Allergic diseases are characterized by an overreaction characterized by Th2-type cell response, and as a consequence, an IgE-switched B cell immunity. Obviously, type-2 cytokines (IL-4, IL-5, IL-9, IL-13) and particularly IL-4 have been identified as potential targets for allergy treatment. While initial experiences using anti-IL-4 principles in asthma were rather ambiguous, more recent studies using an IL-4 mute in blocking the IL-4 and IL-13 receptor have shown promising results. Furthermore, our understanding of IL-4 biology is more specific and may promote more targeted interventions. A key function of IL-4 is the induction of 'master switch' transcription factor GATA3 that drives Th2 differentiation and also effectively inhibits the induction of regulatory T cells. Consequences for treatment of allergic diseases are also discussed.