Feeling good doesn't help but not feeling bad does.

Researchers at the University of Wisconsin asked 52 women to think and write about extremely positive and extremely negative events in their lives whilst they recorded electrical activity in the prefrontal cortex of their brains. Participants were then given a 'flu vaccine. Six months later, the women who had shown the most activity in the right prefrontal cortex during the negative event task had the lowest antibodies, whilst greater activation of the left prefrontal cortex was associated with a stronger immune response. In previous research, regions of the right prefrontal cortex have been found to be active during emotional responses involving anger, fear and sadness, whilst the left prefrontal cortex appears to be more active in association with positive emotions. Brain activity during the positive-emotions test was not linked to differences in antibody levels. (Proceedings of the National Academy of Sciences 2003 100: 11148-11152).