Allergens are molecules normally present in the environment, responsible of inducing the so called atopic diseases. These diseases are characterized by the high production of allergen-specific IgE antibodies, which bind to mast-cells and basophils. The cross-linking of the cell-bound IgE induces the production and/or release of various inflammatory mediators (histamine, leukotrienes, platelet-activating-factor, etc.), which trigger a cascade of events resulting in the allergic syndromes. Once the diagnosis of allergy has been made, two different therapeutic approaches can be followed: a) the pharmacologic management, aimed mainly at preventing or controlling the symptoms resulting from the release of mediators by effector cells; b) the specific immunotherapy, consisting in the administration to the patient of increasing amount of the relevant allergens, opportunely prepared and eventually chemically modified. In order to have the latter approach correctly applied it is necessary to gain a wider knowledge on both regulation of IgE synthesis and allergens characteristics.