LL-37 is a natural protein in our bodies that can kill a wide range of bacteria and some viruses. It also talks to the immune system, sometimes turning it on and sometimes turning it off. When it builds up to fairly high levels (around 1‑10 micromolar) it can start killing human cells, especially those that are already infected, but at very high levels (like 300 micromolar in psoriasis) it may also damage healthy tissue. The peptide’s actions depend a lot on where and how much of it is present.

Human cathelicidin LL-37 shows activity towards both gram-positive and gram-negative bacteria, and it is also active against some types of viruses. Besides its antimicrobial effects, the peptide modulates innate immunity through binding and inactivation of bacterial endotoxins and promoting chemotaxis of immune cells. LL-37 is reported to interact with plasma membrane receptors and mediate import of Ca²⁺. Importantly, LL-37 has both anti- and pro-inflammatory effects. LL-37 is cytotoxic to many different human cell types, particularly infected cells, when administered to the cells at final concentrations of 1-10 µM. In psoriatic lesions very high concentrations (300 µM) of the peptide are detected, and in periodontitis, gingival crevicular fluid contains about 1 µM LL-37, implying high concentrations of the peptide at the site of infection/inflammation which can affect host cell viability locally. Altogether, LL-37 may inhibit and prevent the infection from spreading by direct anti-bacterial and anti-viral effects, but also via anti- and pro-inflammatory mechanisms, and through killing already infected and weakened host cells at the site of infection/inflammation.