There is a need to implement early-life predictors of allergic sensitization, so that at-risk newborns can be accurately defined and preventive measures instituted. The work presented in this book gives an in-sight into the possibility of in utero allergen exposure of the fetus in allergic and non-allergic mothers and suggests that maternal environmental exposure influences innate immune functions of the newborn. Three main findings can be emphasized. First, in-utero exposure to HDM allergens Der p1 & Blo t5 is prevalent, second, in-utero T-cell helper function imbalance due to in-utero allergen exposure and third, the newborn may already be on the way to the development of allergic disease.