EFFECTS OF AGING ON THE LYMPHATIC SYSTEM AND IMMUNITY
Aging appears to have little effect on the lymphatic
system’s ability to remove fluid from tissues, absorb lipids from the digestive
tract, or remove defective red blood cells from the blood. However, aging has a
severe impact on the immune system.
With age, people eventually
lose the ability to produce new, mature T cells in the thymus. By age 40, much
of the thymus has been replaced with adipose tissue, and after age 60, the
thymus decreases in size to the point that it can be difficult to detect. While
the number of T cells remains stable in most individuals due to the replication
(not maturation) of T cells in lymphatic tissues, the T cells are less
functional. In many individuals, the ability of helper T cells to proliferate
in response to antigens decreases. Thus, antigen exposure produces fewer helper
T cells, which results in less stimulation of B cells and cytotoxic T cells.
Consequently, both antibody-mediated immunity and cell-mediated immunity
responses to antigens decrease with age.
Both primary and secondary
antibody responses decrease with age. More antigen is required to produce a
response, the response is slower, less antibody is produced, and fewer memory
cells result. Thus, a person’s ability to resist infections and devel-op
immunity decreases.
The ability of cell-mediated
immunity to resist intracellular pathogens also decreases with age. For
example, the elderly are more susceptible to influenza (flu) and should be
vaccinated every year. Some pathogens cause disease but are not eliminated from
the body; with age, decreased immunity can lead to reactivation of the
pathogen. An example is the virus that causes chickenpox in chil-dren, which
can remain latent within nerve cells, even though the disease seems to have
disappeared. Later in life, the virus can leave the nerve cells and infect skin
cells, causing painful lesions known as herpes zoster, or shingles.
Autoimmune disease occurs
when immune responses destroy otherwise healthy tissue. There is very little
increase in the number of new-onset autoimmune diseases in the elderly.
However, the chronic inflammation and immune responses that began earlier in
life have a cumulative, damaging effect. Likewise, the increased incidence of
cancer in the elderly is likely to be caused by a com-bination of repeated
exposure to and damage from cancer-causing agents and decreased immunity.
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