In terms of transfusion and HDN, the other blood group systems and their antigens become important only when antibody develops. Transfusion service laboratories identify (antibody identification) the specificity and characterize the reactivity of antibodies detected in routine testing (antibody screening). Once this information is known, the blood bank assesses the clinical significance of the antibody and selects the most appropriate blood for transfusion. A detailed description of all the blood group antigens is beyond the scope of this chapter. Because the molecular bases of most blood group antigens and phenotypes are known, DNA analysis can be used to type transfused patients and to identify the fetus at risk for HDN.