LL-37 (Cathelicidin): Clinical Overview and 2026 Status

LL-37 is the only human cathelicidin antimicrobial peptide, generated by cleavage of the precursor protein hCAP18. It is a genuine component of innate immunity, and it is also implicated in the pathogenesis of inflammatory skin disease. Both halves belong in any honest account of it.

This guide situates LL-37 within the broader field of peptide therapy and is written for clinicians. It is clinical education, not medical advice, and nothing here should be read as a treatment recommendation or protocol.

What is LL-37?

LL-37 is a 37-amino-acid peptide beginning with two leucine residues — hence the name. It is produced as part of the precursor protein hCAP18, which is stored in neutrophil granules and expressed by epithelial cells at barrier surfaces including skin, gut and airway, then cleaved to release the active peptide.

It is cationic and amphipathic: positively charged, with distinct water-loving and lipid-loving faces. That architecture allows it to insert into and disrupt bacterial membranes, which carry a net negative charge, while largely sparing host cell membranes.

Its expression is induced by vitamin D. This is one of the better-characterized molecular links between vitamin D status and innate immune function, and it is a more precise statement than most claims made about vitamin D and immunity.

What LL-37 does

Its functions extend well past direct antimicrobial activity. LL-37 acts as a chemoattractant, recruiting neutrophils, monocytes and T cells to sites of injury or infection. It promotes angiogenesis and participates in wound repair and re-epithelialization. It also neutralizes bacterial lipopolysaccharide, damping endotoxin-driven inflammation.

This breadth is the basis for interest in LL-37 as a therapeutic for wound healing and infection, and it is legitimate biology. Antimicrobial peptides are also studied as a potential answer to antibiotic resistance, since membrane disruption is harder to evade than a specific molecular target.

Where LL-37 drives disease

The part omitted from marketing material is that LL-37 is causally implicated in inflammatory skin disease.

In rosacea, abnormal processing of cathelicidin generates peptide forms that are more inflammatory than normal LL-37, and elevated levels of these species are found in affected skin. This is understood as a contributor to the disease process, not an incidental finding.

In psoriasis, LL-37 forms complexes with the patient's own DNA. Those complexes activate plasmacytoid dendritic cells through a pathway that normally responds to microbial DNA, driving interferon production and helping break self-tolerance. LL-37 is, in that setting, functioning as an autoantigen.

The clinical implication is direct: this is an immune-active peptide with documented capacity to drive inflammation, and administering it exogenously is not a neutral act. Any patient with rosacea, psoriasis, or another interferon-driven inflammatory condition warrants particular caution.

Regulatory status — stated precisely

LL-37 was among the twelve peptides removed from FDA Category 2 on April 15, 2026. Category 2 designates substances identified as presenting significant safety risks, effectively barring them from compounding.

Removal is not approval. It lifts the safety-risk designation and returns the substance to pending evaluation; it does not place LL-37 on the 503A bulks list, does not make it an approved drug, and does not by itself establish compounding eligibility. Describing the change as FDA clearance is inaccurate and creates compliance exposure. The same nuance applies across all twelve, as detailed in our PEG-MGF guide.

Our peptide formulary carries current status for every agent, and this category has moved twice in three years.