Antibodies against eosinophil peroxidase (EPX), a major granule protein of eosinophils, have been reported in a subset of patients with eosinophilic and autoimmune disorders. EPX is a heme-containing peroxidase enzyme that contributes to host defense and inflammatory responses through the generation of reactive oxidant species. Autoantibodies directed against EPX may reflect eosinophil activation, tissue damage, and loss of immune tolerance. Elevated levels of anti-EPX antibodies have been associated with chronic inflammatory conditions characterized by eosinophilic infiltration and may serve as a marker of immune dysregulation. EPX belongs to the mammalian peroxidase family and is predominantly expressed in eosinophil granules. The presence of anti-EPX antibodies is being investigated for its potential role in disease pathogenesis, disease activity monitoring, and biomarker development in eosinophil-associated disorders.