T-cells are less responsive to antigens; therefore, fewer cytotoxic T-cells respond to an infection. This is partly because of the gradual decrease in size of the thymus gland and its reduced production of the hormone, thymosin.
Because the helper T-cell production is reduced, B-cells become less responsive, and antibody levels do not rise as quickly after antigen exposure. Depressed lymphocyte function is also accompanied by a decrease in macrophage activity. The result is an increased susceptibility to viral and bacterial illnesses.
Increased cancer incidences also indicate a decline in immune system surveillance, causing tumor cells to proliferate instead of being destroyed.
In addition, the elderly frequently take drugs or have therapies that depress the immune system. For instance, the use of steroids in the treatment of arthritis and the use of drugs and radiation in the treatment of cancers all cause immunosuppression, leaving them open to secondary infections and diseases.
A decreased secretion of growth hormone leads to a decrease in muscle mass while at the same time, increasing fat storage.
Levels of circulating autoantibodies (antibodies directed against self) increase in the elderly and explains why they are more prone to the development of autoimmune diseases.
As the thyroid slows its secretion of thyroxine, the result is a lower basal metabolic rate.