The immune system is a complex surveillance system that fights bacteria, viruses, and other foreign substances that invade the body. This defense relies on the actions of specialized white blood cells called lymphocytes, which circulate through the bloodstream and secrete chemical antibodies. Scientists have discovered that the immune system is linked to other systems of the body, including the brain and nervous system. Psychoneuroimmunology is the study of the relationship between psychological influences, the nervous system, and the immune system.
Researchers in this field have found that psychological factors such as stress can influence immune cell activity and increase vulnerability to physical illness. In controlled animal experiments, rats exposed to overcrowding, noise, or inescapable shocks—and primates separated from social companions—exhibit a significant drop in immune cell activity compared to unstressed animals. In addition, studies on humans have shown that immune cell activity changes in response to divorce, the death of a spouse, loss of employment, and other negative life events. This research helps to explain why stress increases the risk of illnesses ranging from the common cold to certain forms of cancer. It has also sparked interest in how optimism, social support, and other psychological factors can be used to protect the body.