The immune system and the endogenous opioid system are now recognised to be closely and bidirectionally linked, forming an integrated neuroimmune–opioid axis. Rather than operating as separate domains, evidence indicates that immune cells can synthesise, store, and secrete endogenous opioid peptides, thereby contributing directly to peripheral pain modulation and the regulation of inflammatory responses.
This article reviews the key experimental and mechanistic data supporting immune-cell opioid production, including the prominent opioid-precursor families and the immune-cell subtypes involved. It then examines how inflammatory and stress-related mediators (including CRH and IL?1) trigger opioid release, how opioid-containing leukocytes traffic to inflamed tissues, and how locally released peptides activate peripheral opioid receptors to produce clinically relevant analgesia and immunomodulatory effects.
Table of contents
- The neuroimmune-opioid axis
- Mechanisms of immune cell secretion under inflammatory conditions
- Specific cellular sources and migration to inflamed tissues
- Functional outcomes: Immune-derived opioid-mediated analgesia
- Immune system modulation by opioid signalling
- Clinical and therapeutic implications
- Conclusion
- References:
The neuroimmune-opioid axis
The immune system and the endogenous opioid system share a profound and bidirectional relationship, representing one of the most significant discoveries in neuroimmunology. Rather than pain relief and immunity being governed exclusively by their respective systems, research has established that immune cells actively produce, store, and secrete endogenous opioid peptides, thereby linking these two physiological domains. This connection operates through the neuroimmune-opioid axis, in which immune-derived opioids play critical roles in pain modulation, regulation of inflammation, and immune function [1]. [2].
Immune cells synthesise endogenous opioid peptides
The answer to whether immune cells can secrete endogenous opioids is unequivocally yes. Immune cells can synthesise and secrete the full spectrum of endogenous opioid peptides, including three major families derived from distinct precursor proteins [3]. These families are proopiomelanocortin (POMC), which yields ?-endorphin and adrenocorticotropic hormone; proenkephalin (PENK), which produces methionine-enkephalin and leucine-enkephalin; and prodynorphin, which generates dynorphin A and dynorphin B [1].
Evidence for endogenous opioid synthesis by immune cells
The evidence for immune cell synthesis is particularly compelling: using polymerase chain reaction with exacting sensitive methodology, researchers detected full-length mRNA transcripts encoding these opioid precursors in all rat mononuclear leukocytes, confirming that immune cells genuinely synthesise these peptides rather than merely absorbing them from circulating plasma [1].
Which immune cells produce endogenous opoids?
The cellular localisation of opioid peptide production within the immune system is well documented. Multiple subpopulations of T cells and B cells, along with macrophages and monocytes, express homologous mRNA encoding opioid precursors [1].
Specifically, preproenkephalin has been detected in T cells, B cells, macrophages, and granulocytes [1]. Importantly, these opioid peptides are not merely incidental products but are actively processed and released. The appropriate enzymes for posttranslational processing have been identified in immune cells, and immunoreactive enkephalin peptides have been detected in both extracellular supernatants and within cells, indicating active synthesis, storage, and release mechanisms [1].
Mechanisms of immune cell secretion under inflammatory conditions
Under pathological conditions, particularly during inflammation, immune-derived opioids play a substantial role in pain control and immune regulation. A critical prerequisite for these effects appears to be inflammation itself, accompanied by hyperalgesia [1].
CRH and IL-1 stimulate endogenous opioid production
Under stressful stimuli and following exposure to stress-response releasing agents such as corticotropin-releasing hormone (CRH) and interleukin-1 (IL-1), immunocytes can secrete opioids [1]. These peptides activate opioid receptors at peripheral sensory nerve terminals, producing analgesia by inhibiting nerve excitability and suppressing the release of excitatory neuropeptides [1].
Research has specifically demonstrated that lipopolysaccharide stimulation results in dramatic opioid secretion from neutrophils in mouse spleen [1]. The activation and secretion mechanisms involve CRH and interleukin-1 as potent secretagogues. For example, met-enkephalin secretion can be dose-dependently stimulated by CRH, whereas IL-1 produces met-enkephalin release that is antagonised by opioid receptor antagonists, indicating a receptor-mediated autocrine loop [1]. The secretion follows vesicular, endocrine-like pathways, suggesting that immune cells possess a regulated secretory machinery comparable to that of neuroendocrine cells [1].
Specific cellular sources and migration to inflamed tissues
Different immune cell populations express varying amounts of opioid peptides. Under inflammatory conditions, ?-endorphin, dynorphin, and enkephalins have been detected in lymphocytes derived from lymph nodes of rats with injury [1]. Histomorphological procedures and flow cytometry have identified opioid-containing B lymphocytes, granulocytes, and monocytes/macrophages as principal sources [1].
Recent double immunofluorescence studies provide anatomical evidence that activated and memory T lymphocytes are significant opioid-containing cells that reside at or migrate to injury sites [1].
A fascinating aspect of the immune-opioid system involves the orchestrated migration of opioid-containing immune cells to inflamed tissue. This recruitment from the circulation to sites of inflammation begins with endothelial cell attachment, followed by transmigration. The process is mediated by well-orchestrated sets of adhesion molecules, including selectins and other adhesion molecules expressed on opioid-containing vascular endothelium [1].
