Allergen-specific immunotherapy was introduced into clinical practice at the beginning of the 20(th) century and its efficacy in the treatment of seasonal allergic rhinitis has been confirmed in many clinical studies which have shown that it can prevent the onset of new sensitizations to different allergens and reduces the development of asthma in patients with allergic rhinitis. Progress in molecular cloning and characterization of allergens have made it possible to produce single recombinant allergens whose immunological properties have been tested in vitro and in vivo and have demonstrated that they retain properties resembling their natural counterpart. Several rational approaches are being developed to improve the efficacy of SIT by reducing immunoglobulin IgE-mediated adverse reactions. Some of these molecules have been tested in the clinic, demonstrating the feasibility of using biotechnology-derived products as new standardized, improved and safer therapeutic compositions.