The regulation of normal and allergic immune responses to airborne allergens in the mucosa is still poorly understood, and the mechanism of specific immunotherapy (SIT) in normalizing the allergic response to such allergens is currently not clear. Accordingly, we have investigated the immunoregulatory mechanism of both normal and allergic responses to the major house-dust mite (HDM) and birch pollen allergens — Dermatophagoides pteroynyssinus (Der p)1 and Bet v 1, respectively — as well as the immunologic basis of SIT to HDM in rhinitis and asthma patients. In normal immunity to HDM and birch pollen, an allergen-specific peripheral T cell suppression to Der p 1 and Bet v 1 was observed. The deviated immune response was characterized by suppressed proliferative T celland Th1 (IFN-γ) and Th2 (IL-5, IL-13) cytokine responses, and increased IL-10 and TGF-β secretion by allergen-specific T cells. Neutralization of cytokine activity showed that T cell suppression was induced by IL-10 and TGF-β during SIT and in normal immunity to the mucosal allergens. In addition, SIT induced an antigen-specific suppressive activity in CD4+ CD25+ T cells of allergic individuals. Together, these results demonstrate a deviation towards a regulatory/suppressor T cell response during SIT and in normal immunity as a key event for the healthy immune response to mucosal antigens.