LL-37
The body's own antibiotic — the single cathelicidin the human immune system makes, and a host-defence molecule that does far more than kill microbes.
What is LL-37?
The short version. LL-37 is the closest thing the human body has to its own broad-spectrum antibiotic. It belongs to a family of host-defence molecules called cathelicidins — and while around thirty cathelicidins exist across the animal kingdom, humans make exactly one: LL-37. Picture the innate immune system as a building's security: LL-37 is the guard that's both the lock-breaker and the alarm.
Where the name comes from. It's refreshingly literal. The peptide starts with two leucine residues — 'LL' — and is 37 amino acids long, so it was simply called LL-37. It isn't made on its own; it's stored as part of a larger precursor protein (hCAP18) and cut free by enzymes when needed, mostly in neutrophils and the barrier tissues — skin, gut, airways.
How LL-37 Works
The big-picture answer first. LL-37 is a host-defence peptide, which means it has more than one job. It kills microbes directly, it mops up bacterial toxins, and it calls in the immune system. Three tools in one short chain.
The membrane attack. LL-37 is positively charged and folds into an amphipathic helix — one face water-liking, one face fat-liking. That structure lets it anchor into membranes and destabilise them. Microbial membranes carry more negative charge than our own cells, so the peptide is drawn to them preferentially — a key shaped to fit the wrong locks more readily than the right ones.
The toxin sponge. The same helix binds bacterial endotoxin (LPS) and blocks it from triggering the TLR4 inflammatory alarm — dampening the over-reaction that makes some infections dangerous.
The immune beacon. It isn't only a weapon. Through a receptor called FPRL1, LL-37 recruits neutrophils, monocytes and T cells — acting as a signal that pulls immune cells toward a threat.
Where the Research Focuses
The core of the field. LL-37 is studied as a broad-spectrum, membrane-disrupting host-defence peptide active against bacteria, fungi and some enveloped viruses. Its mechanism — physical membrane damage rather than a single molecular target — is part of why microbes struggle to resist it.
Beyond killing microbes, research has examined how LL-37 binds bacterial toxins and recruits immune cells, bridging the innate and adaptive immune systems. It's a recurring example of a peptide that's also a signal.
LL-37 is expressed in skin and epithelial tissues, and studies have looked at its part in wound-repair processes and angiogenesis, as well as its regulation by vitamin D. This is active, mostly preclinical work.
LL-37 isn't a simple 'good' molecule. Research shows that at high levels it can damage host cells, and in some cancers it appears to promote growth rather than suppress it. Its effects are context-dependent — which is itself a major research question.
The Double-Edged Peptide
Most of what makes LL-37 interesting is that it cuts both ways. At high concentrations it can reduce the viability of host cells and push them toward apoptosis, and in certain cancers it has been implicated in promoting tumour growth, angiogenesis and immune-cell recruitment rather than fighting disease. This is why a lot of current work focuses on shorter fragments of the molecule — pieces that aim to keep the useful antimicrobial activity while shedding the toxicity. LL-37 is best understood as a powerful, context-dependent host-defence peptide, not a clean therapeutic — and the research reflects that.
Technical Specifications
Frequently Asked Questions
Research use only. Not for human or veterinary use.
Compiled by the Southern Pacific Labs research team. Last updated June 2026.
Published References
- Agerberth B, Gunne H, Odeberg J, et al. “FALL-39, a putative human peptide antibiotic, is cysteine-free and expressed in bone marrow and testis.” Proceedings of the National Academy of Sciences of the United States of America, 1995. PubMed
- Gudmundsson GH, Agerberth B, Odeberg J, et al. “The human gene FALL39 and processing of the cathelin precursor to the antibacterial peptide LL-37 in granulocytes.” European journal of biochemistry, 1996. PubMed
- De Yang, Chen Q, Schmidt AP, et al. “LL-37, the neutrophil granule- and epithelial cell-derived cathelicidin, utilizes formyl peptide receptor-like 1 (FPRL1) as a receptor to chemoattract human peripheral blood neutrophils, monocytes, and T cells.” The Journal of experimental medicine, 2000. PubMed
- Dürr UH, Sudheendra US, Ramamoorthy A. “LL-37, the only human member of the cathelicidin family of antimicrobial peptides.” Biochimica et biophysica acta, 2006. PubMed
Browse all LL-37 research on PubMed →
Related Research Guides
More in Immune & Defence
A three-amino-acid fragment of alpha-MSH studied for calming inflammation — like LL-37, a small endogenous peptide that works through the immune system rather than against a single target.
Read guide →Copper Peptide ComplexA copper-binding tripeptide tied to skin remodelling and wound repair — a fellow endogenous peptide studied at the barrier tissues where LL-37 is also active.
Read guide →Body Protection CompoundA gastric-derived peptide and one of the most-studied compounds in tissue-repair research — another body-derived protective peptide, approached from the healing rather than the host-defence side.
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