Final answer:
The antibody responsible for crossing the placenta and causing Hemolytic Disease of the Newborn (HDN) when an Rh-negative mother has an Rh-positive baby is anti-Rh factor IgG, which can cause the destruction of fetal red blood cells.
Step-by-step explanation:
If a group O mother gives birth to a group A baby, the antibodies responsible for crossing the placenta and potentially causing Hemolytic Disease of the Newborn (HDN) are anti-Rh factor IgG. HDN is a type II hypersensitivity reaction that can occur when an Rh-negative mother has an Rh-positive fetus.
During the first pregnancy with an Rh-positive fetus, Rh-negative mothers are usually not at risk because there is often not enough time for the mother to produce a significant amount of anti-Rh antibodies. However, a subsequent pregnancy with an Rh-positive fetus can lead to a secondary immune response, where the mother has already been sensitized and produces a larger quantity of anti-Rh factor IgG antibodies.
These antibodies can cross the placenta and target the fetal Rh-positive red blood cells, leading to hemolysis, where the red blood cells are destroyed, and this condition is known as HDN.
To prevent HDN due to Rh factor incompatibility, Rh-negative mothers are treated with Rho(D) immune globulin, which is given during the 28th week of pregnancy and within 72 hours after delivery. This prevents the mother's immune system from being sensitized to Rh-positive cells and producing anti-Rh antibodies that could harm future Rh-positive pregnancies.