Psychology2016

434 CHAPTER 11

cells, encounter an infection in the body. The white blood cells surround the bacteria or other

infectious material and release the chemicals and enzymes into the bloodstream. From there,

these chemicals activate receptor sites on the vagus nerve, the longest nerve that connects

the body to the brain. It is the activation of these receptor sites that signals the brain that the

body is sick, causing the brain to respond by further activation of the immune system.

Stress activates this same system but starts in the brain rather than in the blood-

stream. The same chemical changes that occur in the brain when it has been alerted by

the vagus nerve to infection in the body occurred in laboratory animals when they were

kept isolated from other animals or given electric shocks (Maier & Watkins, 1998). This

has the effect of “priming” the immune system, allowing it to more successfully resist the

effects of the stress, as in Selye’s resistance stage of the GAS.

Hormones also play a part in helping the immune system fight the effects of stress.

Researchers (Morgan et al., 2009) have found that a hormone called dehydroepiandroste-

rone (DHEA), known to provide antistress benefits in animals, also aids humans in stress

toleration—perhaps by regulating the effects of stress on the hippocampus (part of the

limbic system). to Learning Objective 2.11.

So stress actually increases the activity of the immune system?

But then how does stress end up causing those diseases, like high

blood pressure?

The positive effects of stress on the immune system only seem to work when the

stress is not a continual, chronic condition. As stress continues, the body’s resources begin

to fail in the exhaustion phase of the general adaptation to stress (Kiecolt-Glaser et al., 1987,

1995, 1996; Prigerson et al., 1997). In one study, college students who were undergoing a

stressful series of exams were compared to a group of similar students relaxing during a

time of no classes and no exams (Deinzer et al., 2000). The exam group tested significantly

lower for immune system chemicals that help fight off disease than did the relaxing con-

trol group, even as long as 14 days after the exams were over. The suppression of immune

system functioning by stress apparently can continue even after the stress itself is over.

One reason that the early stress reaction is helpful but prolonged stress is not might

be that the stress reaction, in evolutionary terms, is really

only “designed” for a short-term response, such as run-

ning from a predator (Sapolsky, 2004). That level of intense

bodily and hormonal activity isn’t really meant to go on and

on, as it does for human beings in the modern, stress-filled

life we now know. Humans experience the stress reaction

over prolonged periods of time and in situations that are not

necessarily life-threatening, leading to a breakdown in the

immune system. (See Figure 11. 2 )

While it is clear that stress affects the immune sys-

tem and overall health, exactly why this occurs has been a

topic of research. The inf lammatory response happens when

the tissues of the body are injured in some way—bacterial

infections, heat, toxic substances, physical injury, and so

on. Damaged cells release chemicals that then cause the

blood vessels to leak fluids into surrounding tissues, and

this causes swelling or inflammation. Believe it or not, this

inflammation is an important part of the immune system’s

response to invading substances, serving to block access to

other body tissues. Researchers are now finding that inflam-

mation may actually be the means through which stress can

Figure 11.2 Stress Duration and Illness
In this graph, the risk of getting a cold virus increases greatly as the months of
exposure to a stressor increase. Although a stress reaction can be useful in its early
phase, prolonged stress has a negative impact on the immune system, leaving the
body vulnerable to illnesses such as a cold.
SOURCE: Cohen et al. (1998).

4

3

2

1

0

Relative risk of a cold

(1 indicates normal risk of

getting a cold)

No

stressor

11 –6 6–24 over 24

Duration of life stressor (in months)

Interactive