Immune · Cathelicidin antimicrobial peptide · Compounded 503A
LL-37: the human antimicrobial peptide.
LL-37 is the only human cathelicidin antimicrobial peptide — a 37-amino-acid host defense molecule produced by neutrophils, epithelial cells, and macrophages. It acts as a first-line antimicrobial against bacteria, viruses, and fungi while also modulating wound healing and immune signaling.
What LL-37 is
LL-37 (so named for two leucines at the N-terminus and 37 total amino acids) is the active C-terminal fragment of hCAP-18, the only cathelicidin in humans. It is produced by neutrophils, epithelial cells in the skin and gut, and macrophages.
It functions as a broad-spectrum antimicrobial — active against Gram-positive and Gram-negative bacteria, enveloped viruses, and some fungi — and as an immunomodulator that shapes the response to infection and tissue damage.
Vitamin D directly regulates LL-37 expression through the vitamin D response element in the cathelicidin gene — which is why vitamin D deficiency is associated with impaired antimicrobial defense.
How it works
LL-37's primary antimicrobial mechanism is membrane disruption. Its amphipathic alpha-helical structure inserts into negatively charged microbial membranes, forming pores that compromise membrane integrity — a mechanism resistance is slow to develop against.
Beyond direct killing, LL-37 binds the formyl peptide receptor FPR2 on immune cells, modulating chemotaxis, cytokine production, and the resolution of inflammation. This dual antimicrobial-immunomodulatory profile is unusual.
LL-37 also has wound-healing and angiogenic activity, supporting epithelial repair in skin and gut barrier surfaces.
What patients use it for
Antimicrobial defense
Broad-spectrum activity against bacteria, viruses, and fungi — particularly relevant for patients with biofilm-associated or treatment-resistant infections.
Gut barrier and microbiome
LL-37 in the gut epithelium modulates microbiome composition and supports mucosal barrier integrity — relevant for patients with dysbiosis or chronic gut inflammation.
Skin and wound healing
Topical and systemic LL-37 supports epithelial repair, with research applications in chronic non-healing wounds and inflammatory skin conditions.
Immune modulation
Modulates neutrophil function and inflammation resolution — relevant in chronic inflammatory states and post-acute infection recovery.
Evidence summary
Dürr UH et al. (Biochim Biophys Acta, 2006) reviewed the antimicrobial mechanism of LL-37 and its broad-spectrum activity profile.
Liu PT et al. (Science, 2006) demonstrated the vitamin D / cathelicidin axis — vitamin D induces LL-37 in macrophages and is required for Mycobacterium tuberculosis killing.
Vandamme D et al. (Cellular Immunology, 2012) reviewed LL-37's immunomodulatory functions beyond direct antimicrobial activity.
Direct human RCT data for therapeutic injectable LL-37 is limited. The endogenous biology is well-characterized; therapeutic supplementation is investigational.
Dosing and clinical context
General clinical context only. Kindr Health physicians determine the appropriate dose and protocol for each patient based on history and labs. This is not a prescription or dosing recommendation.
Most commonly subcutaneous injection. Nebulized LL-37 has been investigated for pulmonary applications.
Course-based use, typically 4–8 weeks targeted at a specific infectious or inflammatory indication.
Compounded LL-37 from licensed 503A pharmacies is the typical U.S. dispensing pathway. Not FDA-approved.
Safety and contraindications
Human therapeutic safety data is limited. Generally well-tolerated in early clinical experience.
Theoretical concerns: LL-37 has been implicated in some autoimmune conditions (rosacea, psoriasis) as part of dysregulated immune signaling. Patients with active autoimmune disease should be cautious.
Contraindications: pregnancy, active autoimmune flare, known hypersensitivity. Use under physician supervision.
Who it's typically considered for
- Patients with chronic biofilm-associated or treatment-resistant infections under specialist supervision
- Patients with gut dysbiosis, SIBO, or chronic mucosal inflammation
- Adults with chronic non-healing wounds (specialist supervision)
- Patients with documented low vitamin D status and recurrent infection
Frequently asked questions
Is LL-37 FDA-approved?
No. LL-37 is investigational; compounded preparations are dispensed by licensed 503A pharmacies under physician prescription for off-label use.
Will LL-37 disrupt my microbiome?
Endogenous LL-37 helps shape healthy microbiome composition rather than indiscriminately killing all bacteria. Therapeutic dosing should still be physician-supervised for patients with sensitive gut ecology.
LL-37 vs antibiotics — which?
Different roles. Antibiotics are targeted antimicrobials for documented infection. LL-37 is investigational and used in compounded practice for harder-to-treat or biofilm-associated cases — typically alongside, not instead of, conventional therapy.
Can LL-37 cause flare in autoimmune disease?
LL-37 has been implicated in some autoimmune pathways (psoriasis, rosacea). Patients with active autoimmune disease should not use LL-37 without specialist supervision.
How does vitamin D relate to LL-37?
Vitamin D is required for cathelicidin/LL-37 production. Low vitamin D status is associated with low endogenous LL-37 and impaired antimicrobial defense — which is why vitamin D repletion is important first-line.
Is LL-37 anti-viral?
Yes — LL-37 has documented activity against several enveloped viruses in preclinical models. Direct human RCT data in viral indications is limited.
How long are typical courses?
Short, indication-driven courses (4–8 weeks) are typical. Continuous long-term use is uncommon and not well characterized.
Sources
- Dürr UH et al. LL-37, the only human member of the cathelicidin family of antimicrobial peptides. Biochim Biophys Acta (2006). — pubmed.ncbi.nlm.nih.gov/16716248
- Liu PT et al. Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science (2006). — pubmed.ncbi.nlm.nih.gov/16497887
- Vandamme D et al. A comprehensive summary of LL-37, the factotum human cathelicidin peptide. Cellular Immunology (2012). — pubmed.ncbi.nlm.nih.gov/22999958
- Kahlenberg JM, Kaplan MJ. Little peptide, big effects: the role of LL-37 in inflammation and autoimmune disease. Journal of Immunology (2013). — pubmed.ncbi.nlm.nih.gov/23420887
Considering LL-37?
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Medically reviewed by Dr. Ana Lisa Carr, MD, MBA
Board-Certified Family Medicine Physician · Lead Provider / Medical Reviewer
NPI 1689841744 · Last reviewed: May 10, 2026
Last reviewed May 10, 2026. Compounded medications are prepared by FDA-registered 503A pharmacies and are not FDA-approved drug products. Prescriptions require a clinical evaluation; a Kindr Health physician determines eligibility. Not for use in pregnancy. This page provides educational information and is not medical advice.