LL-37

LL-37 is the human body's main cathelicidin — a natural 'host-defense' peptide that forms part of our innate immune front line.

It's a positively charged, membrane-active peptide: it can punch holes in the membranes of bacteria and other microbes, and it also signals to the immune system, guiding immune-cell movement, wound healing, and inflammation. Notably, its inflammatory role is context-dependent and can sometimes be pro-inflammatory.

It's been studied extensively in the lab and in disease associations, but those dual, context-dependent roles make it hard to turn into a clean therapeutic, and it is not a marketed drug.