Red Cell Antigens and Antibodies.

Autoimmune hemolytic anemia (AIHA) is caused by the production of "warm-" or "cold-" reactive autoantibodies directed against RBC antigens that may be of undefined specificity, reacting with all RBCs tested or may have an apparent specificity. Autoantibodies may be of IgG, IgM, or rarely IgA isotypes and their production can be triggered by disease, viral infection, or drugs; from breakdown in immune system tolerance to self-antigens; or from exposure to foreign antigens that induce antibodies that cross-react with self-RBC antigens. Increasingly, AIHA is being reported in patients following allogeneic hematopoietic stem cell transplantation and treatment with anti-cancer checkpoint inhibitors. Autoantibodies, whatever their etiology, interfere with pretransfusion testing of patients requiring RBCs transfusion making compatibility testing complex and labor-intensive. The availability of extended antigen typing by DNA-based assay has made transfusion of RBCs that are selected based on the patient's extended phenotype (e.g., D, C, E, e, K, Jk, Jk, Fy, Fy, S and s) a feasible option and this can provide a significant measure of safety as they avoid the patient being immunized to antigens absent from their RBCs.