Answer
Like most body systems, the immune system declines in efficiency, and effectiveness with age. The reasons for these age-related changes have not been worked out. However, it is known that the thymus begins to atrophy soon after puberty and the production of naive T-cells and naive B-cell decrease with advancing age.
The immune system seems to function effectively until late in life (60s), but with increasing age, problems such as development of cancerous lesions seem to increase. These negative immunological problems may be due partly to declining immune efficiency in addition to increasing development of autoimmunity.
Work Step by Step
Embryonic lymphoid stem cells originate in the liver and the spleen by the 9th week of embryonic development. Later they travel to the bone marrow , the main source of lymphoid cells in adults. In the bone marrow and the thymus the lymphoid cells develop immunocompetence and self tolerance. Then they are distributed to other lymphoid tissues. Newborns are dependent mainly on Type 2 helper cells. Type 1 helper cells must develop in sufficient numbers ( by education through encounter with antigens) or the immune system will be unbalanced and tendencies to autoimmune and allergic reaction will be pronounced in early adulthood.
