This is an amazing study and a great step forward in our understanding of biology and previously mysterious mind-body connections. Besides helping to explain at least part of the classic Placebo Effect, it also opens the door to a great deal of potential both for making existing treatments more efficacious (by supplementing them with reward system intervention) and for new treatments (utilizing the brain reward system to influence immunological response).

The study seems very well-constructed and powerful. They demonstrate that direct stimulation of reward neuralchemistry produces specific and varied immune system responses, and also discover the pathways for this effect. They demonstrate causality well and also show that if the specific pathway they identify (the sympathetic nervous system connecting to lymphatic cells) is blocked, the increased immunological response no longer follows the neural reward stimulation. This seems really, really well done and it's high-impact science.

Upon reading the study, I wondered what might have created this mind-body link, in evolutionary terms. It turns out the authors considered this question as well. From the SciHub piece on this paper: "Many activities that are known to stimulate reward-system signaling in the VTA, including engaging in sex, socializing and traveling, markedly increase exposure to pathogens. Therefore, it can be envisioned, as the authors suggest, that coupling the reward center to the immune system would offer a selective advantage, which helps to explain the intertwining of these two systems from an evolutionary perspective."

Really cool stuff!