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The stress connection
Modern medicine, which once treated the connection
between emotions and physical health with skepticism,
has come to appreciate the closely linked relationship
of mind and body. A wide variety of maladies, including
stomach upset, hives, and even heart disease, are
linked to the effects of emotional stress. But although
the relationship between stress and immune function
is being studied by a number of different types of scientists,
the question of whether emotional stress has an
impact on the immune response remains unresolved.
Studying the relationship between stress and
the immune system presents challenges. For one
thing, stress is difficult to measure precisely in a scientific
way. When people are exposed to situations
they regard as stressful, it is hard to measure exactly
how much stress they feel. The scientist can measure
responses that may reflect stress, such as heart rate, but
such measures also may reflect other factors.
Most scientists studying the relationship of stress
and immune function, however, do not study a sudden,
short-lived stressor; rather, they try to study more
constant and frequent stressors known as chronic stress,
such as that caused by relationships with family, friends,
and co-workers, or sustained challenges to perform well
at one’s work. Some scientists are investigating whether
ongoing stress takes a toll on the immune system.
It is hard to perform controlled experiments in
human beings but repeating the same study many
times in animals and humans and getting the same
results each time is one way to produce reliable results.
Some researchers place animals into stressful situations,
such as being trapped in a small space or being
placed near an aggressive animal. Different functions
of their immune systems, and their health, are
then measured under such stressful conditions. On
the basis of such experiments, some published studies
with animals have yielded the following observations:
• Experimentally created stressful situations delayed
the production of antibodies in mice infected with
influenza virus and suppressed the activity of T cells
in animals inoculated with herpes simplex virus.
• Some forms of stress may be more damaging than
others. In one experiment, mice were put into a cage
with a highly aggressive mouse two hours a day for
six days and repeatedly threatened, but not injured,
by the aggressive mouse—a “social stress.” Other
mice were kept in tiny cages without food and water
for long periods—a “physical stress.” When both
groups of mice were exposed to a bacterial toxin, the
socially stressed animals were twice as likely to die.
• Isolation may suppress immune function. Infant
monkeys separated from their mothers, especially if
they are caged alone rather than in groups, generate
fewer lymphocytes in response to antigens and fewer
antibodies in response to viruses.
Based on studies in people, some researchers
report that stressful situations can reduce various
aspects of human immune response. A research team
from Ohio State University that has long worked in
this field suggests that psychological stress affects
the immune system by disrupting communication
between the nervous system, the endocrine (hormonal)
system, and the immune system. These three
systems “talk” to one another using natural chemical
messages, and must work in close coordination to be
effective. The Ohio State researchers speculate that
long-term stress releases an ongoing trickle of stress
hormones—mainly glucocorticoids. These hormones
affect the thymus, where lymphocytes are produced,
and inhibit the production of cytokines and interleukins,
which stimulate and coordinate white blood cell
activity. This team and others have reported the following
results:
• Elderly people caring for relatives with Alzheimer’s
disease have higher than average levels of cortisol (a
hormone secreted by the adrenal glands) and, perhaps
because of the higher levels of cortisol, make
fewer antibodies in response to influenza vaccine.
• Some measures of T cell activity have been found to
be lower in depressed patients compared with nondepressed
patients, and in men who are separated or
divorced compared with men who are married.
• In a year-long study of people caring for husbands
or wives with Alzheimer’s disease, changes in T cell
function were greatest in those who had the fewest
friends and least outside help.
• Four months after the passage of a major hurricane
in Florida, people in the most heavily damaged
neighborhoods showed reduced activity in several
immune system measurements. Similar results
were found in a study of hospital employees after an
earthquake in Los Angeles.
In all of these studies, however, there was no proof
that the immune system changes measured had any
clear adverse effects on health in these individuals.
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发表于 2011-11-20 14:24