alloimmunization

Alloimmunization is the development of antibodies against antigens from another individual of the same species. It most commonly arises after exposure to foreign red blood cell (RBC) antigens during blood transfusion, pregnancy, or transplantation. The immune response is typically humoral, producing IgG alloantibodies that target specific RBC antigens, such as those in the Rh system (D, C, E), Kell, Duffy, Kidd, and MNS systems. These antibodies can persist and cause problems on subsequent exposures.

In pregnancy, maternal alloimmunization to fetal RBC antigens can lead to hemolytic disease of the fetus and newborn (HDFN) when IgG antibodies cross the placenta and destroy fetal red cells. The risk is highest with Rh(D) incompatibility but other antibodies (e.g., anti-K, anti-E) can also cause fetal injury. In transfusion, alloantibodies can cause acute or delayed hemolytic transfusion reactions and complicate future transfusions by limiting compatible blood.

Diagnosis relies on antibody screening and identification. The indirect antiglobulin test detects alloantibodies in the recipient’s serum; positive tests are followed by antibody identification and selection of antigen-compatible donor units. In pregnancy, fetal antigen testing may inform risk assessment and management.

Prevention and management aim to reduce sensitization and mitigate effects. Rh(D) immune globulin prophylaxis given to Rh-negative pregnant women prevents maternal sensitization. In transfusion practice, extended antigen matching (including C, E, and Kell antigens) for at-risk patients reduces alloimmunization; treating sensitized patients requires antigen-negative transfusions. Neonatal management of HDFN may include phototherapy, intravenous immunoglobulin, and exchange transfusion as needed.