Immunosenescence: ageing of the immune system

“Immunosenescence”, or ageing of the immune system, is the term given to changes (in comparison with younger individuals) observed in the immune systems of elderly individuals. Elderly individuals are predisposed to more severe symptoms from certain infections than young adults, and they do not mount as an effective immune response to vaccination. In addition, the elderly suffer from more malignancies than younger individuals, which may be because of because of failure to provide surveillance against immune system challenges, such as tumours. This decrease in immune function is paradoxically coupled with features of chronic inflammation in the elderly, termed “inflammaging”. The mechanisms of immune senescence are multiple, but seem to be driven largely by changes in T cell-mediated immunity. There are fewer antigen-naive T cells in the peripheral blood of aged individuals than there are in younger individuals. The memory T-cell repertoire is not as broad as that in younger individuals, and the memory T cells demonstrate a poorer functional ability to respond to pathogens. It is likely that these T-cell changes result from the involution of the primary organ of T-cell development, the thymus. Thymic activity is maximal following birth, followed by thymic involution from as early as one year of age. By puberty, adults are left with only a small thymic remnant. While many investigators seek to counteract immune senescence in order to improve vaccine responses or ameliorate various diseases, thymic involution is evolutionarily conserved in vertebrates. Such conservation implies that immunosenenscence is driven by natural selection, and may serve a particular biological function. Such a function may relate to immunity against infections or cancer, as well as to immune tolerance of commensal organisms or the foetus for reproductive compatibility.