The human body produces its own antibiotics. They are called antimicrobial peptides — AMPs — and LL-37 is the most studied of them. It is produced by epithelial cells, neutrophils, macrophages and natural killer cells. It is present in skin, airways, gut and reproductive tract. It is, in effect, a front-line defence molecule the body synthesises and deploys at sites of infection or tissue damage.
LL-37 is a 37-amino acid cationic peptide — the "LL" refers to the two leucine residues at its N-terminus. It is the only known human cathelicidin, a family of antimicrobial peptides found across mammals. It is encoded by the CAMP gene and processed from a larger precursor protein called hCAP-18.
Its antimicrobial mechanism is physical rather than biochemical — it disrupts bacterial cell membranes by inserting into the lipid bilayer and creating pores. This is why, unlike conventional antibiotics, bacteria have shown very limited capacity to develop resistance to LL-37. A bacterium cannot easily evolve a cell membrane that is immune to physical disruption.
LL-37 is now understood to be far more than an antimicrobial molecule. It functions as an immune modulator, a wound healing promoter, and an angiogenesis activator. Specific documented functions include: direct killing of bacteria, fungi and some viruses, modulation of toll-like receptor signalling to balance inflammatory response, promotion of keratinocyte migration and proliferation in wound healing, stimulation of angiogenesis at wound sites, and chemotaxis — attracting immune cells to sites of infection.
This multifunctionality makes LL-37 unusual. Most immune molecules have a primary function and secondary effects. LL-37 appears to have evolved as a broad-spectrum first responder, doing several things simultaneously in response to tissue damage or infection.
LL-37 production is regulated by several factors including vitamin D levels, microbiome composition and inflammatory status. Deficiency states are associated with increased susceptibility to chronic infection, impaired wound healing and certain inflammatory skin conditions. Patients with atopic dermatitis have been found to have significantly reduced LL-37 expression in skin — which may partly explain the increased infection susceptibility characteristic of the condition.
Research has also found reduced LL-37 in patients with chronic lung infections, periodontitis, and some autoimmune conditions. The relationship between LL-37 deficiency and susceptibility to certain infections appears to be causal rather than merely correlational.
LL-37 and its derivatives are in active clinical investigation. Areas of active research include: topical wound healing applications, respiratory infection treatment (nebulised delivery), sepsis management, and as an adjunct in antibiotic-resistant infection treatment.
Several modified versions of LL-37 with enhanced stability and reduced toxicity are in Phase 1 and Phase 2 trials. The challenge with LL-37 as a therapeutic is that it is rapidly degraded in biological fluids — synthetic derivatives that retain antimicrobial activity while improving pharmacokinetics are a major focus of current research.
One particularly interesting finding from 2024 research: LL-37 appears to have activity against certain viral envelopes, including influenza and some coronaviruses, through a similar membrane-disruption mechanism to its antibacterial action.
One practical connection worth noting: vitamin D strongly upregulates LL-37 expression. This is believed to be part of the mechanism by which vitamin D deficiency is associated with increased respiratory infection susceptibility — lower vitamin D reduces LL-37 production in respiratory epithelium, impairing the antimicrobial barrier. Several studies have demonstrated that vitamin D supplementation in deficient individuals measurably increases LL-37 levels in mucosal tissues.
This does not make vitamin D supplementation a substitute for adequate LL-37 — the relationship is one of many factors. But it is a concrete example of how nutritional status affects peptide production in clinically meaningful ways.
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Complete reference entries, protocol guidance and blood work calendars for the compounds in active use.