Antibody tests are done to find antibodies that attack red blood cells. Antibodies are proteins made by the immune system. Normally, antibodies bind to foreign substances, such as bacteria and viruses, and destroy them.

The following conditions cause antibodies to be made.

Transfusion reaction

Human blood is typed by certain markers (called antigens) on the surface of red blood cells. If you get a blood transfusion, the transfused blood must match your type; that is, it must have the same antigens as your red blood cells. If you get a transfusion of blood with antigens different from yours (incompatible blood), your immune system destroys the transfused blood cells. This is called a transfusion reaction and can cause serious illness or even death. This is why matching blood type is so important.

Rh sensitization

Rh is an antigen. The full name for this antigen is Rhesus factor.

If a pregnant woman with Rh-negative blood has a baby (fetus) with Rh-positive blood, Rh sensitization may occur. The baby gets Rh-positive blood from the father's genes. Rh sensitization happens when the baby's blood mixes with the mother's blood during delivery. This causes the mother's immune system to make antibodies against the baby's red blood cells in future pregnancies. This antibody response is called Rh sensitization and, depending on when it happens, can destroy the baby's red blood cells. If sensitization happens, the baby can develop mild to severe problems (called Rh disease, hemolytic disease of the newborn, or erythroblastosis fetalis). In rare cases, if Rh sensitization is not treated, the baby may die.

A woman with Rh-negative blood can get a vaccine called Rh immune globulin (such as RhoGAM) that almost always stops sensitization from occurring. Problems from Rh sensitization have become very rare since the Rh immune globulin vaccine was developed.

Autoimmune hemolytic anemia

A type of hemolytic anemia called autoimmune hemolytic anemia is a rare disease that causes antibodies to be made against a person's own red blood cells.

Two blood tests can check for antibodies that fight against red blood cells: the direct Coombs test and the indirect Coombs test. The direct Coombs test finds antibodies that are already attached to red blood cells. The indirect Coombs test finds antibodies that could bind to certain red blood cells, leading to problems if blood mixing occurs.