TSLP (Thymic Stromal Lymphopoietin) is a secreted cytokine that signals through a complex of TSLP R and IL-7 R alpha/CD127. It acts on thymic dendritic cells which convert potentially autoreactive T cells into regulatory T cells. TSLP promotes fetal liver pro-B cell proliferation as well as large pre-B cell differentiation and B cell lymphopoiesis in adult bone marrow. It is produced by allergen challenged keratinocytes and mucosal epithelial cells. It enhances Th2 biased allergic inflammation by limiting Th1/Th17 inflammation, promoting Th2 differentiation, and enhancing the function of basophils, eosinophils, and mast cells.