LL-37 is the sole human cathelicidin, generated in vivo by proteolytic cleavage of the precursor protein hCAP-18 by the serine protease proteinase 3. The 37-amino acid C-terminal fragment — named for its two N-terminal leucine residues — was first characterized by Birgitta Agerberth, Gudmundur H. Gudmundsson, and colleagues at the Karolinska Institutet in Stockholm in the mid-1990s. It forms an amphipathic alpha-helix in membrane-mimetic environments.

Preclinical investigation has covered direct antimicrobial activity against gram-positive and gram-negative organisms, disruption of bacterial biofilm architecture, modulation of cytokine and chemokine signaling, neutrophil and dendritic cell responses, and wound closure dynamics in murine models. A landmark 2002 study by Scott, Davidson, Bowdish, and Hancock at the University of British Columbia demonstrated that LL-37 functions as a multifunctional modulator of innate immune signaling in vitro. All findings referenced here derive from preclinical models and have not been validated in controlled human clinical trials.

LL-37 has been cited in Journal of Immunology, Frontiers in Immunology, Antimicrobial Agents and Chemotherapy, and PNAS. It is not approved by the U.S. Food and Drug Administration for any diagnostic, therapeutic, or preventive application. It is classified as a research compound only.