A 28-year-old primigravida at 32 weeks of gestation is found to have anti-D antibodies (Rh antibodies) in her serum at a titer of 1:64. She is Rh-negative and has no history of transfusion. Obstetric ultrasound shows fetal hydrops. Which antibody class is responsible for hemolytic disease of the newborn in this scenario, and what is the structural feature that allows transplacental passage?