Alloantibody

Alloantibody is an antibody produced by an individual in response to antigens that are specific to another individual of the same species. These antibodies commonly arise when a person is exposed to foreign blood group antigens through transplantation, pregnancy, or blood transfusion. The most clinically significant alloantibody reactions involve the red blood cell surface antigens of the ABO and Rh systems, but alloantibodies can also target antigens on platelets, white blood cells, and tissues in solid organ transplants.

The development of alloantibodies typically follows antigen exposure. Once the immune system encounters a foreign antigen, B lymphocytes are activated and differentiate into plasma cells that secrete specific immunoglobulin isotypes, most often IgG or IgM. The strength and persistence of the antibody response depend on the immunogenicity of the antigen, the dose of exposure, and the patient’s immunological history. In blood transfusion, even a single transfusion of non-identical blood can elicit multiple alloantibodies, while repeated exposure may lead to higher titres and more robust responses.

Clinical implications are significant in transfusion medicine and transplant biology. Sensitization to ABO or Rh antigens can cause acute hemolytic transfusion reactions or delayed hemolytic transfusion reactions and is a major cause of graft failure in kidney transplantation. For pregnancies, maternal alloantibodies against fetal red blood cell antigens can cause hemolytic disease of the newborn (HDN), a potentially serious condition that may require intrauterine monitoring or neonatal transfusion.

Screening for alloantibodies is performed by antibody panels, cross‑matching techniques, and serologic testing. When a new antigen exposure is anticipated, such as a planned transplant, pre‑transplant testing is critical to identify any preformed alloantibodies that could compromise compatibility. In the hospital setting, awareness of alloantibody status guides blood product selection and informs risk mitigation strategies for both donors and recipients.