The major function of the immune system is defense against infection and disease.

With advancing age, the lymphatic system becomes less effective at combating disease and fighting off infections. T and B cells are the primary cells involved with immunity and attack specific invaders as they penetrate mechanical, chemical, and other cellular barriers.

T cells, the cells responsible for cellular immunity and for coordination and regulation of immunity, become less responsive. As a result, fewer T cells respond to an infection or invasion by a pathogen.

B cells, the cells that produce antibodies, are also less responsive, so antibody levels do not rise as quickly after an infection develops in an elderly person. The net result is an increased susceptibility to viral and bacterial infections.

For this reason, vaccinations for viruses like the flu are strongly recommended for elderly people. In addition, the increased incidence of cancer in the elderly reflects the fact that surveillance by the lymphatic system declines, and tumor cells are not eliminated as effectively.

Two complementary forms of immunity rid humans of pathogens and cancer cells: nonspecific (or innate) defenses and specific (or adaptive) defenses. Nonspecific immunity provides a rapid but incomplete defense against a variety of threatening agents until the slower, specific immune response develops.

Nutrition plays a factor in a healthy immune system as well. In both healthy and nutritionally-deficient older adults, vitamin and other dietary supplements enhance the response of the immune system resulting in fewer days of infectious illnesses.

Older adults often experience loss and stress, and suppressed immunity has been associated with bereavement, depression, and poor social support. Maintaining an active social life and receiving treatment for depression could boost the older adult's immune system.