A 32-year-old female patient

is pregnant for the third time. She presented today for a prenatal check-up at

28 weeks gestation. In her previous pregnancies, she has had problems with

Rh-incompatibility because she is Rh-negative and her husband is Rh-positive.

Therefore, in the past she has received anti-D (Rh) immunoglobulins after

deliveries to neutralize any Rh antigens to prevent immunization and the

production of antibodies against the Rh factor. During this pregnancy, however,

the patient was found to have elevated levels of anti-D (Rh) antibodies,

suggesting that sensitization must have already occurred at some point. The

doctors performed further tests and found that the anti-D (Rh) antibodies,

because of their type, could have crossed the placental barrier to the fetus.

This is a huge risk to the fetus because anti-D antibodies can attack the Rh-positive

blood cells of the fetus, which in extreme cases can lead to hemolytic disease.

Which class of antibodies can cross the placental barrier?