Subjects in a study designed to evaluate the effectiveness of a painkiller experience symptom relief after receiving inactive "sugar pills." People with asthma suffer asthma attacks when they believe they are being exposed to allergens, even though the allergens are purely imaginary. Students experiencing the most psychological stress during exam period come down with more colds than their easygoing friends.

Researchers are beginning to explore the physiological bases of the body-mind relationship and to elucidate the ways our thoughts and feelings can influence the physiological processes that affect our health. Many scientists are discovering that we can best understand physical health with a holistic outlook that includes the functioning of the mind. Psychoneuroimmunology (PNI) is the study of the interrelationships of the three body-mind systems that serve as communication networks in the orchestration of homeostasis: the nervous, endocrine, and immune systems.

At one time physiologists thought the immune system functioned independently. Now, anatomical and physiological evidence points to extensive communication between the immune system and the nervous and endocrine systems. Several organs of the immune system, including the spleen, thymus, and lymph nodes, receive sympathetic nervous system (SNS) innervation. In animal studies, when this innervation is tampered with, changes in immune responses occur. In the spleen and thymus, the SNS nerve endings have a synapse-like connection (the type of connection observed in the nervous system) with lymphocytes (a type of white blood cell), and appear to communicate in some way with these immune cells.

In experiments with laboratory animals, injury to certain areas of the brain leads to immune system changes. These changes affect specific resistance to disease, which refers to the immune system's ability to recognise, attack, and remember particular foreign molecules such as viruses and bacteria. Specific resistance to disease is carried out by several types of white blood cells called B cells and T cells. Lesions to one part of the brain (the anterior hypothalamus) interfere with the activity of antibody-secreting B cells, while damage to another area (the posterior hypothalamus) reduces the effectiveness of the memory T cells.

Neurotransmitters, a type of chemical messenger found in the nervous system, are also found in the immune system, particularly in the bone marrow and thymus gland. Many types of immune cells have receptors for neurotransmitters and hormones, and it has been demonstrated that the immune system can respond to neurotransmitters and hormones. For example, at low concentrations, norepinephrine stimulates the immune system. Cortisol, a hormone secreted by the adrenal cortex in association with the stress response, acts to inhibit immune system activity, perhaps one of its energy conservation effects. An elevation in cortisol has been observed in a significant number of people with major depressive illness, so endocrine effects may help explain the link between depression and decreased immune response.

Some researchers believe the opioid may be another vehicle for communication. Opioid peptides are manufactured by some lymphocytes as well as by nerves in the central nervous system (CNS), which comprises the brain and spinal cord. Opioid receptor sites have been found on some white blood cells.

Communication between the immune system and the CNS goes both ways. The immune system does not merely respond to messages from above, but seems to inform the brain and endocrine organs. Small protein hormones called cytokines are secreted by many types of white blood cells and also in the brain. Cytokines appear to act as chemical messengers in this communication network.

Such physiological data are intriguing, particularly in light of studies on humans investigating changes in immune function following various psychosocial stressors. Most readers are familiar with stories of lowered resistance to disease in people experiencing death of a loved one, divorce, job loss, depression, loneliness, exams, and sleep deprivation. But finding the psychophysiological explanation for these observations still eludes us. The challenge to scientists is to figure out exactly how PNI can explain real-life changes in immune response. Humans are especially difficult to study because when under stress, behavioural variables are apt to change as well: People may get less sleep, cat differently, drink more alcohol, take drugs, and so forth, all of which may influence immune responses.

Despite these obstacles, PNI researchers are making some headway in understanding the types of effects our thoughts and feelings can have on the immune system. One study found that men who exhibited a strong SNS response to frustration showed a greater number of suppressor T cells, which inhibit immune response, and a lower rate of lymphocyte proliferation than men with a milder response. Another study found no relationship between stress and numbers of lymphocytes or other immune cells, but did find certain changes in lymphocyte proliferation and antibody reactions. Stay tuned!

PNI research appears to justify what many health-care workers have observed since the beginning of time: One's thoughts, feelings, moods, and beliefs influence one's level of health of disease. Especially harmful appear to be feelings of helplessness, hopelessness, fear, and social isolation, all common to the experience of being sick, and especially to being a patient in a hospital. While conclusive studies are still lacking, preliminary data suggest that the use of positive imagery, relaxation techniques, psychotherapy, and humour can help patients feel much better, and may have a positive influence on the course of a disease.