[annie123 (OP)]

I just heard someone from the Chiu lab on the radio talking about using pain to suppress the immune system in flesh eating disease. I have written to ask  someone at the lab to elaborate but they probably don't reply to individual lay people so I'm posting here as well. If pain is used to suppress the immune system, why do we use pain killers in diseases that have overactive immune systems like MS, arthritis etc? Wouldn't it be more logical to allow as much pain as possible so that the immune system is suppressed and the damage to joints reduced? rather a bleak presciption of course and one I have never heard recommended cand cannot really understand. If anyone wants to know more or can interpret what I heard more clearly listen to the broadcast Quirks and Quarks CBC radio (canada) May 19 -
http://www.cbc.ca/radio/quirks/flesh-eating-bacteria-stun-the-immune-system-into-silence-with-pain-1.4667343

[RD]

... If pain is used to suppress the immune system, why do we use pain killers in diseases that have overactive immune systems like MS ...

People have exercised to the point it suppresses the immune system (a/k/a overtraining) in an attempt to treat MS.
Physiological-stress increases cortisol production, which suppresses the immune system.

[annie123 (OP)]

I was pleasantly surprised to actually get a reply from Dr Chiu by return. This may be useful for others.

 "It is a really great question and I am happy to try to answer it.  Pain is a normal response of the body to injury or inflammation.  Celsus defined pain as one of the four cardinal signs of inflammation 2000 years ago, and it protects us behaviorally by causing us to withdraw from the dangerous stimuli such as heat or injury, and second causing us to put less pressure on a painful area to allow proper healing processes to occur. A big scientific question is how these pain fibers feedback to regulate the local immune response and inflammation. I think that it will depend on the situation, and is an active and very important area of research.
We found that one of the factors pain fibers release called CGRP, a neuropeptide, shuts down recruitment and function of neutrophils, a very important type of circulating immune cell in necrotizing fasciitis.  In acute injury where there is a sterile wound, I think this suppression of neutrophils is an important part of wound healing because otherwise they could be damaging.  Another situation where nerves shutting down  inflammation could be beneficial is in septic shock, where an overactive immune system is what destroys your organs.
In chronic conditions like what you mentioned such as arthritis, MS, it is not yet clear what pain fibers do to the immune response.  For example, in osteorathritis, there was a recent trial to block nerve growth factor to treat pain.  It was actually quite successful in decreasing pain scores, but joint destruction got worse. Now whether this is because pain fibers were needed to suppress inflammation is not clear, or if people were overusing their joints because of pain blockade.
One very important point to realize is that the painkillers that we have act in very different ways.  Some of them, like opioids, would only shut off the perception of pain because they act in the brain, but not affect how the same nerves signal to the immune system.  Therefore in these cases, it may not make any difference.  However, other drugs may target the peripheral nerves as well, and these could indeed affect the immune response, in some cases potentially making inflammation worse, not better.  Time remains to tell whether pain drives or suppresses immunity in chronic conditions.