The effects of this impaired immune function can be seen in studies showing poor wound-healing in stressed individuals (Arch Gen Psychiatry, 1999; 56: 450-6; Lancet, 1995; 346: 1194-6; J Periodontol, 1999; 70: 786-92; Am Psychol, 1998; 53: 1209-18) as well as an increased susceptibility to colds and flu (N Engl J Med, 1991; 325: 606-12; Psychosom Med, 1999; 61: 175-80; J Psychosom Res, 1997; 43: 271-8; Health Psychol, 1998; 17: 214-23).

We are only just beginning to understand the ways in which stress can alter immunity. Initially, the surge of brain and adrenal hormones that accompanies stress causes an increase in circulating white blood cells. But when cortisol levels remain high, white blood cell numbers decline. The immunosuppressive effect of cortisol has also been found to contribute to reductions in lymphocyte numbers and their effectiveness (J Consult Clin Psychol, 1992; 60: 569-75; Psychol Bull, 1990; 103: 363-82). Cortisol also inhibits the production of cytokines (compounds involved in the inflammatory response) which, in turn, can be detrimental to the body’s response to immune challenges (Psychosom Med, 1999; 61: 175-80).

Poor immunity has been documented in studies of caregivers of those with Alzheimer’s disease (Ann Behav Med, 1987; 9: 16-20). Indeed, along with caring for family members with progressive dementia, other chronic stressors such as poor marital relationships and divorce have been associated with eventual suppression, rather than adaptation, of the immune response (Psychosom Med, 1996; 58: 264-72; Psychosom Med, 1987; 49: 13-34; Psychosom Med, 1988; 50: 213-29; Psychol Inquiry, 1991; 2: 249-51).

There is also an evolving belief that poor immunity as a result of chronic stress may also contribute to the development of cancer. Retrospective studies have found an association between acute loss, bereavement, depression and subsequent development of several types of cancer, including colorectal and prostate cancers (Psychosomatics, 1981; 21: 975-80; Ann NY Acad Sci, 1966; 125: 820-6; Ann NY Acad Sci, 1966; 125: 780-93; Health Psychol, 1999; 18: 482-6).

But it has been the development and progression of breast cancer in relation to life stress that has undergone the most study (BMJ, 1996; 312: 845; Cancer, 1996; 77: 1015-6; J Behav Med, 1978; 1: 133; Science, 1979; 205: 513-5). Women with advanced breast cancer have been found to have high daytime levels of cortisol whereas, in general, cortisol levels are high first thing in the morning, drop during the day and are at their lowest around midnight. Such women die, on average, a year sooner from their condition than those with normal cortisol levels (J Natl Cancer Inst, 2000; 92: 994-1000; Lancet, 1989; ii: 888-91).

Natural killer (NK)-cell activity is important in the prevention of breast cancer metastasis (J Clin Oncol, 1987; 5: 348-53), and there is evidence that women with breast cancer tend to have significantly reduced NK destruction of tumour cells compared with women who have benign breast disease or healthy breasts (Br J Cancer, 1982; 46: 611-6; Health Psychol, 1985; 4: 99-113). In some women, chronic stress can suppress NK activity.

Other evidence suggests that breast cancer patients who have experienced a high number of severe, adverse life events during their illness appear to have shorter remission times than those patients who report fewer or less severe, adverse life events during their illness (BMJ, 1989; 298: 291-3).