LL-37 is the body's own broad-spectrum antibiotic. It is a 37-amino-acid peptide that fights bacteria, fungi, and some viruses while also acting as a signaling molecule in wound healing and inflammation.
Origin and Structure
LL-37, also called cathelicidin, CAP18, or hCAP18, is the active fragment of a larger precursor protein called hCAP18. The body cleaves hCAP18 to release LL-37 when it is needed, mostly in skin, neutrophils (a type of white blood cell), and the mucosal linings of the lungs and gut.
The peptide gets its name from its first two amino acids — two leucines (L-L) — followed by 35 more residues. When LL-37 contacts a membrane, it folds into an amphipathic alpha-helix, meaning one side is positively charged and water-loving while the other is hydrophobic. This split structure is the key to how it works.
Antimicrobial Mechanism
Bacterial membranes carry a negative charge on their outer surface. The positively charged side of LL-37 sticks to that surface, then the hydrophobic side inserts into the membrane and disrupts it. The membrane breaks open, contents leak out, and the bacterium dies.
This mechanism works against gram-positive bacteria, gram-negative bacteria, several fungi, and some enveloped viruses. Dürr and colleagues (2006) reviewed the broad activity profile and the structural reasons LL-37 reaches so many targets.
Because the attack is physical rather than chemical, bacteria struggle to develop resistance. Mutating away from this mechanism would require redesigning the entire membrane, which carries heavy fitness costs. That is the central reason LL-37 has drawn so much interest as antibiotic resistance climbs.
Beyond Killing Bacteria
LL-37 does more than break membranes. It also acts as an immune signaling molecule through a receptor called FPR2. By binding FPR2, the peptide recruits neutrophils and monocytes to sites of injury or infection, modulates inflammation, and triggers wound healing programs.
Research has connected LL-37 to faster epithelial closure in wound models, angiogenesis (new blood vessel formation), and crosstalk with the adaptive immune system. Some studies have explored its role in cancer biology, where its effects vary by tumor type — sometimes anti-tumor, sometimes pro-tumor.
This dual role — direct killer plus signaling molecule — is why some researchers describe LL-37 as a "host defense peptide" rather than just an antimicrobial peptide. The label captures the broader picture.
Why It Matters in Current Research
Antibiotic resistance is one of the most pressing problems in clinical medicine. Many of the antibiotics in use today face rising failure rates, and the pipeline of new small-molecule antibiotics has thinned. Endogenous peptides like LL-37 represent a different chemical class with a different mechanism, which makes them attractive for further study.
Open questions remain. LL-37 is sensitive to degradation by proteases, can be cytotoxic to host cells at higher concentrations, and behaves differently in different tissue environments. Researchers are studying analogs, delivery systems, and combination approaches to address these limitations.
Work continues on what triggers LL-37 expression, how its activity is balanced against host damage, and how its immunomodulatory effects could be harnessed without driving inflammation. These compounds are sold strictly for in vitro laboratory research and are not approved for human consumption.