Stress may play a role in how the immune system responds to cancer. Scientists know that many types of stress activate the body's hormone system, which in turn can cause changes in the immune system, the body's defense against infection and disease, including cancer. However, the immune system is a highly specialized network whose activity is affected not only by stress but also by a number of other factors.

Although studies have shown that stress can alter the way the immune system functions, there is still no scientific evidence of a direct cause-and-effect relationship between stress, immune system changes and the development of cancer.
Of all the immune system cells, the natural killer (NK) cells and Tcells, which are types of white blood cells, have shown the strongest links to destroying cancer cells and preventing the spread of cancer cells. Researchers have found that stress decreases the NK-cell activity in rats and causes an increase in certain types of tumours, as well as promote the spread of cancer.
It is harder to conduct research on humans. The complex interactions of the many systems that affect cancer--from the immune system to the hormone system together with environmental factors makes it difficult to pinpoint the exact role of stress in the formation of cancer.

In addition, researchers can't expose people to tumour cells as they can do with rats. Studies are mainly based on interviewing people with cancer about their stress levels before diagnosis or following large numbers of people for many years until some are shown to develop cancer.

The subject is very controversial among medical professionals, but some studies indicate that there may be some relationship between the onset of cancer and previous breakdown of relationships, loss by death, loss of a job, family disintegration or emotional deprivation. Depression may affect the body's hormones and may inhibit the immune system's ability to counter disease.

Whether we develop clinical cancer or not depends upon the ability of our immune system to destroy these cancer cells. It is only a weakened or unhealthy immune system that makes the body cancer-prone. Many patients are immuno-suppressed due to physical, chemical, mental or environmental causes.