MUC5AC and chronic obstructive pulmonary disease: Chiba et al. (2011) demonstrated that AHR signalling in airway epithelial cells upregulates mucin genes MUC5AC and MUC5B, contributing to mucus hypersecretion (Chiba et al., 2011). In COPD, AHR promotes goblet cell differentiation and airway obstruction, conditions that heighten the risk of COVID-19-related respiratory failure (Bornstein et al., 2021; Steenblock et al., 2021).