Eosinophil peroxidase (EPX) is a cationic granule protein primarily secreted by mature eosinophils that functions in immune regulation and host defense 1. EPX mediates tyrosine nitration of secondary granule proteins and demonstrates antibacterial activity against Mycobacterium tuberculosis through bacterial fragmentation [UniProt/GO annotations]. The protein also suppresses lymphocyte proliferation in vitro at physiologically relevant concentrations, suggesting a regulatory role in immunological responses 2. In asthma, EPX serves as a sensitive biomarker of eosinophilic inflammation. Serum EPX reflects systemic eosinophilic inflammation and normalizes with mepolizumab therapy in 96% of patients, while sputum EPX is more sensitive than sputum eosinophil counts for detecting airway inflammation and frequently remains elevated despite systemic control 3. In asthmatic airways, eosinophils undergo cytolytic degranulation releasing EPX, which neutrophils internalize via CD11b-dependent mechanisms 4. High sputum EPX levels associate with severe airflow obstruction and frequent exacerbations 3. EPX deficiency represents a known disease state. Additionally, airway autoantibodies against EPX occur in 40% of moderate-severe asthmatic patients, persist despite anti-inflammatory treatment, and associate with exacerbations and eosinophil degranulation 5. EPX production is reciprocally regulated with eosinophil cationic protein and influenced by genetic polymorphisms 6.