The Rh system
The Rh system is immunologically different than the ABO system. Unlike the ABO system, there are no naturally occurring (pre-formed) antibodies to the "D" antigen in the Rh system.
Under normal circumstances, Rh negative (Rh-) individuals who
have not been exposed to the Rh antigen will not make anti-D. Such individuals could be safely transfused with either Rh positive or Rh negative RBCs, particularly in an emergency situation.
Forming Anti- D
An Rh- individual may make anti-D only if exposed to the D-antigen via transfusion with Rh+ blood or platelets, or if pregnant with an Rh+ fetus. The probability of forming anti-D after transfusion of Rh+ RBCs is about one-third.
Note, however,
that this immunologic response requires approximately 3 to 4 months to develop and the patient is NOT at immediate risk of hemolysis. Individuals who have formed anti-D must receive only Rh- RBCs.
O Negative Selectivity
Many hospitals in the United States have a limited supply of group O- RBCs. Whenever possible; group 0- RBCs should be used for females of childbearing potential (generally women < 55 years of age) to avoid the possibility of alloimmunization.
In order to effectively maintain an adequate supply of O- RBCs for these and other patients who should or must receive only this type, it is common practice to transfuse Rh+ RBCs to Rh- males as well as females of non-childbearing potential
who require urgent or continued RBC transfusions.
