The Leucocyte Protein L1 (Calprotectin): A Putative Nonspecific Defence Factor at Epithelial Surfaces

The leucocyte antigen L1 is a calcium-binding, highly immunogenic molecule which predominantly consists of different polypeptide chains adding up to an Mr≈36 K noncovalently stabilized complex.1 Peptides I and II are virtually identical (L1H chain) whereas the smaller peptide III is structurally and antigenically in the main different (L1L chain).2 L1 was discovered more than 10 years ago as a major cytosol protein fraction (50–60%) of neutrophilic granulocytes.3 In addition to being expressed by most circulating (and emigrated) neutrophils and monocytes4, it is also found in a subset of reactive tissue macrophages and many tissue eosinophils.5 Its abundant occurrence in vivo as an Mr≈36 K peptide complex both in neutrophils and monocytes has recently been confirmed6,7.