IL4 and infection: Because an IL-4-mediated decrease in ceramides could cause abnormal barrier functions—induction of excessive transepidermal water loss, and an increased risk of pathogenic infection—PS likely accelerates the enhancement of the skin barrier function by a prompt increase in ceramide content due to the SM hydrolysis pathway, through activation of SMases in the plasma membrane as well as the SPT/CerS-mediated de novo synthesis pathway under stressed conditions, compared with un-stressed conditions, in which the SPT-mediated de novo synthesis pathway is only operated.