1. Stress can affect natural killer cell cytotoxicity - which is of major importance in the defence against various infections and cancer. 2. Reduced levels of natural killer cell cytotoxicity have been found in people who are highly stressed - including students facing important examinations - bereaved persons - and those who are severely depressed.(c)Lifestyle is the overall pattern of decisions and behaviours that determine a person's health and quality of life. 4. Studies reveal that immune functioning is better in individuals receiving social support. 5. Also - changes in the immune system will have more effect on health among those whose immune systems are already weakened.