The detection of antibodies against human erythrocytes is important in the diagnosis of hemolytic anemia, erythroblastosis fetalis, and transfusion reactions and is essential for crossmatching blood before transfusion. Such antibodies belong to the immunoglobulin group of proteins. The immunoglobulin-S are IgG, the most common; IgM; and IgA. Tests for blood group antibodies must detect antibody protein of all three immunoglobulin types. While no one test will do this for every antibody, the Coombs' (antiglobulin) test is the best because it reacts with most antibodies in each class of immunoglobulin. The test is done with antiglobulin serum, an anti-antibody type of reagent produced in laboratory animals. Rabbits injected with human serum produce antibodies to human proteins. Serum from such immunized animals, when properly prepared, can be used as an anti-antibody to detect human immunoglobulins, particularly those that have reacted with erythrocytes. In the Coombs' test, erythrocytes coated with antibody are agglutinated