Allergic disorders are characterized by the prevalence of immunoglobulin (Ig) isotype E antibodies, and are considered to result from enhanced T helper type-2 (Th2) responses to allergens. A Th2 response is characterized by enhanced humoral responses and the production of IL-4 and IL-5 by CD4+ T cells (Th2) in response to antigen (1 ,2 ). These “Th2 cytokines” enhance the allergic response by inducing B cell isotype switching to IgE (3 –5 ), by inducing undifferentiated Th0 cells to further differentiate into Th2 cells, and by inducing eosinophil growth, and differentiation (5 ). In addition, the Th2 cytokines inhibit Th0 differentiation into Th1 cells, thereby reducing the recruitment of interferon gamma (IFN-γ) producing Th1 cells that could then down-regulate or modulate the Th2 responses (3 ,5 ). Consequently, the ability of a Th2 response to allergens to exert a positive feedback effect leads to a vicious cycle and may explain the exacerbation of allergic responses that follows continued exposure to allergens in atopic humans (2 ,6 ).