Since D is the most immunogenic antigen in the Rh system, testing for the presence or absence of the antigen is routinely required. Immunogenicity is the ability of an antigen to stimulate an immune response. It has been shown that exposure to as little as 0.1 mL of Rh-positive red cells can stimulate production of anti-D in an Rh-negative individual. c, E, C, and e antigens - in that order - have all been found to be highly capable of eliciting an immune response. During the course of a workup for identification of an immune antibody, for example, the patient's red cells may be tested for all five antigens in the Rh system. In clinical situations such as sickle cell disease or thalassemia requiring ongoing transfusion of red cells, some institutions proactively match the patient and donor for Rh and K phenotypes to prevent the formation of multiple antibodies.
