The immune system changes throughout life.

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 people: 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 antigens) 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 people encounter a new antigen, the body is less able to remember and defend against it.
• Older people have smaller amounts of complement proteins and do not produce as many of these proteins as younger people do in response to bacterial infections.
• The amount of antibody produced in response to an antigen is less, and the antibodies are less able to attach to the antigen. These changes may partly explain why pneumonia, influenza, infectious endocarditis, and tetanus are more common among older people and result in death more often. These changes may also partly explain why vaccines are less effective in older people.

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

Last full review/revision March 2013 by Peter J. Delves, PhD