Effects of Aging

The immune system changes throughout life. At birth, specific immunity is not fully developed. However, newborns have some antibodies, which crossed the placenta from the mother during pregnancy. These antibodies protect newborns against infections until their own immune system fully develops. Breastfed newborns also receive antibodies from the mother in the breast milk.

As people age, the immune system becomes less effective. It becomes less able to distinguish self from nonself. As a result, autoimmune disorders become more common. Macrophages destroy bacteria, cancer cells, and other antigens more slowly. This slowdown may be one reason that cancer is more common among older people. T lymphocytes respond less quickly to antigens, and there are fewer lymphocytes capable of responding to new antigens. Thus, when older people encounter a new antigen, the body is less able to recognize and defend against it.

Older people have smaller amounts of complement proteins than younger people, especially during bacterial infections. The amount of antibody produced in response to an antigen and the antibody's ability to attach to the antigen are reduced. These changes may partly explain why pneumonia, influenza, infectious endocarditis, and tetanus are more common among older people and result in death more often. Also, vaccines are less likely to produce immunity in older people.

These changes in immune function may contribute to the greater susceptibility of older people to some infections and cancers.