Besides being a natural antibiotic, the human cathelicidin LL37, produced by epithelial cells and neutrophils, is an important immune-modulator. LL37 alone, or in complex with DNA, can activate inflammatory pathways in psoriasis, Systemic Lupus Erythematosus (SLE) and Rheumatoid Arthritis (RA). In this work, we describe the capacity of the recombinant monoclonal antibody RB137, previously shown to specifically recognize LL37 in its native form by ELISA, to detect LL37 by immunofluorescence in human tissues derived from SLE and RA patients. In these tissues, LL37 decorates DNA filaments resembling neutrophil-extracellular-trap (NET) structures. This antibody represents a valuable tool to detect the presence, the native state and the exact localization of LL37 in human tissues in health and disease.