Psychoneuroimmunology is the branch
of biomedical science that explores the relationships between
the nervous system, emotions, and the immune system. Stress
has been suspected of increasing susceptibility to various infectious
diseases or cancer for years. Now evidence is mounting that
the immune system and the nervous system may be inextricably
interconnected.
Research has shown that a wide range of stresses,
from losing a spouse to facing a tough examination, can deplete
immune resources, causing levels of B and T cells to drop, natural
killer cells to become less responsive, and fewer IgA antibodies
to be secreted in the saliva.

Biological
links between the immune system and the central nervous system
exist at several levels. One well-known pathway involves the
adrenal glands, which, in response to stress messages from the
brain, release corticosteroid hormones into the blood. In addition
to helping a person respond to emergencies by mobilizing the
body’s energy reserves, these “stress hormones”
decrease antibodies and reduce lymphocytes in both number and
strength.
More recently, it has become apparent that
hormones and neuropeptides, which convey messages to other cells
of the nervous system and organs throughout the body, also “speak”
to cells of the immune system. Macrophages and T cells carry
receptors for certain neuropeptides; natural killer cells, too,
respond to them. Even more surprising, some macrophages and
activated lymphocytes actually manufacture typical neuropeptides.
At the same time, some lymphokines, secreted by the activated
lymphocytes such as interferon and the interleukins, can transmit
information to the nervous system. Hormones produced by the
thymus, too, act on cells in the brain.
In addition, the brain may directly influence
the immune system by sending messages down nerve cells. Networks
of nerve fibers have been found that connect to the thymus gland,
spleen, lymph nodes, and bone marrow. Moreover, experiments
show that immune function can be altered by actions that destroy
specific brain areas.
The closely interlocked system facilitates
a two-way flow of information. Primarily through the language
of hormones and secondarily through the immune cells, may function
in a sensory capacity, detecting the arrival of foreign invaders
and relaying chemical signals to alert the brain. The brain,
for its part, may send signals that guide the traffic of cells
through the lymphoid organs.