The invention is in the field of molecular immunology, more in particular in the field of medical treatment of animals such as humans suffering from unwanted immune reactions. The invention relates to methods for the treatment of unwanted immune reactions and provides means and methods for suppressing an immune response. The present invention relates in particular to regulatory T cells and methods of long-term, culture-expanding, activating and using same in immunotherapy and for the suppression of autoimmune responses, allergies and inflammatory diseases. The invention provides a sia alpha 2,3-conjugated antigen for use in the suppression of an immune response in a patient in need of such a treatment.