LL-37

LL-37 is a 37-amino-acid antimicrobial peptide and the only member of the cathelicidin family found in humans ^[1]. It is produced from a precursor protein called hCAP-18, from which the active LL-37 fragment is cleaved; the name reflects that the peptide begins with two leucine residues and is 37 amino acids long ^[1][4]. LL-37 is a natural component of human innate immunity, expressed by neutrophils and other leukocytes and by epithelial cells of the skin, gut, and respiratory tract, and it is secreted into wound fluid and airway surface liquid ^[1][4].

The peptide is strikingly multifunctional. Its best-known activity is direct antimicrobial action against a broad range of bacteria, but research has established that it also regulates inflammation, recruits immune cells, neutralizes bacterial endotoxin, and actively promotes tissue repair ^[1][2][4]. In human skin, LL-37 is strongly induced upon wounding and participates in re-epithelialization, the process by which new epithelium covers a wound; its levels are low in chronic, non-healing ulcers, and blocking it impairs wound closure ^[2][3]. It additionally promotes angiogenesis, the growth of new blood vessels needed for healing ^[4].

Because it is a human host-defense molecule with combined antimicrobial, immunomodulatory, and regenerative activities, LL-37 is studied as a template for treating stubborn infections, chronic wounds, and inflammatory and gut conditions. It is a potent, biologically active peptide, and therapeutic use remains in the research stage; context matters, because in some settings LL-37 can also amplify inflammation. It is generally supplied as a synthetic peptide for research.

LL-37 is a human antimicrobial and immunomodulatory peptide with no approved therapeutic form and no established safety or dosing for supplemental use. Beyond its antimicrobial role it is a potent immune-signaling molecule, and elevated LL-37 has been implicated in inflammatory and autoimmune conditions such as psoriasis, rosacea, and lupus, as well as in promoting angiogenesis and the growth of certain tumors. Its net effect from exogenous administration in humans is unknown.

LL-37 is the active fragment of hCAP18, the protein product of the human cathelicidin gene (CAMP), and is the only cathelicidin peptide found in humans. It was characterized in the mid-1990s, when researchers describing the antibacterial C-terminal region of hCAP18 named the parent 39-residue peptide FALL-39; the cleaved mature form was designated LL-37 for the two leucine residues that begin its 37-amino-acid chain. Its function was initially understood as antimicrobial defense, but in 2003 Heilborn and colleagues at the Karolinska Institute demonstrated a distinct role in wound re-epithelialization, showing that the peptide is strongly induced in healing skin and is deficient in chronic ulcers that fail to close ^[1]. Subsequent work established its additional actions in angiogenesis and immune signaling.

LL-37 enjoys a strong reputation as a genuinely multifunctional molecule, appealing precisely because it is a defender the human body already produces and trusts. Researchers highlight its combination of broad antimicrobial activity, promotion of wound healing and new blood-vessel growth, and fine-tuning of immune signaling, an unusually versatile profile for a single peptide. This breadth makes it a fascinating subject for research into infection, tissue repair, and inflammation. In honesty, LL-37 remains at the research stage with no approved therapeutic form; its effects are context-dependent, and at some concentrations it can contribute to inflammation rather than resolve it, so its clinical use is still being worked out.