This is the 2007 version. Click here for the 2017 chapter 05 table of contents.

Conditional Immune Response

A potentially important application of Pavlovian conditioning involves the body's immune system. Like other body systems, it can be activated or suppressed through classical conditioning. This has exciting implications. If learning can stimulate immune system activity, people should be able to arrange conditions to improve health or healing. Perhaps humans have already been doing this for thousands of years. Classical conditioning may shed a light on healing rituals and trances practiced by pre-modern cultures.

An early experiment reported by Schmeck (1985) involved a team of researchers at the University of Alabama medical school. They studied effects of classical conditioning on activity of natural killer cells (NK cells) that destroy germs and other invaders in the body. In the experiment, mice were exposed for three hours at a time to a powerful odor (camphor ). Exposure to this odor, by itself, had no effect on the mice.

Next, the odor was made to predict a significant biological event. After exposure to the odor, mice in an experimental group were given injections of a synthetic chemical called poly I:C (for polyinosinic-polycytidilic acid) that stimulates activity of natural killer cells. Mice in the control group did not receive the poly I:C.

How did researchers demonstrate a conditional immune response in mice?

For the experimental group, the odor of camphor was paired with exposure to Poly I:C nine times. In the 10th session, the mice were exposed only to the odor of camphor. Every mouse in the experimental group showed large increases in natural killer cell activity. Their bodies were "predicting" the injection of poly I:C and responding with immune system activity. In the control group, which was exposed only to the odor of camphor, no such response occurred.

This is typical of research on classical conditioning. It is capable of demonstrating remarkable, subtle biological effects. However, analyzing the exact mechanism can be difficult. How exactly does a mouse's "knowledge" that poly I:C is about to be delivered to its bloodstream stimulate the production of NK cells? If researchers knew that, perhaps they could help human patients boost production of NK cells when needed, as well.

Researchers suspect that neuroimmunomodulation takes place at every level of the nervous system. The word neuroimmunomodulation contains the word roots for nerve (neuro) and immune (immuno). So neuroimmunomodulation means modulating immune system activity with nervous system activity. The discipline of psychoneuroimmunology-arose in the 1970s and 1980s to study psychological influences on immune system functioning. However, research in this area has produced disappointing results. The positive effects of psychological intervention on health are easy to document, but evidence relating these benefits to immune system changes has been elusive (see the section on psychoneuroimmunology in Chapter 14).

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