Homeostasis is critically dependent on communication between the immune, endocrine and nervous systems, and also the opioid system. Opioids can affect immune processes directly through the activation of opioid receptors on leukocytes or indirectly through the stimulation of the corticosteroid and catecholamine release. Leukocytes can also synthesize and release opioid peptides, which act as analgesic factors during inflammatory process. Many data indicate that opioids are also immunomodulators, and can affect leukocyte migration, killing activity and apoptosis.