Aging of the immune system across different species

Aging of the immune system, called immunosenescence, has been linked to reduced ability to resist infection, and increased mortality and morbidity from infectious diseases, which consistently rank in the top five causes of death in the old age, even in industrial societies. This review summarizes our knowledge about the evolution of the innate and adaptive immune systems in light of the longevity of the organism, as well as the current state of our understanding of age-related changes in each of the arms of the immune system in various model organisms. It is clear that adaptive immunity, which we propose evolved as an essential function that provides longevity, eventually erodes in the old age. The jury is still out as to whether innate immunity undergoes a similar decay, whether it stays the same, or whether, perhaps, it is able to compensate for the loss of adaptive immune function.