<P> There is growing evidence that thymic involution is plastic and can be therapeutically halted or reversed in order to help boost the immune system . In fact, under certain circumstances, the thymus has been shown to undergo acute thymic involution (alternatively called transient involution). For example, transient involution has been induced in humans and other animals by stresses such as infections, pregnancy, and malnutrition . The thymus has also been shown to decrease during hibernation and, in frogs, change in size depending on the season, growing smaller in the winter Studies on acute thymic involution may help in developing treatments for patients, who for example are unable to restore immune function after chemotherapy, ionizing radiation, or infections like HIV . </P> <P> Thymic involution remains an evolutionary mystery since it occurs in most vertebrates despite its negative effects . Since it is not induced by senescence, many scientists have hypothesized that there may have been evolutionary pressures for the organ to involute . A few hypotheses are as follows: Developing T cells that interact strongly with antigen being presented within the thymus are induced to undergo programmed cell death . The intended effect is deletion of self - reactive T cells . This works well when the antigen being presented within the thymus is truly of self origin, but antigen from pathogenic microbes that happens to infiltrate the thymus has the potential to subvert the entire process . Rather than deleting T cells that would cause autoimmunity, T cells capable of eliminating the infiltrating pathogen are deleted instead . It has been proposed that one way to minimize this problem is to produce as many long - lived T cells as possible during the time of life when the thymus is most likely to be pristine, which generally would be when organisms are very young and under the protection of a functional maternal immune system . Thus, in mice and humans, for example, the best time to have a prodigiously functional thymus is prior to birth . In turn, it is well known from Williams' theory of the evolution of senescence that strong selection for enhanced early function readily accommodates, through antagonistic pleiotropy, deleterious later occurring effects, thus potentially accounting for the especially early demise of the thymus . The disposable soma hypothesis and life history hypothesis say similarly that tradeoffs are involved in thymic involution . Since the immune system must compete with other bodily systems, notably reproduction, for limited physiological resources, the body must invest in the immune system differentially at different stages of life . There is high immunological investment in youth since immunological memory is low . There are also hypotheses that suggest that thymic involution is directly adaptive . For example, some hypotheses have proposed that thymic involution may help in avoidance of autoimmunity or other dangers, prevention of infection, and production of an optimal repertoire of T - cells . Zinc deficiency may also play a role </P>

Where do t cells mature after thymic atrophy