LPO is physiologically overexpressed (at protein and/or mRNA level) as a defence mechanism in response to airway infections (both bacterial and viral)54,55; the degree of overexpression may depend on the specific pathogen (e.g. patients with influenza had the highest LPO upregulation among tested infectious diseases; Fig. 3E) or on disease severity (e.g. patients admitted to ICU had higher levels of LPO than hospitalised ones; Fig. 3B). Here, LPO is linked to infectious disease.