Which substance is NOT known to cross the placental barrier?

The correct answer is that allergens are not typically known to cross the placental barrier. The placenta acts as a selective barrier, meaning it regulates what substances can pass from the maternal circulation to the fetus. While it allows essential compounds like oxygen, nutrients, and certain medications to cross, it generally does not permit larger or complex molecules such as allergens to pass through easily.

Medications can differ widely in their ability to cross the placenta, depending on their molecular size and characteristics. Antibodies, particularly immunoglobulin G (IgG), can cross the placenta to provide passive immunity to the fetus. Oxygen is a vital gas that also crosses the placenta to sustain fetal life.

In contrast, allergens, which may be proteins that can trigger an immune response, are usually too large or complex to cross the placental barrier, thus helping to protect the fetus from potential allergic reactions or immune responses that the mother might encounter.