The nature of this script, sometimes called the biological clock, is not determined, but it is suspected that it is part of the brain's function. Organs and tissues often undergo changes because the cells used to form them no longer function in the same manner that they once did when young, or in the first years of duplication. For example, ovaries are not activated until puberty arrives. Then the ovaries begin to sputter to a stop. The timing of these events depends on the genetic script a person inherits at birth.
Some investigators believe the immune system is responsible for aging.
As people get older, two things happen related to this. The body produces more autoimmune antibodies. Sometimes these antibodies attack the body's own tissues as if they were foreign substances or bacteria. In extreme states they cause many diseases, called autoimmune diseases. Other changes occur when the body's immune defenses decrease.