Rh antigens, also known as Rhesus factors, are proteins found on the surface of red blood cells. They are inherited from both parents and play a role in determining a person's blood type. There are several Rh antigens, but the most important one is the RhD antigen. People who have the RhD antigen are Rh-positive, while those who don't have it are Rh-negative.

Rh antigens are important in pregnancy because they can cause a condition called Rh incompatibility. This occurs when a Rh-negative mother is carrying a Rh-positive fetus. The mother's immune system can see the RhD antigen on the fetus's red blood cells as foreign and produce antibodies against it. These antibodies can cross the placenta and attack the fetus's red blood cells, causing hemolytic anemia.

To prevent Rh incompatibility, Rh-negative mothers are given a medication called RhoGAM. This medication helps to block the mother's immune system from producing antibodies against the RhD antigen.

Rh antigens are also important in blood transfusions. People who are Rh-negative can only receive blood from other Rh-negative people. If a Rh-negative person receives blood from a Rh-positive person, their immune system can produce antibodies against the RhD antigen. These antibodies can cause a transfusion reaction, which can be fatal.