**1. Introduction**

Allergic diseases are manifested in susceptible individual by exposure to proteins named allergens that induce an immune response mediated by IgE antibody. Numerous allergens from different sources such as plants, insects, mites and mammals have been obtained as recombinant molecules by molecular cloning. These types of molecules have shown molecular, functional and immunological properties similar to the corresponding natural allergens and, therefore, could be used for in vitro and in vivo diagnosis test of allergy. An important step was done with the development of variants of allergens with reduced allergenicity and preserved immunogenicity, which paved the way toward its rational use in allergen specific immunotherapy to treat allergies. Few of the allergens cloned have been developed to a stage at which they are suitable for use in clinical studies. However, today the academic and scientific communities note a broad and important activity to offer in the near future preparations with enhanced clinical efficacy and safety. In this work, basic aspects and experimental and clinical results of this process are presented.
