Since nobody has mentioned it yet:
Opiates are also generally immunosuppressive
Researchers discover how opiates cause immunosuppression
Opioid therapy and immunosuppression: a review. - PubMed - NCBI
This is actually a great find. If you look at the studies associated with the news piece above, you will see that the FAS protein, which opioids increase the expression of is a member of the TNF super family (TNFSF). So, very similar to TNF-a. Histamine is one of the strongest promoters of the TNFSF, including TNF-a, so may be that is how opioid go about increasing FAS and subsequently immune cell death.
Fas-mediated cell death promoted by opioids : Abstract : Nature
Incidentally, many auto-immune patients are being given opioids for pain. In light of the connection between opioids and immune system suppression, this may seem like a great result but the TNF promotion is the exact opposite of what you want in an auto-immune condition. So, it seems opioids are like cortisol - masks the symptoms but destroys the organism in the long run.