Abstract It is well documented that exposure to ultraviolet (UV) radiation in sunlight before immunization suppresses systemic as well as local immune responses. We have previously shown that administrating UV irradiation 7 days after immunization also suppresses Th1- and Th2-driven antibody (Ab) via generation of antigen (Ag)-specific CD4 + regulatory T cells. In this study, we specifically show that IL-10, which is produced by CD4 + regulatory T cells generated in mice that received UV irradiation after immunization, mediates the suppression of Ab responses by inhibiting Th cell activation. In addition, IL-10 produced upon Ag-specific activation by UV-induced regulatory T cells also mediates bystander suppression. Furthermore, because UV irradiation after immunization effectively dampens both Th1 and Th2 immune responses, we further demonstrated that mice receiving UV irradiation after allergen sensitization had reduced Th2-driven airway inflammation and airway hyperreactivity (AHR). These results suggest that UV irradiation in pre-sensitized individuals induces Ag-specific IL-10 producing regulatory T cells representing type 1 regulatory T cells that suppress Th2 immunity and may have therapeutic potential for asthmatic patients.