Aging of the vertebrate immune system

We have summarized current knowledge on the aging of the immune system in three vertebrate groups: fish, amphibians and birds. Available data are few due to difficulties in studying ageing in natural populations and in accurately determining age. In all vertebrates, the most obvious evidence of the senescence of lymphoid tissue is the involution of thymus, which courses with decreased numbers of thymocytes, and loss of the histological organization of gland. On the other hand, there is little information on aged secondary lymphoid organs. Possible influence of the endocrine system in the changes observed in aged lymphoid organs is also discussed.