The main role of endogenous opioid peptides is the modulation of pain. Opioid peptides exert their analgesic
activity by binding to the opioid receptors distributed widely in the central nervous system (CNS). However, opioid
receptors are also found on tissues and organs outside the CNS, including the cells of the immune system, indicating that
opioids are capable of exerting additional effects in periphery. Morphine, which is a gold standard in the treatment of
chronic pain, is well-known for its immunosuppressive effects. Much less is known about the immunomodulatory effects
exerted by endogenous (enkephalins, endorphins, dynorphins and endomorphins) and synthetic peptides activating opioid
receptors. In this review we tried to summarize opioid peptide-mediated modulation of immune cell functions which can
be stimulatory as well as inhibitory.