The three major types of opioid receptors (, , and ) are members of the DRY (Asp-Arg-Tyr)-containing subfamily of seven transmembrane spanning receptors. Opioid receptors on cells of the immune system are virtually identical to those on neuronal cells. The activation of opioid receptors in the CNS leads to analgesia while activation of opioid receptors on immune cells can enhance or suppress immune function depending on the target cell and immune parameter measured.