There are many transmitters and peptides etc which are usually thought of as relating to neurotransmission and affective states, many (if not all) of these are spread out through the whole body. The topic implies one aspect of what these neurotransmitters might be doing in the external body- regulating immune function through the mood state of the organism. Or it might be a conservation of sorts whereby a particular chemical has many different functions throughout the body so that it only needs to produce a certain number of chemicals for these transmissions.

Which of these or both of them or other theories are correctish?
Why do macrophages have opiate receptors?