Biological immortality refers to a stable (or decreasing) rate of mortality as a function of chronological age. Some individual cells and entire organisms in some species achieve this state either throughout their existence or after living long enough. This requires that death occur from injury or disease rather than deterioration, i.e., the absence of cellular senescence. However, 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, also 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 fact that some multi-cellular organisms do not experience senescence implies that aging is not an Aristotelian inevitability and refutes the prior hypothesis that aging follows a Gompertz model. No actual organism or individual cell is inviolably immortal (i.e. "invincible" or "indestructible"). A biologically immortal living thing can die, for example, upon receiving sufficient injury or otherwise having its body destroyed or diseased.
Biologists have chosen the word immortal to designate cells that are not limited by the Hayflick limit (where cells no longer divide because of DNA damage or shortened telomeres). Prior to Leonard Hayflick, Alexis Carrel hypothesized that all normal somatic cells are immortal.
The term immortalization was first applied to cancer cells that expressed the telomere-lengthening enzyme telomerase, and thereby avoided apoptosis (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. 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).
Immortality may not be desirable for multi-cellular organisms, as the main controls over cancer are the apoptotic mechanisms.
Tardigrades, otherwise known as "water bears", are highly resilient microscopic animals. Capable of surviving extremes such as heat, radiation, drought, and even the vacuum of space 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, fresh-water 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 (aging), 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 () species of jellyfish which 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, capture, etc. See Lobster longevity.
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.".
Some life extensionists, such as those who practice cryonics, have the hope that humans may someday become immortal through technological developments which 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 ELPIs (Extreme Lifespans through Perpetual-equalising Interventions) 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.
- DNA damage theory of aging
- American Academy of Anti-Aging Medicine
- Strategies for Engineered Negligible Senescence (SENS)
- Immortality Institute
- Maximum lifespan
- Methuselah Foundation
- Reliability theory of aging and longevity
- Negligible senescence
- 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.
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