β-Endorphin is an endogenous opioid peptide that acts as an agonist at μ-opioid receptors (μORs) and exhibits immunomodulatory, neuromodulatory, antidepressant, and antinociceptive/analgesic activities. In vivo, β-endorphin increases levels of B-cells and production of antibodies as well as secretion of IL-4 and proliferation of splenocytes, shifting the immune response toward Th2-specific mediators.