LL-37

What is LL-37?

LL-37 is a natural antimicrobial peptide made by the human body. It is the only member of the cathelicidin family in humans. Its name comes from its structure: it is 37 amino acids long and begins with two leucines (L). It is cut from a larger precursor protein called hCAP18, which is made by white blood cells called neutrophils and by cells lining the skin, gut and airways. The precursor was described in the mid-1990s.

LL-37 is part of the body's first line of defense against infection.⁸ It is not an approved drug. Small early-phase trials have tested it applied to chronic leg ulcers, but it is not approved for that or any other use. Online it is sold as a research chemical and marketed for immune support, chronic infections, biofilms and wound healing.

How LL-37 works

LL-37 does two different jobs. First, it kills microbes directly. It carries a positive charge and is attracted to the negatively charged outer surfaces of bacteria, where it punches holes in their membranes. Second, it acts as a messenger for the immune system. It calls in immune cells, changes how cells respond to bacterial fragments and influences wound repair.

At first, scientists studied cathelicidins mainly for their killing power. Today their immune-signaling roles are seen as just as important.⁷ That second role is where things get complicated, because the same signals that help fight infection can also drive unwanted inflammation.

In more precise terms, LL-37 is an amphipathic, cationic alpha-helical peptide. It binds bacterial lipopolysaccharide and other microbial molecules, and it changes how Toll-like receptors detect them. For lipid ligands such as LPS it tends to dampen signaling, while for nucleic acids such as self-DNA and RNA it can enhance it.⁷ Its production is switched on by vitamin D. In immune cells, active vitamin D directly induces the gene for LL-37, which is a key part of antibacterial and antiviral defense.²

What the research shows on LL-37

Most of what is known about LL-37 comes from studying the body's own peptide in health and disease, not from giving it as a treatment.

Its protective role is well supported. LL-37 is one of the key antimicrobial molecules in the layered defenses of the skin.⁸ In the lungs of people with cystic fibrosis, neutrophils release LL-37 as part of the response to infection, but Pseudomonas aeruginosa can resist these defenses.³ In people with eczema who develop a severe skin herpes infection called eczema herpeticum, levels of LL-37 and other antimicrobial peptides are reduced.⁶

Its harmful role is just as well supported. In rosacea, increased cathelicidin in the skin is one of the drivers of inflammation.¹ Recent work describes a TLR2, LL-37 and mTORC1 signaling axis as a core pathway in the disease.⁴ In psoriasis, LL-37 released from neutrophils binds the body's own DNA, which helps activate immune cells and the IL-17 pathway that drives the disease.⁵ Reviews link cathelicidins to worse inflammation in autoimmune conditions more broadly.⁷

As a treatment, the evidence is thin. Small trials of topical LL-37 in chronic leg ulcers exist, but no large trial has shown benefit, and there are no human trials of injected LL-37 for immune support, chronic infection or biofilms. Vitamin D is sometimes promoted as a natural way to raise LL-37. Low vitamin D status is associated with greater susceptibility to viral infections,² but an association is not proof that supplements prevent infection in people with normal levels. Lab studies showing that LL-37 kills bacteria in a dish do not show that injecting it clears an infection in the body.

Studied for: Antimicrobial research, Biofilm research, Wound healing.

LL-37 reference data

LL-37 dosage in published research

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How LL-37 is used in practice

LL-37 has no approved medical use. Some integrative and functional medicine clinics have offered it, usually as a short course of injections under the skin, for chronic infections, Lyme-related symptoms, mold illness or recurring infections. These uses are not supported by clinical trials. It is sometimes pitched as an alternative to antibiotics, which it is not. Compounding access is restricted because of FDA concerns about bulk peptides.

Most LL-37 is bought online as "research use only" powder. It is a fairly long peptide that is harder to make cleanly than short ones, and online products are not made to pharmaceutical standards. Buyers cannot verify identity, purity, sterility or contamination with bacterial toxins. Any injected product with bacterial endotoxin can itself trigger fever and inflammation.

LL-37 risks and side effects

Reported side effects include redness and inflammation at the injection site, flu-like symptoms and flushing. These fit with LL-37's role as an immune signal.

The main concern is that more LL-37 may worsen certain diseases. Because it drives inflammation in rosacea and psoriasis,¹⁵ people with those conditions, or with other autoimmune or inflammatory skin diseases, should avoid it. Its effects on autoimmune disease more generally are a reasonable concern.⁷ At high concentrations in the lab, LL-37 can also damage human cells, not just bacteria.

There are no data on long-term use, safe doses or interactions. It should not be used in pregnancy. Using an unproven peptide in place of proper diagnosis and antibiotics for a real infection can let that infection progress.

Reported side effects

Alternatives to LL-37

For bacterial infections, the right antibiotic, chosen with cultures when possible, remains the standard. Chronic or biofilm-related infections, such as those around implants or in chronic wounds, usually need specialist care, which may include surgical cleaning or device removal along with targeted antibiotics.

For chronic wounds, proven measures include compression for venous ulcers, offloading for diabetic foot ulcers, debridement and infection control. Becaplermin (Regranex) is an FDA-approved growth factor gel for certain diabetic foot ulcers. For people with low vitamin D, correcting the deficiency supports the body's own LL-37 production,² although supplements are not a cure for infection.

Questions for a clinician

• What evidence applies to my situation, and at what level?
• What established alternatives should I consider first?
• What monitoring would be necessary, and how often?
• Can this be prescribed at all where I live, or only sold for research?