Maternal IgE activates fetal mast cells Mast cells (MCs) are immune cells that participate in allergic reactions through their activation by immunoglobulin E (IgE) antibodies. MCs arise early during mammalian development, but it is unclear whether IgE-mediated activation occurs in fetal tissues and what the source of IgE stimulation is. Msallam et al. show that human and mouse fetal MCs can be sensitized by IgE of maternal origin, which crosses the placental barrier through the fetal neonatal Fc receptor (see the Perspective by Rothenberg). Prenatal maternal sensitization conferred transient allergen sensitivity after birth and resulted in the development of postnatal skin and airway inflammation in the offspring after their first exposure to allergen. Thus, both maternal IgE and fetal MCs may influence mother-to-child transmission of allergic disease during gestation. Science , this issue p. 941 ; see also p. 907