alloantibodies

Alloantibodies are antibodies directed against antigens from a different individual of the same species. They arise after exposure to non-self antigens through transfusion of red blood cells or platelets, pregnancy with fetal antigens, or organ transplantation. They are distinct from autoantibodies, which target the individual's own antigens.

Most clinically significant alloantibodies are of the IgG class and can cross the placenta; IgM alloantibodies may activate complement but often do not cause pregnancy complications. Alloantibodies target blood group antigens such as Rh, Kell, Duffy, Kidd, and MNS, or human leukocyte antigen (HLA) antigens.

Clinical significance varies by context. In transfusion, alloantibodies against red blood cell antigens can cause acute or delayed hemolytic transfusion reactions, necessitating antigen-matched or phenotypically compatible units. In pregnancy, maternal anti-HLA or anti-HPA antibodies can cause fetal/neonatal alloimmune thrombocytopenia or hemolytic disease of the fetus and newborn. In transplantation, donor-specific anti-HLA antibodies are associated with graft rejection—hyperacute, acute, or chronic—and poorer outcomes; their presence is assessed with sensitive antibody testing such as flow cytometry or solid-phase assays.

Detection and testing involve antibody screening with indirect antiglobulin tests, panel reactive antibody measurements, and crossmatching (major, minor, and flow cytometry-based crossmatches). Specificity is often determined by high-resolution assays such as Luminex single antigen bead panels.

Management and prevention focus on minimizing exposure to relevant non-self antigens when possible, transfusing compatible or antigen-mashed units, and using desensitization strategies (e.g., plasmapheresis and intravenous immunoglobulin) in transplantation. In pregnancy, monitoring and counseling address alloimmune risks, while ongoing monitoring for signs of alloimmune disease or graft rejection is essential in affected recipients.