alloimmunized

Alloimmunized describes a person who has formed alloantibodies against antigens on the cells of other individuals of the same species, typically red blood cells, due to exposure to foreign antigens through blood transfusion, pregnancy, or organ or bone marrow transplantation. Alloantibodies are usually IgG and can react at body temperature, causing clinically significant immune responses when the person is exposed again to matched donor cells carrying the corresponding antigen. Alloimmunization can complicate transfusion and obstetric care by causing hemolytic transfusion reactions or hemolytic disease of the fetus and newborn, and it can contribute to graft rejection after transplantation.

Common sensitization targets include red blood cell antigens such as the Rh system (notably D), Kell, Duffy, Kidd, and MNS antigens; HLA antigens can also be targets in organ or stem cell transplantation and in platelet transfusion refractoriness.

Diagnosis is based on antibody screening and identification (antibody screen, panel reactive factor, extended antigen typing). Management aims to prevent exposure to known antigens by providing antigen-negative or extensively matched units, or by reducing risk in pregnancy through prophylaxis and monitoring. In pregnancy, RhD immunoglobulin prophylaxis in RhD-negative mothers has markedly reduced alloimmunization against fetal D antigen. In transfusion medicine, patients who are alloimmunized may require antigen-negative units or donor units matched beyond the standard ABO/RhD compatibility. The term emphasizes the immune response to non-self antigens rather than autoimmunity and is typically a chronic condition requiring coordinated transfusion and obstetric planning.