A
active immunity Long-term, acquired immune
protection. Active immunity, also called acquired
immunity, results from fighting an INFECTION or
receiving a VACCINE that stimulates ANTIBODY
response. In many circumstances active immunity
is lifelong.
See also ANTIBODY-MEDIATED IMMUNITY; CELL-MEDI-
ATED IMMUNITY; PASSIVE IMMUNITY.
aging, effects on immune response At birth the
IMMUNE SYSTEMis fairly undeveloped. The infant
relies largely on the carryover of maternal
immune components for about the first six weeks
of its life, while the infant’s body builds its own
immune system. By age four months, maternal
IMMUNITYwears off and the infant’s immune sys-
tem is on its own (though an infant who is BREAST-
FEEDINGcontinues to receive antibodies and limited
immune support from his or her mother). Immu-
nity reaches full STRENGTH in early childhood, a
level at which it continues until about age 40.
After age 40, the effectiveness of the immune
system begins to diminish. T-cell lymphocytes and
macrophages respond more slowly. Levels of com-
plement (protein factors essential for ANTIBODY–
ANTIGENbinding) and of antibodies drop off. The
immune system is slower to differentiate B-cell
lymphocytes to antigen-producing PLASMA cells,
and plasma cells produce lower quantities of anti-
bodies. The immune response to disease as well as
to vaccines becomes slower and less effective,
increasing susceptibility to serious INFECTION(such
as INFLUENZAand PNEUMONIA) from pathogens. The
amounts and activity of MUCOSA-ASSOCIATED LYM-
PHOID TISSUE(MALT) decrease in areas such as the
LUNGS, further reducing the body’s ability to reject
infection from invading pathogens. Decades of
exposure to antigens mean more lymphocytes are
sensitized for specific antigens, leaving fewer to
become sensitized to new antigens. The IMMUNE
RESPONSEsummons T-cell lymphocytes less quickly
to the scene of an infection.
Changes in antigens and antigen recognition
also occur, resulting in a decreased ability of the
immune system to distinguish between self and
nonself antigens. Cells may acquire a mix of anti-
gens that makes them appear foreign, initiating an
inappropriate immune response (autoimmune dis-
order) that damages an organ or structure. Or the
immune system may fail to detect the change in
antigens on the surfaces of cell membranes of cells
that become cancerous, allowing cancer tumors to
develop. AUTOIMMUNE DISORDERSand cancer conse-
quently become more common with advancing
age.
Measures to prevent infection can help offset
age-related immune changes to some degree. Dili-
gent HAND WASHINGand avoiding exposure to other
people who have COLDSor influenza may prevent
the spread of these infections. ECHINACEA and
GOLDENSEALare herbal remedies that can boost
immune function after an exposure to common
pathogens. GAMMAGLOBULIN may boost the
immune response in circumstances such as expo-
sure to HEPATITIS. Older people often benefit from
more aggressive antibiotic therapy—ANTIBIOTIC
MEDICATIONS administered early in the infection
process—to help them fight infections they do
develop.
See also ANTIBODY-MEDIATED IMMUNITY; CELL-MEDI-
ATED IMMUNITY; COMPLEMENT CASCADE; HEALING;
IMMUNOSENESCENCE; LYMPHOCYTE; MACROPHAGE; T-CELL
LYMPHOCYTE.allergen A harmless substance, also called a hap-
ten, that causes an exaggerated response from the239