What is being tested?The indirect antiglobulin test (IAT) looks for circulating
antibodies in the blood directed against red blood cells (RBCs). The primary reasons that you may have RBC antibodies circulating in your blood are because you have been exposed through blood transfusion or through pregnancy to RBCs other than your own (foreign RBCs).
RBCs normally have structures on their surface called antigens. You have your own individual set of antigens on your RBCs, determined by inheritance from your parents. The two major antigens or surface identifiers on human RBCs are the A and B antigens, and your blood is grouped 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 your red blood cells, your blood type is Rh+ (positive); if it is absent, your blood is type Rh- (negative). (For more on these, 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 you may begin to produce antibodies against certain 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, your ABO group and Rh type is matched with that of donor blood to prevent a serious transfusion reaction from occurring. That is, the donor’s blood must be the same ABO group and Rh type as yours so that your antibodies do not react with and destroy donor blood cells. However, if you receive a blood transfusion, your body may recognize antigens from other blood groups (such as Kell or Kidd) that you do not have as foreign. You then may produce antibodies to attack these foreign antigens. People who have many transfusions are more likely to make antibodies to RBCs because they are exposed to more foreign RBC antigens.
With mother-baby blood type incompatibility
A baby may inherit antigens from its father that are not on its 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 IAT can help determine if the mother has produced RBC antibodies.
The first time you are exposed to a foreign RBC antigen, by transfusion or pregnancy, you may begin to produce antibodies but your 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 hemolyze the transfused RBCs or the baby’s RBCs.
The IAT screens for the presence of RBC antibodies (other than ABO antibodies) in your blood. RBC antibodies that are detected with the IAT can be identified with an antibody identification test (see Blood Banking for more information).
