LL-37 is a member of the large cationic family of anti-microbial peptides called cathelicidins which have broad-spectrum anti-microbial activity and are expressed in many species. The only cathelicidin found in humans is LL-37, this is produced in epithelial cells, by proteolytic cleavage from the C-terminal of the hCAP-18 protein. LL-37 can be processed into different forms of anti-microbial peptides. As well as its anti-microbial properties LL-37 also regulates many aspects of the innate immune system and overexpression of LL-37 has been linked to autoimmune diseases such as asthma and psoriasis, making LL-37 the most studied form of the human cathelicidin peptides.

More recently, studies have shown that LL-37 binds to SARS-CoV-2 S protein and inhibits binding to its receptor hACE2, which may inhibit viral entry into the cell. LL-37 is upregulated by vitamin D, therefore this may be one mode of action for the positive outcomes seen with vitamin D treatment for Covid-19.