Other theories hold that human aging results from fundamental changes in immune function. Immune surveillance declines with age, resulting in greater susceptibility to cancer, infectious disease, and auto-immune conditions. Administration of thymic extracts or specifically bioengineered monoclonal antibodies might help buttress a flagging immune function. Maladaptive reactions to stress have also been shown to accelerate the aging process. Most individuals who survive into their 80s or 90s embody personal traits of resilience and stress-coping that have sheltered them from the adverse effects of rampaging catecholamines.
Evidence also suggests a role for hormones in the process of aging.
By contrast, levels of insulin tend to rise in response to dietary excess and sedentary lifestyle. It is also clear that certain aspects of aging are genetically programmed. For example, the presence of a gene called Apo-E4 predicts susceptibility to Alzheimer's Disease. Among the most popular is Deprenyl, a prescription medication used to slow the progression of Parkinson's disease. It is held by some anti-aging researchers that Deprenyl and other drugs can be used to halt the decline in mental functioning that is the hallmark of aging.