Researchers in this literature review provide a fairly comprehensive account of relevant acupuncture research studies grouped by the issue being treated - low back pain, stroke recovery, depression and IBS. Beyond just an account of the treatment methods and results for each set of issues, they delve deeply into the known and/or suspected biological mechanisms involved in the results from acupuncture.
They recognize some of the known/explored effects of acupuncture - "analgesic, muscle relaxing, anti-inflammatory, mild anxiolytic, and antidepressant actions, with possible biological mechanisms such as central sensitization, neurotransmitters, the intestinal flora, immune regulation, oxidative stress, and neuroinflammation."