The clinical efficacy of the allergen-specific immunotherapy (AIT) has been well-documented using inhalant or hymenoptera-derived allergens in atopic patients with corresponding specific IgE antibodies. AIT is considered as the unique treatment that is capable of modifying the natural course of the allergic disease because it induces a variety of immunological mechanisms, with emphasis in the production of blocking IgG antibodies by IL-10-stimulated B cells due to the generation of Treg, Breg, or even Th2 cells. Thus, the measurement of specific IgG subclasses, particularly IgG4, to the crude extract or more importantly to allergen components, might be a useful and potential tool to follow-up objectively the patients undergoing AIT in addition to clinical parameters. In this chapter, the authors have emphasized a very sensitive and highly specific reverse ELISA, developed by them, to measure IgG subclasses directed to clinically relevant natural allergens that are undoubtedly better when compared to those obtained with recombinant counterparts. Such a technique may produce more authentic results taking into account the IgG subclass binding capacity to a particular allergen and might be a valuable and alternative method for monitoring activation of tolerance-inducing mechanisms in patients under AIT.