**8. Discussion: ACOS as a phenotype or endotype of obstructive airway disease.**

Asthma and COPD are themselves heterogeneous disorders that comprise several phenotypes and endotypes. If we admit that ACOS should be characterized by the presence of inflamma‐ tory features of both COPD (mainly Th1) and asthma (mainly Th2), we could argue that COPD patients with sputum eosinophilia and with asthma, with a mixed neutrophilic–eosinophilic pattern are, in fact, patients with ACOS, regardless of their clinical presentation. So changing our point of view, from an initial clinically based classification of obstructive airways diseases to another centered on inflammatory underlying mechanisms (endotypes), would allow us to tailor and optimize treatments, leaving behind the rigid categorization of patients into existing diagnostic labels of either asthma or COPD, which do not fully recognize the molecular and clinical heterogeneity of chronic obstructive airway diseases [39].

We need to move toward a new taxonomy of airway diseases that takes into account the underlying pathogenic mechanisms. In this new scenario, ACOS would be an endotype, like early-onset allergic asthma or emphysema. However, things are not so straightforward, and there are several important issues that complicate the settling-in of this new approach:


It is to be expected that in a near future, advances in the identification of inflammatory patterns can help us to adequately classify patients with chronic obstructive airway disease and offer them the best therapeutic option in each case.
