Clinical Laboratory - Transfusion Medicine

Alloimmunization and Hemolytic Disease of the Newborn

Alloimmunization, which is an immune response to foreign antigens from the same species, can occur with the transfusion of PRBCs and platelets.

PRBCs

RBC transfusions can expose the patient to RBC antigens not recognized as self. If an antibody is produced, future transfusions can be delayed because extended donor blood typing will be required to identify compatible units.

O negative blood released uncrossmatched in emergencies could result in a hemolytic transfusion reaction if the patient has an alloantibody produced after a previous transfusion.

Alloantibody production in a female can result in hemolytic disease of the newborn.

Platelets

Platelets contain HLA and A & B antigens. Prior exposure to non-self HLA antigens (from WBC contamination of red cell products) can result in antibodies that will render future platelet transfusions useless.

Hemolytic Disease of the Newborn

Maternal IgG antibodies can cross the placenta and provide the fetus with passive immunity following birth. However, these antibodies could be directed at antigens on fetal cells. When directed at red blood cell antigens, then hemolysis can occur, with fetal hydrops and/or neonatal jaundice. Both major (A and B) and minor (Rh and others) antigens may be targeted.

• There is usually little IgG directed at major A and B antigens, so maternal-fetal ABO incompatibility is usually nonexistent or mild. However, maternal exposure to Rh antigens, typically with prior pregnancy, or prior transfusion, can induce higher IgG titers that can lead to significant hemolysis pre and postnatally in subsequent pregnancies.

• Most cases are due to antibodies to the "D" antigen, termed "Rh Positive." The Rh negative mother forms antibodies on exposure to Rh positive RBCs. However, antibodies may also be directed at other Rh antigens and other minor blood group antigens such as Kell.

Screening for these alloantibodies can be done with a Coombs test. To prevent alloimmunization at the time of birth, when fetal-maternal hemorrhage may occur, exposing the mother to fetal RBC antigens, the product RhoGAM (Rh immune globulin) can be given.