Expression of Th2 immunity against environmental Ags is the hallmark of the allergic phenotype and contrasts with the Th1-like pattern, which is stably expressed in healthy adults throughout life. Epidemiological studies indicate that the prenatal environment plays an important and decisive role in the development of allergy later in life. Since the underlying mechanisms were unclear, an animal model was developed to study the impact of maternal allergy on the development of an allergic immune response in early life. An allergic Th2 response was induced in pregnant mice by sensitization and aerosol allergen exposure. Both, IgG1 and IgG2a, but not IgE, Abs cross the placental barrier. Free allergen also crosses the placental area and was detected in serum and amniotic fluids of neonatal F1 mice. These F1 mice demonstrated a suppressed Th1 response, as reflected by lowered frequencies and reduced levels of IFN-γ production. Development of an IgE response against the same allergen was completely prevented early in life. This effect was mediated by diaplacental transfer of allergen-specific IgG1 Abs. In contrast, allergic sensitization against a different allergen early in life was accelerated in these mice. This effect was mediated by maternal CD4 and OVA-specific Th2 cells induced by allergic sensitization during pregnancy. These data indicate a critical role for maternal T and B cell response in shaping pre- and postnatal maturation of specific immunity to allergens.