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One of the major characteristics of vertebrate immunology is thymic involution, the shrinking (involution) of the thymus with age, resulting in changes in the architecture of the thymus and a decrease in tissue mass. [1] This process is genetically regulated, with the nucleic material responsible being an example of a conserved sequence — one maintained through natural selection (though the pressures shaping this are unclear as will be discussed) since it arose in a common ancestor of all species now exhibiting it, via a phenomenon known to bioinformaticists as an orthologic sequence homology. The thymus involutes in almost all vertebrates, from birds, teleosts, amphibians to reptiles, though the thymi of a few species of sharks are known not to involute. [1][2] T-cells are named for the thymus where T-lymphocytes migrate from the bone marrow to mature. Its regression has been linked to the reduction in immunosurveillance [3] and the rise of infectious disease and cancer incidence in the elderly (in some cases risk is inversely proportional to thymus size). [4] Though thymic involution has been linked to immunosenescence, it is not induced by senescence as the organ starts involuting from a young age: [5] in humans, as early as the first year after birth. [6]
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