LL-37 — Host defense / antimicrobial peptide — sole human cathelicidin; innate immunity effector peptide

LL-37 is the C-terminal 37-amino acid peptide cleaved from the precursor protein hCAP18 (human cathelicidin antimicrobial protein 18) by proteinase 3 in neutrophils and kallikrein 5 in skin. It is the only cathelicidin in the human genome. Mechanisms: (1) Direct membrane disruption of gram-positive and gram-negative bacteria, fungi, and some enveloped viruses by forming amphipathic alpha-helical structures that insert into and disrupt lipid bilayers; (2) Chemotaxis — recruits neutrophils, monocytes, and mast cells to infection sites via formyl peptide receptor-like 1 (FPRL-1) binding; (3) Wound healing — activates keratinocyte migration and proliferation via EGFR transactivation; (4) Immunomodulation — suppresses LPS-induced TLR4 signaling, reducing excessive inflammation; (5) Angiogenesis — promotes VEGF-A expression and endothelial tube formation. Vitamin D3 is the primary regulator of LL-37 gene (CAMP gene) expression — critically important clinical context.