Background: Elevation of sIgG4 is accompanied by increase in IgG-dependent serum inhibitory activity for IgE-facilitated allergen binding (IgE-FAB) assay during specific allergen immunotherapy (SIT).Objectives: we investigated the time course of serum inhibitory activity for IgE-FAB during different period of Dermatophagoides pteronyssinus subcutaneous immunotherapy (Der p-SIT) in rhinitis and/or asthma patient.Methods: This study involved 20 adult patients with allergic rhinitis and/or asthma receiving a 156-week course of Der p-SIT, and 20 adult patients with allergic rhinitis and/or asthma receiving drug therapy only as control. Symptom and medication scores, forced expiratory volume in one second (FEV1), Der p-sIgG4 levels and the serum inhibitory activity of Der p-IgE binding to B-cells at weeks 0, 4, 12, 16, 52, 104 and 156 were analyzed.Results: Symptom score showed improvement at week 52, 104 and 156 than 0 week with significant difference in SIT patients (p<0.05), and not significant difference between 104 week and 156 week (p>0.05). Levels of Der p-sIgG4 showed a significant increase after 16 weeks of Der p-SIT (p < 0.01) and continued to increase during the 156-week SIT period. Serum obtained during Der p-SIT significantly inhibited Der p-IgE binding to B-cells (IgE-FAB) after 16 weeks of SIT (% relative Der p-IgE complex binding to B cells: 0 w = 104±24%; 4 w =104±20%, 12 w =105±17%, 16 weeks = 53±22%; 52 w = 35±22%; 104 w = 29±18%; 156 w = 27±15%; P<0.001).Conclusion: Serum sIgG4 levels and inhibitory activity for IgE-FAB increased significantly during Der p-SIT. IgE-FAB may be more relevant for symptom scores of Der p-SIT.Jing Li: Corresponding author.