In aging there is a lot of correlation - many biological systems that are traditionally studied separately are declining at once, so most researchers are only looking in detail at one tiny part of aging. Part of the challenging facing researchers has always been how to figure out what is cause and what is consequence given the need for specialization to make progress in any given small area of the biology of aging. Here, researchers focus on the immune system, and you can see the biases inherent in being a specialist: "The aging process is accompanied by an impairment of the physiological systems including the immune system. This system is an excellent indicator of health. We have also observed that several functions of the immune cells are good markers of biological age and predictors of longevity. In agreement with the oxidation-inflammation theory that we have proposed, the chronic oxidative stress that appears with age affects all cells and especially those of the regulatory systems, such as the nervous, endocrine and immune systems and the communication between them. This fact prevents an adequate homeostasis and, therefore, the preservation of health. We have also proposed an involvement of the immune system in the aging process of the organism, concretely in the rate of aging, since there is a relation between the redox state and functional capacity of the immune cells and the longevity of individuals. A confirmation of the central role of the immune system in oxi-inflamm-aging is that several lifestyle strategies such as the administration of adequate amounts of antioxidants in the diet, physical exercise, physical and mental activity through environmental enrichment and hormetic interventions improve functions of immune cells, decreasing their oxidative stress, and consequently increasing the longevity of individuals."