New molecular and pharmacological tools have made it possible for
us to identify the intricate network that exists between the immune
system and the brain [particularly the stress pathway], a network that
allows the two systems to signal each other continuously and rapidly.
Chemicals produced by immune cells signal the brain, and the brain
in turn sends chemical signals to restrain the immune system.... Dis-
ruption of this communication network in any way, whether inher-
ited or through drugs, toxic substances or surgery, exacerbates the
diseases that these systems guard against: infectious, inflammatory,
autoimmune, and associated mood disorders.
Cortisol has [from the stress pathway] a double-edged effect on the
immune system. Too much of it suppresses immune function and
makes us more vulnerable to infections. Yet in the short term, a burst
of cortisol helps the immune system respond to an infection or injury.
It sends the white blood cells, the body’s main line of defense against
injury and infection, to their battle stations.... Cortisol also signals
when the level of immune activity is adequate. It sends this message
via the brain, which relays the information through the hypothala-
mus to the pituitary gland; the stress response is then adjusted accord-
ingly. Cortisol’s checks-and-balances effect is what makes it such a suc-
cessful treatment for problems resulting from a hyperactive immune
system, such as rashes or allergies, and for autoimmune conditions in
which the immune system attacks the body’s own healthy tissue.
When we put cortisone cream on a rash or take steroids orally to fight
inflammation, we are only supplementing what our own cortisol nor-
mally does.
Cytokines from the body’s immune system can send signals to the
brain in several ways. Ordinarily, a “blood-brain barrier” shields the
central nervous system from potentially dangerous molecules in the
bloodstream. During inflammation or illness, however, this barrier be-
comes more permeable, and cytokines may be carried across into the
brain with nutrients from the blood. Some cytokines, on the other
hand, readily pass through leaky areas in the blood-brain barrier at
any time. But cytokines do not have to cross the blood-brain barrier to
exert their effects. Cytokines can attach to their receptors in the lining
of blood vessels in the brain and stimulate the release of secondary
chemical signals in the brain tissue around the blood vessels.Cytokines can also signal the brain and affect nerve pathways,
such as the vagus nerve, which innervates the heart, stomach, small
54 J. Fraser Mustard