Thymic involution is the shrinking (involution) of the thymus with age, resulting in changes in the architecture of the thymus and a decrease in tissue mass.[1] Thymus involution is one of the major characteristics of vertebrate immunology, and occurs 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] 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.
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]