The immune system is the body's defense against foreign or dangerous invaders. Such invaders include
Microorganisms (commonly called germs, such as bacteria, viruses, and fungi)
Parasites (such as worms)
Cancer cells
Transplanted organs and tissues
The immune system changes throughout life. (See also Overview of the Immune System.)
Newborns
At birth, acquired (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 breast milk.
Older adults
As people age, the immune system becomes less effective in the following ways:
The immune system becomes less able to distinguish self from nonself (that is, to identify foreign antigens). As a result, autoimmune disorders become more common.
Macrophages (which ingest bacteria and other foreign cells) destroy bacteria, cancer cells, and other antigens more slowly. This slowdown may be one reason that cancer is more common among older people.
T cells (which remember antigens they have previously encountered) respond less quickly to the antigens.
There are fewer white blood cells capable of responding to new antigens. Thus, when older adults encounter a new antigen, the body is less able to remember and defend against it.
Older adults have smaller amounts of complement proteins and do not produce as many of these proteins as younger people do in response to bacterial infections.
Although the amount of antibody produced in response to an antigen remains about the same overall, the antibodies become less able to attach to the antigen. This change may partly explain why pneumonia, influenza, infective endocarditis, and tetanus are more common among older adults and result in death more often. These changes may also partly explain why vaccines are less effective in older adults and thus why it is important for older adults to get booster shots (which are available for some vaccines).
These changes in immune function may contribute to the greater susceptibility of older adults to some infections and cancers.
