What is being tested?
The RBC antibody screen looks for circulating antibodies in the blood directed against red blood cells (RBCs). The primary reason that a person may have RBC antibodies circulating in the blood is because the person has been exposed, through blood transfusion or through pregnancy, to RBCs other than his or her own (foreign RBCs). These antibodies have the potential to cause harm if a person is transfused with red blood cells that the antibodies may target or if a pregnant woman has antibodies that target the red cells of her developing baby.
RBCs normally have structures on their surface called antigens. People have their own individual set of antigens on their RBCs, determined by inheritance from their parents. The major antigens or surface identifiers on human RBCs are the O, A, and B antigens, and a person’s blood is grouped into an A, B, AB, or O blood type according to the presence or absence of these antigens.
Another important surface antigen is Rh factor, also called D antigen. If it is present on a person’s red blood cells, that person’s blood type is Rh+ (positive); if it is absent, the blood is type Rh- (negative). (For more on these antigens, see the article on Blood Typing). In addition, there are many other types of RBC antigens that make up lesser-known blood groups, such as Kell, Lewis, and Kidd blood groups.
There are a few reasons why someone may produce antibodies against RBC antigens.
- Following blood transfusions: Antibodies directed against A and B red cell antigens are naturally-occurring; we produce them without having to be exposed to the antigens. Before receiving a blood transfusion, a person’s ABO group and Rh type are matched with that of donor blood to prevent a serious transfusion reaction from occurring. That is, the donor’s blood must be compatible with the recipient’s so that antibodies do not react with and destroy donor blood cells.If someone receives a blood transfusion, the person’s body may also recognize other RBC antigens from other blood groups (such as Kell or Kidd) that the person does not have as foreign. The recipient may produce antibodies to attack these foreign antigens. People who have many transfusions make antibodies to RBCs because they are exposed to foreign RBC antigens with each transfusion.
- During pregnancy, with blood type incompatibility between mother and baby: A baby may inherit antigens from the father that are not on the mother’s RBCs. The mother may be exposed during pregnancy or at delivery to the foreign antigens on her baby’s RBCs when some of the baby’s cells enter the mother’s circulation as the placenta separates. The mother may begin to produce antibodies against these foreign RBC antigens. This can cause hemolytic disease of the newborn, usually not affecting the first baby but affecting subsequent children when the mother’s antibodies cross the placenta, attach to the baby’s RBCs, and hemolyze them. An RBC antibody screen can help determine if the mother has produced RBC antibodies outside of the ABO blood group.
The first time a person is exposed to a foreign RBC antigen, by transfusion or pregnancy, the person may begin to produce antibodies but his or her cells do not usually have the time during the first exposure to make enough antibodies to actually destroy the foreign RBCs. When the next transfusion or pregnancy occurs, the immune response may be strong enough for enough antibodies to be produced, attach to, and break apart (hemolyze) the transfused RBCs or the baby’s RBCs. Antibodies to the ABO antigens are naturally-occurring so do not require exposure to foreign RBCs.
