Rh sensitization is an antibody response that may occur when a woman with Rh-negative blood has been exposed to blood from her Rh-positive fetus. Once she is sensitized, her immune system can make antibodies that can destroy Rh-positive red blood cells.

An Rh factor (Rh antigen) is one of the markers on the surface of red blood cells that the immune system can recognize. A person whose blood contains the Rh factor is Rh-positive. A person whose blood does not contain the Rh factor is Rh-negative.

Most women who become sensitized do so during childbirth, when their blood mixes with Rh-positive fetal blood. However, blood mixing sometimes occurs during a miscarriage, an abortion, or an injury (especially to the woman's abdomen).

Rh sensitization is no danger to the pregnant woman and usually is no harm to the first Rh-positive fetus. However, future Rh-positive fetuses are in danger of having their red blood cells destroyed before birth by the pregnant woman's immune system. This danger can usually be prevented by giving the Rh-negative woman an Rh immune globulin injection (such as RhoGAM) at 24 to 28 weeks of pregnancy, and after delivery if the baby is Rh-positive.

An Rh-negative woman can only become pregnant with an Rh-positive fetus if the father is Rh-positive.