Allergy, from the Greek “alios” (other) and “ergeon” (action), can be defined as a separate behavior in comparison with the classical immune response. It is a modification of the human or animal organism reactivity to a particular immunogen, the ‘ ‘allergen.' ' The molecule responsible for the sensitization was clearly defined at the beginning of the twentieth century by Clemens von Pirquet and then by Paul Portier and Charles Richet in their works on anaphylaxis. Either for sensitization or desensitization an allergen can act at a very low concentration. To become sensitized to an allergen one must be genetically programmed and able to synthesize immunoglobulins of the particular IgE class in response to an antigen. However, an antigen can only be considered as an allergen when it is able not only to induce an IgE response but also to subsequently provoke an anaphylactic or/and inflammatory reaction. Desensitization is a peculiar immunotherapy that leads the patient to synthesize IgG instead of IgE in response to repeated injections of very low doses of allergen that are practically unable to induce any anaphylactic reaction. In this case, the best molecule to be used should be one whose structure would have been modified to be nonreactive with the specific IgE present on the mast cells and thus to prevent the cascade of events leading to the inflammation process by inducing an IgG response.