Which of the following conditions causes erythroblastosis foetalis?

Mother is Rh^-ve and foetus is Rh^+ve in case of erythroblastosis foetalis.

Sensitisation: If an Rh-negative mother carries an Rh-positive fetus, there is a risk of Rh sensitisation during pregnancy or childbirth. This means that a small amount of Rh-positive blood from the foetus can enter the maternal bloodstream, usually during delivery or other events like miscarriage or abortion.

Maternal Antibody Production: When the mother is exposed to the Rh antigen (present on Rh-positive red blood cells), her immune system may produce antibodies against it. These antibodies are of the IgG type and are capable of crossing the placenta.

Subsequent Pregnancies: The problem arises in subsequent pregnancies if the mother becomes pregnant with another Rh-positive fetus. The antibodies produced during the first pregnancy can cross the placenta and attack the Rh-positive red blood cells of the fetus. This can lead to hemolysis (destruction of red blood cells) in the fetus, resulting in anemia and other complications, including erythroblastosis fetalis.