The adverse changes in the cellular environment cause ageing.
Some biologists believe that ageing in animals is to be dependent on an intrinsic genetic property of the cells of the body.
It advocates that ageing is an outcome of interaction between the genes present in the body of an individual and the environment in which the individual lives.
It postulates that the animals with a high rate of metabolic activity age earlier and die sooner than with a lower rate of metabolism.
According to this theory, the cells and tissues of animals are continuously subjected to the wear and tear processes due to various intrinsic (genetic) and extrinsic (environment) factors and thus the animals show signs of ageing ultimately leading to death.
According to this recently postulated theory, the gradual atrophy and disappearance of thymus gland disturbs the defence mechanism of the body of combating germs and pathogens.
With the disappearance of this gland, the body produces a great number of harmful abnormal cells which cause the increased rate of the change and destruction of tissues. Due to these reasons old people contract infectious diseases more often than the young people.
This theory holds that the primary defects due to ageing develop at certain centres of the brain which control the function of the endocrine glands. Such defects results in hormonal malfunctions and imbalances in the body which initiate changes characterising ageing.
For example, many mammals, including man, show signs of ageing with the reduced production of sex hormones in the body. Immunologic and neurohormonal theories are pacemaker theories of ageing.