The blockade of L-selectin with fucoidin decreases ?-endorphin-containing infiltrating leukocytes, and this reduced recruitment results in diminished opioid content and concurrent abolishment of stress-induced analgesia [1].
Functional outcomes: Immune-derived opioid-mediated analgesia
The peripheral analgesic system mediated by immune cells represents an important endogenous mechanism of pain control. Following transendothelial migration of opioid-containing leukocytes into sites of inflammation, these cells encounter a harsh microenvironment characterised by increased temperature, low pH, and high proteolytic activity [4]. Despite these challenging conditions, proximity between immune cells and peripheral sensory nerve fibres has been observed, enabling local paracrine secretion of opioids that produces significant analgesia [4].
IL-4 and its anti-inflammatory effect
The specificity and efficacy of immune-derived opioid analgesia have been demonstrated experimentally. When IL-4, a pleiotropic anti-inflammatory cytokine, is applied to injured nerves in mice, it induces M2 macrophages that continuously produce opioid peptides, including met-enkephalin, ?-endorphin, and dynorphin A [5]. This IL-4-induced shift in macrophage phenotype produces long-lasting attenuation of neuropathy-induced mechanical hypersensitivity that extends well beyond the duration of treatment [5]. Local opioid receptor systems specifically mediate the analgesia, as it is decreased by application of opioid peptide antibodies and receptor antagonists [5].
Immune system modulation by opioid signalling
Beyond pain relief, immune-derived opioids also modulate immune cell functions themselves.
Opioid-mediated actions include modulation of immune cell proliferation and their functions, including chemotaxis, superoxide production, cytokine production, and mast cell degranulation [1]. These immunomodulatory actions involve both stimulatory and inhibitory effects attributed to opioid receptor activation [1]. In particular, endogenous opioids exert opposite actions compared to exogenous opioids. While acute and chronic exogenous opioid administration has inhibitory effects on humoral responses, including antibody production and natural killer cell activity, endogenous opiates generally exert opposite actions, supporting immune function [6].
Clinical and therapeutic implications
The discovery that immune cells secrete endogenous opioids has profound clinical implications. Because these peptides act peripherally, they are largely devoid of central nervous system side effects, such as respiratory depression, sedation, dysphoria, and dependence [1]. This makes targeting opioid-containing immune tissues a novel approach for pain control and has opened new therapeutic avenues. Clinical observations support this: in postoperative pain situations, immune cell-derived opioids tonically contribute to pain relief, and blocking these endogenous opioids with naloxone increases postoperative pain [1].
Endogenous opioids and IBD
The immune-opioid axis also provides insights into disease pathogenesis. For example, in chronic inflammatory bowel disease models in which immune suppression is involved, mucosal CD4+ T cells produce endogenous opioids that suppress visceral hypersensitivity during active inflammation [7]. This suggests that endogenous opioid production by immune cells represents an adaptive mechanism to counteract inflammatory pain.
Conclusion
The evidence unequivocally demonstrates that immune cells synthesise, store, and secrete endogenous opioid peptides, including ?-endorphins, enkephalins, and dynorphins.
This secretion is particularly pronounced under inflammatory conditions when immunocytes are stimulated by stress hormones and inflammatory cytokines such as CRH and IL-1.
These immune-derived opioids migrate with immune cells to inflamed tissues, where they bind to peripheral opioid receptors on sensory nerve terminals, producing potent local analgesia without the systemic side effects associated with exogenous opioids.
This immune-opioid connection exemplifies neuroimmune integration, in which the immune system functions as both a producer and a regulator of neuropeptides, maintaining homeostasis under both health and disease.
References:
The neuroimmune-opioid axis
[1] H. Machelska and C. Stein, “Immune Mechanisms in Pain Control,” Lippincott Williams & Wilkins, Oct. 2002, https://journals.lww.com/anesthesia-analgesia/citation/2002/10000/immune_mechanisms_in_pain_control.39.aspx
Immune cells synthesise endogenous opioid peptides.
[3] Courier Pharmacy (n.d.) How does LDN work? Available at: https://courierpharmacy.co.uk/how-does-ldn-work/ (Accessed: 21 December 2025).
Functional outcomes: Immune-derived opioid-mediated analgesia
[4] S. Hua, “Neuroimmune Interaction in the Regulation of Peripheral Opioid-Mediated Analgesia in Inflammation,” Frontiers Media, Aug. 2016, https://pmc.ncbi.nlm.nih.gov/articles/PMC4970451/
[5] M. Celik, D. Abuz, J. Keye, R. Glauben, and H. Machelska, “IL-4 induces M2 macrophages to produce sustained analgesia via opioids,” American Society for Clinical Investigation, Feb. 2020, https://pmc.ncbi.nlm.nih.gov/articles/PMC7101153/
Immune system modulation by opioid signalling
[6] R. Vallejo, O. A. D. Len-Casasola, and R. Benyamin, “Opioid Therapy and Immunosuppression,” Lippincott Williams & Wilkins, Sep. 2004, https://journals.lww.com/americantherapeutics/abstract/2004/09000/opioid_therapy_and_immunosuppression__a_review.5.aspx
Are weight loss treatments making you tired, or have they led to a sudden increase in hair loss? Do you struggle with sleep?
Get a free month’s supply of one of our compounded treatments for energy, hair loss or sleep, with your first purchase of Mounjaro or Wegovy from Medical Mojo.
Claim your FREE offer
