appear unless the carrier has been exposed to Rh antigens. Another difference between the two systems lies in the fact that most of the antibodies of the rh system are incomplete IgG antibodies, which, in contrast to the complete IgM antibodies of ABO agglutinins, are small enough to pass the placental barrier. Before giving a transfusion to a person, it is necessary to do tests to determine the blood type including ABO and rh, and a cross-matched testappear unless the carrier has been exposed to Rh antigens. Another difference between the two systems lies in the fact that most of the antibodies of the Rh system are incomplete IgG antibodies, which, in contrast to the complete IgM antibodies of ABO agglutinins, are small enough to pass the placental barrier. Before giving a transfusion to a person, it is necessary to do tests to determine the blood type including ABO and Rh, and a cross-matched test. 13