To put in brief, the body has immune reactions to acute, inflammatory injuries as well as to long-term neural pain and damage. White blood cells, inflammatory mediators, and other structures travel to those sites and interact with the tissue and nerve cells in the area and attempt to repair.
The study shows that many of the white blood cells that go to work at an area of pain have a positive effect, often going as far as to release pain-dampening opioid chemicals. However, there is also a line to be crossed with the immune response. In acute injuries, prolonged inflammation and the inflammatory cells that act in the area can further potentiate pain signals. In chronic, neural pain, immune cells can completely change the way that both our peripheral nerves (through the body and tissue) and central nerves (in the brain and spinal cord) respond to stimuli, heightening sensitivity to pain in many situations.
The specific processes and variety of cells involved in this phenomenon is exceedingly complex and well out of the scope of any manual therapist to thoroughly understand or attempt to address. However, it's important to be aware of the mechanisms in place, as it's suggested that any activation of the immune system, such as general illness and disease, might play a role in pain propagation.
While we certainly do not want to be shutting down and suppressing our immune systems in order to try and control pain, researchers are working on ways to isolate the individual immune mechanisms to alter pain sensations. Although no solutions are hitting the applied practices of medicine yet, this information certainly has major implications when it comes to factoring in general health, chronic disease, and even diet into the reasons why complex pain occurs.