Thymic involution

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. 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. 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 and the rise of infectious disease and cancer incidence in the elderly (in some cases risk is inversely proportional to thymus size). Though thymic involution has been linked to immunosenescence, it is not induced by senescence as the organ starts involuting from a young age: in humans, as early as the first year after birth.