3-Bromotyrosine, a product of protein oxidation, is generated after eosinophil activation during allergic responses. It serves as a biomarker for eosinophil peroxidase-induced protein oxidation both in vitro and in vivo. Beyond its association with eosinophils, 3-bromotyrosine levels are significantly elevated in peritoneal exudate from K. pneumoniae-infected mice, indicating a fivefold increase compared to uninfected controls, which implies that neutrophils may release increased levels of oxidized products during inflammation. Furthermore, in humans, allergen-challenged lung samples from patients with allergen-induced asthma show a tenfold increase in 3-bromotyrosine, highlighting its potential as a marker for oxidative stress in allergic conditions.