Cellular and systemic defense system against age-promoting stimuli.

Although much progress has been achieved in aging research using lower animals, especially yeast and C. elegans, aging in humans remains puzzling. Here I offer my hypothesis of host defense against age-promoting stimuli, which holds that the cell itself has a defense system and multi-organism, an even more sophisticated one against age-promoting stimuli. Here I review recent achievements in aging research and try to explain their findings in the light of my hypothesis. Age-promoting stimuli, including reactive oxygen species, telomere shortening or external stimuli such as UV or radiation induce stress responses in cells. The cellular defense system operates to overcome stimuli or repair damaged cellular components. Stress-induced damaged cells or infectious stimuli activate systemic defense systems, especially the innate immune system. Macrophages in the innate immune system are especially active not only in clearing damaged cells and repairing damaged tissue, but unfortunately, also in inducing age-related diseases.