Changes in Eosinophil Cationic Protein and Sputum Eosinophils Following Allergen Immunotherapy

A significant subset of asthma patients exhibits a type 2 inflammatory phenotype characterized by eosinophilic airway inflammation and IgE-mediated immune responses. This phenotype is driven by Th2 cytokines, including IL-4, IL-5, and IL-13, which promote eosinophil recruitment, activation, and survival within the airways. Activated eosinophils release cytotoxic granule proteins such as eosinophil cationic protein (ECP), which contributes to epithelial injury and reflects disease activity. In addition, sputum eosinophil counts are widely validated biomarkers of airway inflammation and are strongly associated with asthma severity and exacerbation risk. Allergen immunotherapy (AIT) is the only disease-modifying treatment for IgE-mediated allergic diseases and represents a cornerstone in the management of allergic asthma. It induces long-term immune tolerance through modulation of allergen-specific immune responses, including suppression of Th2-driven inflammation and enhancement of regulatory immune pathways. Both subcutaneous immunotherapy (SCIT) and sublingual immunotherapy (SLIT) have demonstrated clinical efficacy in improving asthma outcomes. Recent advances have further elucidated the immunological mechanisms underlying successful AIT, including immune deviation, induction of regulatory T cells, and increased production of blocking antibodies. In addition, personalized medicine approaches increasingly emphasize the role of biomarkers in predicting and monitoring treatment response. Standardization of outcome measures in allergen immunotherapy studies has been strongly recommended to improve comparability across clinical trials and real-world studies. Moreover, real-world evidence continues to support the effectiveness of AIT, although inter-individual variability in response remains a significant clinical challenge. Despite well-established long-term benefits of AIT, early immunologic changes following initiation of SCIT remain insufficiently characterized. Therefore, this study aimed to evaluate early changes in serum eosinophil cationic protein (ECP) and sputum eosinophils following subcutaneous allergen immunotherapy in patients with allergic asthma and to assess their relationship with clinical outcomes.