LL-37

LL-37 is a 37-amino-acid peptide derived from the human cathelicidin precursor protein hCAP18 and is the best-characterized cathelicidin antimicrobial peptide in humans. It is produced by several immune and epithelial cell types and is found in tissues including the skin, respiratory tract, gastrointestinal tract, and other epithelial surfaces. LL-37 is part of the innate immune system, where it has been studied for direct antimicrobial activity as well as for broader host-defense signaling.

Its biological activity is more complex than simply disrupting microbial membranes. LL-37 has been reported to interact with bacterial membranes and endotoxins, influence chemotaxis of immune cells, modulate inflammatory responses, and participate in epithelial repair. Research also suggests roles in angiogenesis, cell migration, barrier integrity, and wound closure. Importantly, LL-37 can have both pro-inflammatory and anti-inflammatory effects depending on concentration, tissue environment, receptor activity, and disease context, which is one reason the peptide remains scientifically interesting but biologically complex.

Much of the mechanistic evidence comes from cell, tissue, and animal models rather than established therapeutic use in humans. Researchers continue to investigate how LL-37 activity changes across different concentrations and biological environments, including how it interacts with immune signaling pathways, epithelial cells, microbial membranes, and inflammatory mediators.