The IMMUNISATION of a person by an ANTIGEN they do not have but which is present in other people. For example, a rhesus-negative mother does not carry the rhesus antigen. If she carries a rhesus-positive baby, passage of the rhesus antigen from the baby into the mother's circulation before birth may cause her to be iso-immunised. Her immune system (see IMMUNITY) may be provoked into producing ANTIBODIES to the rhesus antigen. When she next becomes pregnant, if the baby is again rhesus positive, the mother will produce large amounts of anti-Rh antibodies in early pregnancy which can enter the fetal circulation and cause its blood cells to break up. (See HAEMOLYTIC DISEASE OF THE NEWBORN; BLOOD GROUPS.)