In a sense, aging of the immune system actually begins before birth, when nonself T cells are selected for destruction, via programmed cell death (apoptosis), in the thymus. The immune system begins to decline early in life. The thymus gland reaches its maximal size in adolescence and then slowly shrinks. By age seventy, the thymus is one-tenth the size it was at the age of ten, and the immune system is only 25% as powerful.
The declining strength of the immune response is why elderly people have a higher risk of developing cancer and succumb more easily to infections that they easily fought off at an earlier age, such as influenza, tuberculosis, and pneumonia. When West Nile virus struck New York City in 1999, the seven people who died from associated encephalitis were elderly. Most younger individuals develop only flulike symptoms from this infection. HIV infection progresses to AIDS faster in people older than forty. AIDS is more difficult to diagnose in older people, sometimes because physicians do not initially suspect the condition, instead attributing the fatigue, confusion, loss of appetite, and swollen glands to other causes. However, 11% of new cases of AIDS occur in those over age fifty.
Interestingly, numbers of T cells diminish only slightly with increasing age, and numbers of B cells not at all. However, activity levels change for both types of lymphocytes. Because T cell function controls production of B cells, effects on B cells are secondary. The antibody response to antigens is slower, and as a result, vaccines that would ordinarily be effective in one dose may require an extra dose. The proportions of the different antibody classes shift, with IgA and IgG increasing, and IgM and IgE decreasing. A person may produce more autoantibod-ies than at a younger age, increasing the risk of developing an autoimmune disorder.
Because of the declining function of the immune system, elderly people may not be candidates for certain medical treatments that suppress immunity, such as cancer chemotherapy and steroids to treat inflammatory disorders. Overall, the immune system makes it possible for us to survive in a world that is also home to many microorganisms. Clinical Application 16.2 looks at the devastation of immunity that is AIDS.
H When is maximum size of the thymus reached?
Explain the decline in strength of the immune response in elderly people.
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