Major pathways thought to regulate the development of T cells with the Th2 phenotype. Th2 cells typically promote allergic symptoms and asthma, as well as the systemic responses that help combat parasitic responses. As discussed in the text, it is still uncertain whether the effects of various factors are most consistent with an instructive or a selective route to T cell differentiation. Nevertheless, the transcription factor GATA-3 appears to be central to this response. Binding of GATA-3 to genes encoding the characteristic Th2 cytokines is an essential first step in the activation of these genes. In addition, GATA-3 autoactivates its own gene expression, thus stabilizing the Th2 phenotype through an intracellular (intrinsic) positive feedback loop.