What is the mechanism explaining the effect of a positive attitude on immune system functioning? It seems likely that a positive attitude can speed up the healing process. but I can't decide by what mechanism it would operate by. 

Questions


Does having a positive attitude improve immune system functioning?
If so, what is the mechanism by which this occurs?


Initial thoughts: Is it the inhibition of some sort of stress cortisol? 
The answer seems like it would have to do with how the placebo effect or hypnosis suggestion functions.
As far as I know, it has not been shown that a positive attitude has any effect on the immune system (ignoring the less significant placebo effect).  What has been shown is that long-term stress has a negative effect on the immune system.  Short-term stress actually has a positive effect on the immune system, but long-term stress has been correlated with several affecting mechanisms: lower white blood cell count, too much cytokine production.  

The following study found lots of mixed results.  A typical sign that there's really no clear answer to the question.  When there are so many confounding results, it's easy for people to only take the half they agree with, so we should be wary of our propensity to do so when we want to hear a particular answer.


  The failure of optimism to consistently predict better physical health indicators and outcomes is consistent with this pessimistic view of optimism: “The best doesn't always occur. When things go wrong in a big way, the optimist may be particularly vulnerable.”


...


  I have suggested an alternative hypothesis to explain negative effects of optimism on the immune system (see Fig. 1; Segerstrom, 2001; Segerstrom et al., 2003; Solberg Nes et al., in press). This hypothesis specifies that under difficult circumstances, more optimistic people remain engaged with those circumstances whereas more pessimistic people disengage, avoid, or give up. Giving up can be a physiologically protective response because stressor exposure is minimized in the short term by giving up rather than remaining engaged...


http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1948078/