!. July 27, 2012. Retrieved January 31, 2013.
- "Как долго хотят жить россияне" (in Russian). Левада-Центр. February 6, 2012. Retrieved January 31, 2013.
- https://acrobat.com/#d=MAgyT1rkdwono-lQL6thBQ[dead link]
- Kyriazis, Marios (March 4, 2011). "Indefinite Lifespans: A Natural Consequence of the Global Brain". H+. Humanity+. Retrieved January 31, 2013.
- Llewellyn-Smith, Julia (October 25, 2011). "Has the fountain of youth been found?". The Daily Telegraph.
- The ELPIs Theory. Retrieved January 31, 2013.
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- James L. Halperin. The First Immortal, Del Rey, 1998. ISBN 0-345-42092-6
- Robert Ettinger. The Prospect of Immortality, Ria University Press, 2005. ISBN 0-9743472-3-X
- Dr. R. Michael Perry. Forever For All: Moral Philosophy, Cryonics, and the Scientific Prospects for Immortality, Universal Publishers, 2001. ISBN 1-58112-724-3
- Martinez, D.E. (1998) "Mortality patterns suggest lack of senescence in hydra." Experimental Gerontology 1998 May;33(3):217–225. Full text.
- Rose, Michael; Rauser, Casandra L.; Mueller, Laurence D. (Spring 2011). Does Aging Stop?. Oxford University Press.
- Biological Immortality Slide show by Michael R. Rose
- Cellular Senescence and Apoptosis in Aging
- Cellular Senescence Information Center
- Geron Announces Launch of Telomerase-Immortalized Cell Line Geron Corporation
- Late Life: A New Frontier for Physiology study indicates biological immortality in humans late in life
- No Limit to Lifespan if We're Machines (pdf) - Betterhumans, August 26, 2004
- More on human biological immortality