Changes in the aging immune system.

The functional capacity of the immune system gradually declines with age. T lymphocytes are more severely affected than B cells or antigen-presenting cells. This is mainly due to the involution of the thymus which is almost complete at the age of 60. The host is then dependent on the T cell pool generated in earlier life. Continuous activation, clonal expansion and elimination of T cells of various specificities eventually leads to changes in the T cell repertoire. CD45RA+ "naive" cells are replaced by CD45RA- "memory" cells and a T cell receptor oligoclonality develops. At the same time, T cells with signal transduction defects accumulate. Age-related T cell alterations lead to a decreased clonal expansion and a reduced efficiency of T cell effector functions such as cytotoxicity or B cell help. Decreased antibody production and a shortened immunological memory are the consequence. Changes in the aging immune system represent a permissive factor for the frequent occurrence and the severity of disease. Efficient protection of elderly individuals by suitable vaccination strategies is therefore a matter of great importance.