T cells and its aging.

The function of immune system peaks at around puberty and gradually declines thereafter with advance in age. The age-related decline of immunological function primarily occurs in T cell-dependent immune system and is generally associated with increase in susceptibility to infections as well as in incidence of autoimmune phenomena in the elderly. The age-related change in T cell-dependent immune functions can be ascribed to the physiological thymic atrophy which starts in an early stage of life. Emigration of T cells from the thymus to the periphery mainly takes place in the late fetal and newborn stage, and dramatically declines after puberty. In other word, the thymic capacity to promote T cell differentiation starts to change in the early stage of life in terms of quantity and quality of T cells. Age-related quantitative changes of aging T cells are seen in their number and subsets. Thus, the composition of T cell-subsets in the periphery gradually changes with age, resulting in the alteration of T cell functions in the elderly. In relation to the qualitative change of aging T cells, disturbance of intracellular signal transduction is responsible for the impairment of proliferation and cytokine production upon antigenic stimulation. However, age-related change of immune functions are controlled not only by T cells but also by endogenous and exogenous factors. In other words, the age-related alteration of immune functions in T cells might be further explained by proper understanding of the relationship between the neuroendocrine and the immune system.