Secondly, from the results of the current study, the question about the cell types responsible for the increased MMP and YKL-40 production arises, and although we were not able to allocate the MMP and YKL-40 secretion to a certain cell type, it can be speculated that the observed increases in PEx reflect the extent of neutrophilic airway inflammation. This evidence concerns the gene CHI3L1 and inflammatory response.