PTGDR2 and chronic obstructive pulmonary disease: Interestingly, ILC2s within inflamed tissue of chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) patients are found to exhibit slightly different phenotypic characteristics: here ILC2s have been reported to be either positive for CRTH2 or ST2 (IL-33 receptor chain).